Showing 1–50 of 79 results for author: Kopatskaya, E N

Multiwavelength quasi-periodic variability of the blazar Ton 599

Authors: Yu. V. Sotnikova, T. V. Mufakharov, A. E. Volvach, V. V. Vlasyuk, M. L. Khabibullina, A. G. Mikhailov, T. An, D. O. Kudryavtsev, Yu. A. Kovalev, Y. Y. Kovalev, A. V. Popkov, S. S. Savchenko, A. K. Erkenov, D. A. Morozova, T. A. Semenova, O. I. Spiridonova, M. A. Kharinov, I. A. Rakhimov, T. S. Andreeva, L. Cui, X. Wang, N. Chang, R. Yu. Udovitskiy, P. G. Zhekanis, G. A. Borman , et al. (12 additional authors not shown)

Abstract: During the last 40-50 years, the TeV blazar Ton 599 has systematically experienced major outbursts detected in a wide wavelength range from radio to $γ$-rays. In this work, we present an analysis of Ton 599 quasi-periodic variability across multiple wavelengths using an observing baseline from 1983 to 2025. The $γ$-ray, optical, and radio emissions are found to be highly correlated with time lags… ▽ More During the last 40-50 years, the TeV blazar Ton 599 has systematically experienced major outbursts detected in a wide wavelength range from radio to $γ$-rays. In this work, we present an analysis of Ton 599 quasi-periodic variability across multiple wavelengths using an observing baseline from 1983 to 2025. The $γ$-ray, optical, and radio emissions are found to be highly correlated with time lags $\sim0$-$360$ days, which indicates that they are triggered by the same population of particles. Using the Lomb-Scargle periodogram and the Weighted Wavelet Z-transform, we revealed several periodic components with characteristic periods of 1.4, 1.7, 2.3, 6.5, and 7.5 yrs. The result is consistent with the detection of periodic components in the 1997-2011 light curves, which means that we observe the same mechanism causing long-term periodic variability. A model of a binary supermassive black hole (SMBH) with a precessing jet, applied to the radio light curves of Ton 599, yields frequency-dependent best-fitting parameters with orbital periods ranging from $\sim$1.2 to 1.7 yrs and precession periods from $\sim$5.8 to 7.7 yrs. This result implies the existence of an SMBH system modulating emission through both the orbiting and jet precession effects, with differing observed periods possibly reflecting frequency-dependent emission regions along a structured, stratified jet. Nonetheless, the short-term periodicity and exceptionally strong flares likely arise from internal jet shocks, which aligns with typical blazar behavior. We suggest that the multiband quasi-periodicity of Ton 599 is reasonably described by a combination of geometric effects (orbital motion and precession) and stochastic processes. △ Less

Submitted 5 March, 2026; originally announced March 2026.

Comments: 42 pages. Published in MNRAS

First detection of VHE gamma-ray signal from the FSRQ TON 0599

Authors: MAGIC Collaboration, K. Abe, S. Abe, J. Abhir, A. Abhishek, V. A. Acciari, A. Aguasca-Cabot, I. Agudo, T. Aniello, S. Ansoldi, L. A. Antonelli, A. Arbet Engels, C. Arcaro, K. Asano, A. Babić, U. Barres de Almeida, J. A. Barrio, L. Barrios-Jiménez, I. Batković, J. Baxter, J. Becerra González, W. Bednarek, E. Bernardini, J. Bernete, A. Berti , et al. (223 additional authors not shown)

Abstract: TON\,0599 (z=0.7247) belongs to the few flat spectrum radio quasars (FSRQs) detected in the very high energy (VHE, $E > 100$\,GeV) gamma-ray band. Its redshift makes it currently one of the farthest VHE gamma-ray sources. It was detected for the first time with the MAGIC telescopes on 2017 December 15, and observed until December 29. The flux reached a maximum of about 50 per cent of the Crab Nebu… ▽ More TON\,0599 (z=0.7247) belongs to the few flat spectrum radio quasars (FSRQs) detected in the very high energy (VHE, $E > 100$\,GeV) gamma-ray band. Its redshift makes it currently one of the farthest VHE gamma-ray sources. It was detected for the first time with the MAGIC telescopes on 2017 December 15, and observed until December 29. The flux reached a maximum of about 50 per cent of the Crab Nebula flux above 80 GeV on the second night of observation, after which we witnessed a gradual decrease of the flux. The VHE gamma-ray spectrum connects smoothly to the one in the high energy ($E > 100$\,MeV) band obtained from simultaneous observations with {\textit Fermi}-LAT. It features a cut-off at energies around 50\,GeV, indicating the location of the gamma-ray emission zone beyond the broad line region. In addition, we were able to follow the spectral evolution during the fading phase of the flare. Multiwavelength analysis based on observations in optical, near-infrared, and radio bands acquired by the Whole Earth Blazar Telescope (WEBT) Collaboration from November to March, as well as observations in X-ray and optical--UV bands with instruments on board the \textit{Swift} satellite, shows strong correlation between different bands. We model the broadband emission with a simple one-zone leptonic model, where the high-energy peak is predominantly produced by external Compton (EC) scattering of photons from the dusty torus. △ Less

Submitted 14 March, 2026; v1 submitted 7 January, 2026; originally announced January 2026.

Comments: 17 pages, 8 figures, 8 tables, Published in Monthly Notices of the Royal Astronomical Society

Journal ref: MNRAS, 546, 4, (2026) stag032

Multiband optical variability on diverse timescales of the blazar Ton 599 from 2011 to 2023

Authors: O. Vince, C. M. Raiteri, M. Villata, A. C. Gupta, J. Kovačević-Dojčinović, M. Lakićević, L. Č. Popović, P. Kushwaha, D. O. Mirzaqulov, S. A. Ehgamberdiev, D. Carosati, S. G. Jorstad, A. P. Marscher, Z. R. Weaver, J. R. Webb, P. S. Smith, W. P. Chen, A. Tsai, H. C. Lin, G. A. Borman, T. S. Grishina, V. A. Hagen-Thorn, E. N. Kopatskaya, E. G. Larionova, V. M. Larionov , et al. (50 additional authors not shown)

Abstract: (Shortened)Context: We analyze the optical variability of the FSRQ Ton 599 using BVRI photometry from the WEBT collaboration (2011-2023), complemented by photometric and spectroscopic data from the Steward Observatory.\\ Aims: To characterize short- and long-term optical variability -- including flux distributions, intranight changes, color evolution, and spectra -- to constrain physical parameter… ▽ More (Shortened)Context: We analyze the optical variability of the FSRQ Ton 599 using BVRI photometry from the WEBT collaboration (2011-2023), complemented by photometric and spectroscopic data from the Steward Observatory.\\ Aims: To characterize short- and long-term optical variability -- including flux distributions, intranight changes, color evolution, and spectra -- to constrain physical parameters and processes in the central engine.\\ Methods: We tested flux distributions in each filter against normal and log-normal, explored the RMS-flux relation and derived PSDs. We quantified intranight variability using a $χ^2$ test and fractional variability. From variability timescales, we estimated the emitting region size and magnetic field. Long-term variability was studied by segmenting the light curve into 12 intervals and analyzing flux statistics. For multi-filter flares, we computed spectral slopes, redshift-corrected fluxes, monochromatic luminosities and generated Color-magnitude and color-time diagrams. From low-flux spectra, we measured Mg II line and estimated the black hole mass.\\ Results: Ton 599 showed strong optical variability. Log-normal distributions fit the fluxes better, and all bands show a positive RMS-flux relation with red-noise PSDs.Intranight variability is detected, and used in constraining the emission region and magnetic field.The R band reaches a peak flux of 23.5 mJy, corresponding to a monochromatic luminosity of $log(νLν)= 48.48 [erg/s]$. We found a redder-when-brighter trend at low fluxes (thermal), achromatic behavior at intermediate levels (possibly due to jet orientation changes), and a bluer-when-brighter trend at high fluxes (synchrotron). Long-term color changes are modest, short-term are significant, with a negative correlation between the amplitude of color changes and the average flux. The estimated SMBH mass is order of $10^8 M_\odot$. △ Less

Submitted 26 December, 2025; originally announced December 2025.

Comments: 22 pages, 12 figures (+1 in Appendix), 9 tables (+1 in Appendix)

Journal ref: A&A, 703 (2025) A259

Detection of Compton scattering in the jet of 3C 84

Authors: Ioannis Liodakis, Sudip Chakraborty, Frédéric Marin, Steven R. Ehlert, Thibault Barnouin, Pouya M. Kouch, Kari Nilsson, Elina Lindfors, Tapio Pursimo, Georgios F. Paraschos, Riccardo Middei, Anna Trindade Falcão, Svetlana Jorstad, Iván Agudo, Yuri Y. Kovalev, Jacob J. Casey, Laura Di Gesu, Philip Kaaret, Dawoon E. Kim, Fabian Kislat, Ajay Ratheesh, M. Lynne Saade, Francesco Tombesi, Alan Marscher, Francisco José Aceituno , et al. (55 additional authors not shown)

Abstract: 3C 84 is the brightest cluster galaxy in the Perseus Cluster. It is among the closest radio-loud active galaxies and among the very few that can be detected from low frequency radio up to TeV $γ$-rays. Here we report on the first X-ray polarization observation of 3C~84 with the Imaging X-ray Polarimetry Explorer, for a total of 2.2 Msec that coincides with a flare in $γ$-rays. This is the longest… ▽ More 3C 84 is the brightest cluster galaxy in the Perseus Cluster. It is among the closest radio-loud active galaxies and among the very few that can be detected from low frequency radio up to TeV $γ$-rays. Here we report on the first X-ray polarization observation of 3C~84 with the Imaging X-ray Polarimetry Explorer, for a total of 2.2 Msec that coincides with a flare in $γ$-rays. This is the longest observation for a radio-loud active galaxy that allowed us to reach unprecedented sensitivity, leading to the detection of an X-ray polarization degree of $\rmΠ_X=4.2\pm1.3\%$ ($\sim3.2σ$ confidence) at an X-ray electric vector polarization angle of $\rm ψ_X=163^{\circ}\pm9^{\circ}$, that is aligned with the radio jet direction on the sky. Optical polarization observations show fast variability about the jet axis as well. Our results strongly favor models in which X-rays are produced by Compton scattering from relativistic electrons -- specifically Synchrotron Self-Compton -- that takes places downstream, away from the supermassive black hole. △ Less

Submitted 11 November, 2025; v1 submitted 17 October, 2025; originally announced October 2025.

Comments: 21 pages, 13 figure, 1 Table, published in APJL

Journal ref: ApJL 994 (2025) L9

Unveiling blazar synchrotron emission: a multiwavelength polarimetric study of HSP and LSP populations

Authors: Sara Capecchiacci, Ioannis Liodakis, Riccardo Middei, Dawoon E. Kim, Laura Di Gesu, Ivan Agudo, Beatriz Agis-Gonzalez, Axel Arbet-Engels, Dmitry Blinov, Chien-Ting Chen, Steven R. Ehlert, Ephraim Gau, Lea Heckmann, Kun Hu, Svetlana G. Jorstad, Philip Kaaret, Pouya M. Kouch, Henric Krawczynski, Elina Lindfors, Frederic Marin, Alan P. Marscher, Ioannis Myserlis, Stephen L. O'Dell, Luigi Pacciani, David Paneque , et al. (74 additional authors not shown)

Abstract: Polarimetric properties of blazars allow us to put constraints on the acceleration mechanisms that fuel their powerful jets. By studying the multiwavelength polarimetric behaviour of high synchrotron peaked (HSP) and low synchrotron peaked (LSP) blazars, we aim to explore differences in their emission mechanisms and magnetic field structure in the acceleration region. In this study, we take advant… ▽ More Polarimetric properties of blazars allow us to put constraints on the acceleration mechanisms that fuel their powerful jets. By studying the multiwavelength polarimetric behaviour of high synchrotron peaked (HSP) and low synchrotron peaked (LSP) blazars, we aim to explore differences in their emission mechanisms and magnetic field structure in the acceleration region. In this study, we take advantage of several X-ray polarisation observations of HSP by the IXPE, including four new observations of Mrk 501, and optical polarisation observations of LSP from RoboPol and many others. We find that the polarisation degree (PD) distribution of HSP in X-rays is systematically higher than in optical and mm-radio wavelengths, as reported in previous IXPE publications. The distribution of the X-ray electric vector position angles (PA) is centered around the jet axis with most of the observations consistent with zero difference within uncertainties. In fact, the distribution of the offset of the PA from the jet axis is consistent between the LSP and HSP populations (with PA measured in optical for the first, X-ray for the latter), suggesting a common magnetic field structure close to the acceleration region, in strong support of the emerging energy stratified picture of particle acceleration followed by energy loss in blazar jets. △ Less

Submitted 19 August, 2025; originally announced August 2025.

IXPE Observations of the Blazar Mrk 501 in 2022: A Multiwavelength View

Authors: L. Lisalda, E. Gau, H. Krawczynski, F. Tavecchio, I. Liodakis, A. Gokus, N. Rodriguez Cavero, M. Nowak, M. Negro, R. Middei, M. Perri, S. Puccetti, S. G. Jorstad, I. Agudo, A. P. Marscher, B. Agís-González, A. V. Berdyugin, M. I. Bernardos, D. Blinov, G. Bonnoli, G. A. Borman, I. G. Bourbah, C. Casadio, V. Casanova, A. J. Castro-Tirado , et al. (135 additional authors not shown)

Abstract: The blazar Markarian 501 (Mrk 501) was observed on three occasions over a 4-month period between 2022 March and 2022 July with the Imaging X-ray Polarimetry Explorer (IXPE). In this paper, we report for the first time on the third IXPE observation, performed between 2022 July 9 and 12, during which IXPE detected a linear polarization degree of $Π_X=6\pm2$ per cent at a polarization angle, measured… ▽ More The blazar Markarian 501 (Mrk 501) was observed on three occasions over a 4-month period between 2022 March and 2022 July with the Imaging X-ray Polarimetry Explorer (IXPE). In this paper, we report for the first time on the third IXPE observation, performed between 2022 July 9 and 12, during which IXPE detected a linear polarization degree of $Π_X=6\pm2$ per cent at a polarization angle, measured east of north, of $Ψ_X=143^\circ\pm11^\circ$ within the 2-8 keV X-ray band. The X-ray polarization angle and degree during this observation are consistent with those obtained during the first two observations. The chromaticity of the polarization across radio, optical, and X-ray bands is likewise consistent with the result from the simultaneous campaigns during the first two observations. Furthermore, we present two types of models to explain the observed spectral energy distributions (SEDs) and energy-resolved polarization: a synchrotron self-Compton model with an anisotropic magnetic field probability distribution in the emitting volume, as well as an energy-stratified shock model. Our results support both the shock scenario as well as support that small levels of magnetic field anisotropy can explain the observed polarization. △ Less

Submitted 9 July, 2025; originally announced July 2025.

Comments: 17 pages, 8 figures, 4 tables. Accepted for publication in Monthly Notices of the Royal Astronomical Society

Determining the origin of the X-ray emission in blazars through multiwavelength polarization

Authors: Ioannis Liodakis, Haocheng Zhang, Stella Boula, Riccardo Middei, Jorge Otero-Santos, Dmitry Blinov, Iván Agudo, Markus Böttcher, Chien-Ting Chen, Steven R. Ehlert, Svetlana G. Jorstad, Philip Kaaret, Henric Krawczynski, Abel L. Peirson, Roger W. Romani, Fabrizio Tavecchio, Martin C. Weisskopf, Pouya M. Kouch, Elina Lindfors, Kari Nilsson, Callum McCall, Helen E. Jermak, Iain A. Steele, Ioannis Myserlis, Mark Gurwell , et al. (26 additional authors not shown)

Abstract: The origin of the high-energy emission in astrophysical jets from black holes is a highly debated issue. This is particularly true for jets from supermassive black holes that are among the most powerful particle accelerators in the Universe. So far, the addition of new observations and new messengers have only managed to create more questions than answers. However, the newly available X-ray polari… ▽ More The origin of the high-energy emission in astrophysical jets from black holes is a highly debated issue. This is particularly true for jets from supermassive black holes that are among the most powerful particle accelerators in the Universe. So far, the addition of new observations and new messengers have only managed to create more questions than answers. However, the newly available X-ray polarization observations promise to finally distinguish between emission models. We use extensive multiwavelength and polarization campaigns as well as state-of-the-art polarized spectral energy distribution models to attack this problem by focusing on two X-ray polarization observations of blazar BL Lacertae in flaring and quiescent $γ$-ray states. We find that regardless of the jet composition and underlying emission model, inverse-Compton scattering from relativistic electrons dominates at X-ray energies. △ Less

Submitted 12 June, 2025; v1 submitted 19 May, 2025; originally announced May 2025.

Comments: 10 pages, 15 figures, 4 Tables, published in A&A

Journal ref: A&A 698, L19 (2025)

High optical to X-ray polarization ratio reveals Compton scattering in BL Lacertae's jet

Authors: Ivan Agudo, Ioannis Liodakis, Jorge Otero-Santos, Riccardo Middei, Alan Marscher, Svetlana Jorstad, Haocheng Zhang, Hui Li, Laura Di Gesu, Roger W. Romani, Dawoon E. Kim, Francesco Fenu, Herman L. Marshall, Luigi Pacciani, Juan Escudero Pedrosa, Francisco Jose Aceituno, Beatriz Agis-Gonzalez, Giacomo Bonnoli, Victor Casanova, Daniel Morcuende, Vilppu Piirola, Alfredo Sota, Pouya M. Kouch, Elina Lindfors, Callum McCall , et al. (125 additional authors not shown)

Abstract: Blazars, supermassive black hole systems (SMBHs) with highly relativistic jets aligned with the line of sight, are the most powerful long-lived emitters of electromagnetic emission in the Universe. We report here on a radio to gamma-ray multiwavelength campaign on the blazar BL Lacertae with unprecedented polarimetric coverage from radio to X-ray wavelengths. The observations caught an extraordina… ▽ More Blazars, supermassive black hole systems (SMBHs) with highly relativistic jets aligned with the line of sight, are the most powerful long-lived emitters of electromagnetic emission in the Universe. We report here on a radio to gamma-ray multiwavelength campaign on the blazar BL Lacertae with unprecedented polarimetric coverage from radio to X-ray wavelengths. The observations caught an extraordinary event on 2023 November 10-18, when the degree of linear polarization of optical synchrotron radiation reached a record value of 47.5%. In stark contrast, the Imaging X-ray Polarimetry Explorer (IXPE) found that the X-ray (Compton scattering or hadron-induced) emission was polarized at less than 7.4% (3sigma confidence level). We argue here that this observational result rules out a hadronic origin of the high energy emission, and strongly favors a leptonic (Compton scattering) origin, thereby breaking the degeneracy between hadronic and leptonic emission models for BL Lacertae and demonstrating the power of multiwavelength polarimetry to address this question. Furthermore, the multiwavelength flux and polarization variability, featuring an extremely prominent rise and decay of the optical polarization degree, is interpreted for the first time by the relaxation of a magnetic "spring" embedded in the newly injected plasma. This suggests that the plasma jet can maintain a predominant toroidal magnetic field component parsecs away from the central engine. △ Less

Submitted 3 May, 2025; originally announced May 2025.

Comments: Accepted for publication in ApJ Letters

Polarization Studies in Different Blazar Types. Part I: Correlation between Optical Brightness and Polarization Degree

Authors: E. A. Shkodkina, S. S. Savchenko, D. A. Morozova, S. G. Jorstad, G. A. Borman, T. S. Grishina, E. N. Kopatskaya, E. G. Larionova, A. A. Vasilyev, I. S. Troitskiy, Yu. V. Troitskaya, P. A. Novikova, E. V. Shishkina, A. V. Zhovtan

Abstract: We present an analysis of the cross-correlation between optical brightness and polarization degree in different types of blazars. The aim is to identify objects with simultaneous and consistent changes in characteristics and to determine if this behavior relates to the types of objects studied. The analysis includes 23 objects: 11 FSRQ, 11 BL Lac, and 1 radio galaxy. Dense overlapping observation… ▽ More We present an analysis of the cross-correlation between optical brightness and polarization degree in different types of blazars. The aim is to identify objects with simultaneous and consistent changes in characteristics and to determine if this behavior relates to the types of objects studied. The analysis includes 23 objects: 11 FSRQ, 11 BL Lac, and 1 radio galaxy. Dense overlapping observation series in the R band were used, collected over more than 10 years as part of a monitoring program for bright blazars at St. Petersburg State University. The cross-correlation analysis procedure is detailed, including a method for assessing significance based on Monte Carlo simulations of synthetic light curves modeled using a Damped Random Walk. Significant correlations were found for 5 FSRQ and 1 BL Lac. No significant correlation was detected for 10 BL Lac and 6 FSRQ. One object did not yield a reliable estimate. Based on the current results, we cannot claim that the observed difference in the behavior of these emission characteristics for different classes of blazars is significant. It is possible that observed correlations may be explained by the contribution of simultaneous flare events to the changes in flux and polarization degree curves, which occur more frequently in FSRQ objects. △ Less

Submitted 25 April, 2025; originally announced April 2025.

Comments: 16 pages, 6 figures, 3 tables

Multiband Optical Variability of the Blazar 3C 454.3 on Diverse Timescales

Authors: Karan Dogra, Alok C. Gupta, C. M. Raiteri, M. Villata, Paul J. Wiita, S. O. Kurtanidze, S. G. Jorstad, R. Bachev, G. Damljanovic, C. Lorey, S. S. Savchenko, O. Vince, M. Abdelkareem, F. J. Aceituno, J. A. Acosta-Pulido, I. Agudo, G. Andreuzzi, S. A. Ata, G. V. Baida, L. Barbieri, D. A. Blinov, G. Bonnoli, G. A. Borman, M. I. Carnerero, D. Carosati , et al. (57 additional authors not shown)

Abstract: Due to its peculiar and highly variable nature, the blazar 3C 454.3 has been extensively monitored by the WEBT team. Here, we present for the first time these long-term optical flux and color variability results using data acquired in B, V, R, and I bands over a time span of $\sim$ 2 decades. We include data from WEBT collaborators and public archives such as SMARTS, Steward Observatory, and ZTF.… ▽ More Due to its peculiar and highly variable nature, the blazar 3C 454.3 has been extensively monitored by the WEBT team. Here, we present for the first time these long-term optical flux and color variability results using data acquired in B, V, R, and I bands over a time span of $\sim$ 2 decades. We include data from WEBT collaborators and public archives such as SMARTS, Steward Observatory, and ZTF. The data are binned and segmented to study the source over this long term when more regular sampling was available. During our study, the long-term spectral variability reveals a redder when brighter (RWB) trend, which, however, stabilizes at a particular brightness cutoff $\sim$ 14.5 mag in the I-band, after which it saturates and evolves into a complex state. This trend indicates increasing jet emission dominance over accretion disk emission until jet emission completely dominates. Plots of the spectral index variation (following $F_ν \propto ν^{-α}$) reveal a bimodal distribution using a one-day binning. These correlate with two extreme phases of 3C 454.3, an outburst or high flux state and quiescent or low flux state, which are respectively jet and accretion disk dominated. We have also conducted intra-day variability studies of nine light curves and found that six of them are variable. Discrete Correlation Function (DCF) analysis between different optical waveband pairs peak at zero lags, indicating co-spatial emission in different optical bands. △ Less

Submitted 14 December, 2024; originally announced December 2024.

Comments: 18 pages, 6 figures, 5 tables

Journal ref: ApJS(2025) 276:1

Multi-wavelength picture of the misaligned BL Lac object 3C 371

Authors: J. Otero-Santos, C. M. Raiteri, A. Tramacere, J. Escudero Pedrosa, J. A. Acosta-Pulido, M. I. Carnerero, M. Villata, I. Agudo, I. A. Rahimov, T. S. Andreeva, D. V. Ivanov, N. Marchili, S. Righini, M. Giroletti, M. A. Gurwell, S. S. Savchenko, D. Carosati, W. P. Chen, S. O. Kurtanidze, M. D. Joner, E. Semkov, T. Pursimo, E. Benítez, G. Damljanovic, G. Andreuzzi , et al. (30 additional authors not shown)

Abstract: The BL Lac object 3C 371 is one of the targets that are regularly monitored by the Whole Earth Blazar Telescope (WEBT) Collaboration to study blazar variability on both short and long timescales. We aim to evaluate the long-term multiwavelength (MWL) behaviour of 3C 371, comparing it with the results derived for its optical emission in our previous study. For this, we make use of the multi-band ca… ▽ More The BL Lac object 3C 371 is one of the targets that are regularly monitored by the Whole Earth Blazar Telescope (WEBT) Collaboration to study blazar variability on both short and long timescales. We aim to evaluate the long-term multiwavelength (MWL) behaviour of 3C 371, comparing it with the results derived for its optical emission in our previous study. For this, we make use of the multi-band campaigns organized by the WEBT Collaboration in optical and radio between January 2018 and December 2020, and of public data from Swift and Fermi satellites and the MOJAVE Very Large Interferometry programme. We evaluate the variability shown by the source in each band with the amplitude variability quantification, as well as possible interband correlation using the z-Discrete Correlation Function. We also present a deep analysis of the optical-UV, X-ray and $γ$-ray spectral variability. With the MOJAVE data we perform a kinematics analysis, looking for components propagating along the jet, calculating its kinematics parameters. This set of parameters is later used for the interpretation of the source MWL behaviour, modelling the broadband spectral energy distribution (SED) of the source with theoretical blazar emission scenarios. △ Less

Submitted 13 December, 2024; v1 submitted 5 December, 2024; originally announced December 2024.

Comments: Accepted in A&A, 23 pages, 15 figures

IXPE Observation of the Low-Synchrotron Peaked Blazar S4 0954+65 During An Optical-X-ray Flare

Authors: Pouya M. Kouch, Ioannis Liodakis, Francesco Fenu, Haocheng Zhang, Stella Boula, Riccardo Middei, Laura Di Gesu, Georgios F. Paraschos, Iván Agudo, Svetlana G. Jorstad, Elina Lindfors, Alan P. Marscher, Henric Krawczynski, Michela Negro, Kun Hu, Dawoon E. Kim, Elisabetta Cavazzuti, Manel Errando, Dmitry Blinov, Anastasia Gourni, Sebastian Kiehlmann, Angelos Kourtidis, Nikos Mandarakas, Nikolaos Triantafyllou, Anna Vervelaki , et al. (112 additional authors not shown)

Abstract: The X-ray polarization observations made possible with the Imaging X-ray Polarimetry Explorer (IXPE) offer new ways of probing high-energy emission processes in astrophysical jets from blazars. Here we report on the first X-ray polarization observation of the blazar S4 0954+65 in a high optical and X-ray state. During our multi-wavelength campaign on the source, we detected an optical flare whose… ▽ More The X-ray polarization observations made possible with the Imaging X-ray Polarimetry Explorer (IXPE) offer new ways of probing high-energy emission processes in astrophysical jets from blazars. Here we report on the first X-ray polarization observation of the blazar S4 0954+65 in a high optical and X-ray state. During our multi-wavelength campaign on the source, we detected an optical flare whose peak coincided with the peak of an X-ray flare. This optical-X-ray flare most likely took place in a feature moving along the parsec-scale jet, imaged at 43 GHz by the Very Long Baseline Array. The 43 GHz polarization angle of the moving component underwent a rotation near the time of the flare. In the optical band, prior to the IXPE observation, we measured the polarization angle to be aligned with the jet axis. In contrast, during the optical flare the optical polarization angle was perpendicular to the jet axis; after the flare, it reverted to being parallel to the jet axis. Due to the smooth behavior of the optical polarization angle during the flare, we favor shocks as the main acceleration mechanism. We also infer that the ambient magnetic field lines in the jet were parallel to the jet position angle. The average degree of optical polarization during the IXPE observation was (14.3$\pm$4.1)%. Despite the flare, we only detected an upper limit of 14% (at 3$σ$ level) on the X-ray polarization degree; although a reasonable assumption on the X-ray polarization angle results in an upper limit of 8.8% ($3σ$). We model the spectral energy distribution (SED) and spectral polarization distribution (SPD) of S4 0954+65 with leptonic (synchrotron self-Compton) and hadronic (proton and pair synchrotron) models. The constraints we obtain with our combined multi-wavelength polarization observations and SED modeling tentatively disfavor hadronic models for the X-ray emission in S4 0954+65. △ Less

Submitted 10 March, 2025; v1 submitted 25 November, 2024; originally announced November 2024.

Comments: Accepted for publication in A&A, 18 pages, 5 figures, and 8 tables

Journal ref: A&A 695, A99 (2025)

A wiggling filamentary jet at the origin of the blazar multi-wavelength behaviour

Authors: C. M. Raiteri, M. Villata, M. I. Carnerero, S. O. Kurtanidze, D. O. Mirzaqulov, E. Benítez, G. Bonnoli, D. Carosati, J. A. Acosta-Pulido, I. Agudo, T. S. Andreeva, G. Apolonio, R. Bachev, G. A. Borman, V. Bozhilov, L. F. Brown, W. Carbonell, C. Casadio, W. P. Chen, G. Damljanovic, S. A. Ehgamberdiev, D. Elsaesser, J. Escudero, M. Feige, A. Fuentes , et al. (74 additional authors not shown)

Abstract: Blazars are beamed active galactic nuclei known for their strong multi-wavelength variability on timescales from years down to minutes. We aim to investigate the suitability of the twisting jet model presented in previous works to explain the multi-wavelength behaviour of BL Lacertae, the prototype of one of the blazar classes. According to this model, the jet is inhomogeneous, curved, and twistin… ▽ More Blazars are beamed active galactic nuclei known for their strong multi-wavelength variability on timescales from years down to minutes. We aim to investigate the suitability of the twisting jet model presented in previous works to explain the multi-wavelength behaviour of BL Lacertae, the prototype of one of the blazar classes. According to this model, the jet is inhomogeneous, curved, and twisting, and the long-term variability is due to changes in the Doppler factor due to variations in the orientation of the jet-emitting regions. We analysed optical data of the source obtained during monitoring campaigns organised by the Whole Earth Blazar Telescope (WEBT) in 2019-2022, together with radio data from the WEBT and other teams, and gamma-ray data from the Fermi satellite. In this period, BL Lacertae underwent an extraordinary activity phase, reaching its historical optical and gamma-ray brightness maxima. The application of the twisting jet model to the source light curves allows us to infer the wiggling motion of the optical, radio, and gamma-ray jet-emitting regions. The optical-radio correlation shows that the changes in the radio viewing angle follow those in the optical viewing angle by about 120 days, and it suggests that the jet is composed of plasma filaments, which is in agreement with some radio high-resolution observations of other sources. The gamma-ray emitting region is found to be co-spatial with the optical one, and the analysis of the gamma-optical correlation is consistent with both the geometric interpretation and a synchrotron self-Compton (SSC) origin of the high-energy photons. We propose a geometric scenario where the jet is made up of a pair of emitting plasma filaments in a sort of double-helix curved rotating structure, whose wiggling motion produces changes in the Doppler beaming and can thus explain the observed multi-wavelength long-term variability. △ Less

Submitted 29 October, 2024; originally announced October 2024.

Comments: In press for A&A

Journal ref: A&A 692 (2024) A48

A Two-Week $IXPE$ Monitoring Campaign on Mrk 421

Authors: W. Peter Maksym, Ioannis Liodakis, M. Lynne Saade, Dawoon E. Kim, Riccardo Middei, Laura Di Gesu, Sebastian Kiehlmann, Gabriele Matzeu, Iván Agudo, Alan P. Marscher, Steven R. Ehlert, Svetlana G. Jorstad, Philip Kaaret, Herman L. Marshall, Luigi Pacciani, Matteo Perri, Simonetta Puccetti, Pouya M. Kouch, Elina Lindfors, Francisco José Aceituno, Giacomo Bonnoli, Víctor Casanova, Juan Escudero, Beatriz Agís-González, César Husillos , et al. (131 additional authors not shown)

Abstract: X-ray polarization is a unique new probe of the particle acceleration in astrophysical jets made possible through the Imaging X-ray Polarimetry Explorer. Here we report on the first dense X-ray polarization monitoring campaign on the blazar Mrk 421. Our observations were accompanied by an even denser radio and optical polarization campaign. We find significant short-timescale variability in both X… ▽ More X-ray polarization is a unique new probe of the particle acceleration in astrophysical jets made possible through the Imaging X-ray Polarimetry Explorer. Here we report on the first dense X-ray polarization monitoring campaign on the blazar Mrk 421. Our observations were accompanied by an even denser radio and optical polarization campaign. We find significant short-timescale variability in both X-ray polarization degree and angle, including a $\sim90^\circ$ angle rotation about the jet axis. We attribute this to random variations of the magnetic field, consistent with the presence of turbulence but also unlikely to be explained by turbulence alone. At the same time, the degree of lower-energy polarization is significantly lower and shows no more than mild variability. Our campaign provides further evidence for a scenario in which energy-stratified shock-acceleration of relativistic electrons, combined with a turbulent magnetic field, is responsible for optical to X-ray synchrotron emission in blazar jets. △ Less

Submitted 25 October, 2024; originally announced October 2024.

Comments: 23 pages, including 8 pages of appendices. 12 figures, 3 tables. Submitted to ApJ

The Method of Searching for Rotations of the Polarization Position Angle of Quasars

Authors: S. S. Savchenko, D. A. Morozova, S. G. Jorstad, D. A. Blinov, G. A. Borman, A. A. Vasilyev, T. S. Grishina, A. V. Zhovtan, E. N. Kopatskaya, E. G. Larionova, I. S. Troitskiy, Yu. V. Troitskaya, E. V. Shishkina, E. A. Shkodkina

Abstract: Observations of quasars show that the polarization position angle of the emission coming from them varies greatly over time, including periods called rotations during which the angle changes in an orderly manner. The study proposes a method for identifying such events and assessing their statistical significance. The operation of the method is demonstrated using the example of long-term polarimetr… ▽ More Observations of quasars show that the polarization position angle of the emission coming from them varies greatly over time, including periods called rotations during which the angle changes in an orderly manner. The study proposes a method for identifying such events and assessing their statistical significance. The operation of the method is demonstrated using the example of long-term polarimetric observations of the blazars CTA 102, 3C 454.3, and OT 081. During the analysis of light curves, 51 rotations of the polarization position angle were found and it was shown that for CTA 102 and 3C 454.3 the rotations are predominantly oriented in one direction. △ Less

Submitted 13 September, 2024; originally announced September 2024.

Comments: 21 pages, 12 figures

Journal ref: Astrophysical Bulletin, Volume 79, Issue 2, pp. 186-202, 2024

X-ray and multiwavelength polarization of Mrk 501 from 2022 to 2023

Authors: Chien-Ting J. Chen, Ioannis Liodakis, Riccardo Middei, Dawoon E. Kim, Laura Di Gesu, Alessandro Di Marco, Steven R. Ehlert, Manel Errando, Michela Negro, Svetlana G. Jorstad, Alan P. Marscher, Kinwah Wu, Iván Agudo, Juri Poutanen, Tsunefumi Mizuno, Pouya M. Kouch, Elina Lindfors, George A. Borman, Tatiana S. Grishina, Evgenia N. Kopatskaya, Elena G. Larionova, Daria A. Morozova, Sergey S. Savchenko, Ivan S. Troitsky, Yulia V. Troitskaya , et al. (121 additional authors not shown)

Abstract: We present multiwavelength polarization measurements of the luminous blazar Mrk~501 over a 14-month period. The 2--8 keV X-ray polarization was measured with the Imaging X-ray Polarimetry Explorer (IXPE) with six 100-ks observations spanning from 2022 March to 2023 April. Each IXPE observation was accompanied by simultaneous X-ray data from NuSTAR, Swift/XRT, and/or XMM-Newton. Complementary optic… ▽ More We present multiwavelength polarization measurements of the luminous blazar Mrk~501 over a 14-month period. The 2--8 keV X-ray polarization was measured with the Imaging X-ray Polarimetry Explorer (IXPE) with six 100-ks observations spanning from 2022 March to 2023 April. Each IXPE observation was accompanied by simultaneous X-ray data from NuSTAR, Swift/XRT, and/or XMM-Newton. Complementary optical-infrared polarization measurements were also available in the B, V, R, I, and J bands, as were radio polarization measurements from 4.85 GHz to 225.5 GHz. Among the first five IXPE observations, we did not find significant variability in the X-ray polarization degree and angle with IXPE. However, the most recent sixth observation found an elevated polarization degree at $>3σ$ above the average of the other five observations. The optical and radio measurements show no apparent correlations with the X-ray polarization properties. Throughout the six IXPE observations, the X-ray polarization degree remained higher than, or similar to, the R-band optical polarization degree, which remained higher than the radio value. This is consistent with the energy-stratified shock scenario proposed to explain the first two IXPE observations, in which the polarized X-ray, optical, and radio emission arises from different regions. △ Less

Submitted 15 July, 2024; originally announced July 2024.

Comments: Accepted for publication in ApJ

Testing particle acceleration in blazar jets with continuous high-cadence optical polarization observations

Authors: Ioannis Liodakis, Sebastian Kiehlmann, Alan P. Marscher, Haocheng Zhang, Dmitry Blinov, Svetlana G. Jorstad, Iván Agudo, Erika Benítez, Andrei Berdyugin, Giacomo Bonnoli, Carolina Casadio, Chien-Ting Chen, Wen-Ping Chen, Steven R. Ehlert, Juan Escudero, Tatiana S. Grishina, David Hiriart, Angela Hsu, Ryo Imazawa, Helen E. Jermak, Jincen Jose, Philip Kaaret, Evgenia N. Kopatskaya, Bhavana Lalchand, Elena G. Larionova , et al. (22 additional authors not shown)

Abstract: Variability can be the pathway to understanding the physical processes in astrophysical jets, however, the high-cadence observations required to test particle acceleration models are still missing. Here we report on the first attempt to produce continuous, >24 hour polarization light curves of blazars using telescopes distributed across the globe and the rotation of the Earth to avoid the rising S… ▽ More Variability can be the pathway to understanding the physical processes in astrophysical jets, however, the high-cadence observations required to test particle acceleration models are still missing. Here we report on the first attempt to produce continuous, >24 hour polarization light curves of blazars using telescopes distributed across the globe and the rotation of the Earth to avoid the rising Sun. Our campaign involved 16 telescopes in Asia, Europe, and North America. We observed BL Lacertae and CGRaBS J0211+1051 for a combined 685 telescope hours. We find large variations in the polarization degree and angle for both sources in sub-hour timescales as well as a ~180 degree rotation of the polarization angle in CGRaBS J0211+1051 in less than two days. We compared our high-cadence observations to Particle-In-Cell magnetic reconnection and turbulent plasma simulations. We find that although the state of the art simulation frameworks can produce a large fraction of the polarization properties, they do not account for the entirety of the observed polarization behavior in blazar jets. △ Less

Submitted 21 June, 2024; originally announced June 2024.

Comments: 20 pages, 15 figures, 2 tables, accepted for publication in A&A. The data used in the paper are available here: https://doi.org/10.7910/DVN/IETSXS

Journal ref: A&A 689, A200 (2024)

Optical variability of the blazar 3C 371: from minute to year timescales

Authors: J. Otero-Santos, C. M. Raiteri, J. A. Acosta-Pulido, M. I. Carnerero, M. Villata, S. S. Savchenko, D. Carosati, W. P. Chen, S. O. Kurtanidze, M. D. Joner, E. Semkov, T. Pursimo, E. Benítez, G. Damljanovic, G. Apolonio, G. A. Borman, V. Bozhilov, F. J. Galindo-Guil, T. S. Grishina, V. A. Hagen-Thorn, D. Hiriart, H. Y. Hsiao, S. Ibryamov, R. Z. Ivanidze, G. N. Kimeridze , et al. (19 additional authors not shown)

Abstract: The BL Lac object 3C 371 has been observed by the Transiting Exoplanet Survey Satellite (\textit{TESS}) for approximately a year, between July 2019 and July 2020, with an unmatched 2-minute observing cadence. In parallel, the Whole Earth Blazar Telescope (WEBT) Collaboration organized an extensive observing campaign, providing three years of continuous optical monitoring between 2018 and 2020. The… ▽ More The BL Lac object 3C 371 has been observed by the Transiting Exoplanet Survey Satellite (\textit{TESS}) for approximately a year, between July 2019 and July 2020, with an unmatched 2-minute observing cadence. In parallel, the Whole Earth Blazar Telescope (WEBT) Collaboration organized an extensive observing campaign, providing three years of continuous optical monitoring between 2018 and 2020. These datasets allow for a thorough investigation of the variability of the source. The goal of this study is evaluating the optical variability of 3C 371. Taking advantage of the remarkable cadence of \textit{TESS} data, we aim to characterize the intra-day variability (IDV) displayed by the source and identify its shortest variability timescale. With this estimate, constraints on the size of the emitting region and black hole mass can be calculated. Moreover, WEBT data is used to investigate long-term variability (LTV), including understanding spectral behaviour of the source and the polarization variability. Based on the derived characteristics, information on the origin of the variability on different timescales is extracted. We evaluated the variability applying the variability amplitude tool that quantifies how variable the emission is. Moreover, we employed common tools like ANOVA (ANalysis Of VAariance) tests, wavelet and power spectral density (PSD) analyses to characterize the shortest variability timescales present in the emission and the underlying noise affecting the data. Short- and long-term colour behaviours have been evaluated to understand the spectral behaviour of the source. The polarized emission was analyzed, studying its variability and possible rotation patterns of the electric vector position angle (EVPA). Flux distributions of IDV and LTV were also studied with the aim of linking the flux variations to turbulent and/or accretion disc related processes. △ Less

Submitted 4 April, 2024; v1 submitted 26 March, 2024; originally announced March 2024.

Comments: Accepted in A&A, 20 pages, 19 figures

Journal ref: A&A 686, A228 (2024)

The variability patterns of the TeV blazar PG 1553+113 from a decade of MAGIC and multi-band observations

Authors: MAGIC Collaboration, H. Abe, S. Abe, J. Abhir, V. A. Acciari, I. Agudo, T. Aniello, S. Ansoldi, L. A. Antonelli, A. Arbet Engels, C. Arcaro, M. Artero, K. Asano, D. Baack, A. Babić, A. Baquero, U. Barres de Almeida, I. Batković, J. Baxter, J. Becerra González, E. Bernardini, J. Bernete, A. Berti, J. Besenrieder, C. Bigongiari , et al. (242 additional authors not shown)

Abstract: PG 1553+113 is one of the few blazars with a convincing quasi-periodic emission in the gamma-ray band. The source is also a very high-energy (VHE; >100 GeV) gamma-ray emitter. To better understand its properties and identify the underlying physical processes driving its variability, the MAGIC Collaboration initiated a multiyear, multiwavelength monitoring campaign in 2015 involving the OVRO 40-m a… ▽ More PG 1553+113 is one of the few blazars with a convincing quasi-periodic emission in the gamma-ray band. The source is also a very high-energy (VHE; >100 GeV) gamma-ray emitter. To better understand its properties and identify the underlying physical processes driving its variability, the MAGIC Collaboration initiated a multiyear, multiwavelength monitoring campaign in 2015 involving the OVRO 40-m and Medicina radio telescopes, REM, KVA, and the MAGIC telescopes, Swift and Fermi satellites, and the WEBT network. The analysis presented in this paper uses data until 2017 and focuses on the characterization of the variability. The gamma-ray data show a (hint of a) periodic signal compatible with literature, but the X-ray and VHE gamma-ray data do not show statistical evidence for a periodic signal. In other bands, the data are compatible with the gamma-ray period, but with a relatively high p-value. The complex connection between the low and high-energy emission and the non-monochromatic modulation and changes in flux suggests that a simple one-zone model is unable to explain all the variability. Instead, a model including a periodic component along with multiple emission zones is required. △ Less

Submitted 4 March, 2024; originally announced March 2024.

Comments: Accepted for publication in Monthly Notices of the Royal Astronomical Society. 19 pages, 9 figures. Corresponding authors: Elisa Prandini, Antonio Stamerra, Talvikki Hovatta

Optical circular polarization of blazar S4 0954+65 during high linear polarized states

Authors: I. Liodakis, E. Shablovinskaya, D. Blinov, S. S. Savchenko, E. Malygin, S. Kotov, S. Kiehlmann, A. C. S. Readhead, S. B. Potter, F. M. Rieger, T. S. Grishina, V. A. Hagen-Thorn, E. N. Kopatskaya, E. G. Larionova, D. A. Morozova, I. S. Troitskiy, Y. V. Troitskaya, A. A. Vasilyev, A. V. Zhovtan, G. A. Borman

Abstract: Optical circular polarization observations can directly test the particle composition in black holes jets. Here we report on the first observations of the BL Lac type object S4 0954+65 in high linear polarized states. While no circular polarization was detected, we were able to place upper limits of <0.5% at the 99.7% confidence. Using a simple model and our novel optical circular polarization obs… ▽ More Optical circular polarization observations can directly test the particle composition in black holes jets. Here we report on the first observations of the BL Lac type object S4 0954+65 in high linear polarized states. While no circular polarization was detected, we were able to place upper limits of <0.5% at the 99.7% confidence. Using a simple model and our novel optical circular polarization observations we can constrain the allowed parameter space for the magnetic field strength and composition of the emitting particles. Our results favor models that require magnetic field strengths of only a few Gauss and models where the jet composition is dominated by electron-positron pairs. We discuss our findings in the context of typical magnetic field strength requirements for blazar emission models. △ Less

Submitted 6 November, 2023; originally announced November 2023.

Comments: 4 pages, 1 figure, 1 table, accepted for publication in A&A letters

Quasi-simultaneous Optical Flux and Polarization Variability of the Binary Super Massive Black Hole Blazar OJ 287 from 2015 to 2023: Detection of an Anticorrelation in Flux and Polarization Variability

Authors: Alok C. Gupta, Pankaj Kushwaha, Mauri J. Valtonen, Sergey S. Savchenko, Svetlana G. Jorstad, Ryo Imazawa, Paul J. Wiita, Minfeng Gu, Alan P. Marscher, Zhongli Zhang, Rumen Bachev, G. A. Borman, Haritma Gaur, T. S. Grishina, V. A. Hagen-Thorn, E. N. Kopatskaya, V. M. Larionov, E. G. Larionova, L. V. Larionova, D. A. Morozova, T. Nakaoka, A. Strigachev, Yulia V. Troitskaya, I. S. Troitsky, M. Uemura , et al. (3 additional authors not shown)

Abstract: We study the optical flux and polarization variability of the binary black hole blazar OJ 287 using quasi-simultaneous observations from 2015 to 2023 carried out using telescopes in the USA, Japan, Russia, Crimea, and Bulgaria. This is one of the most extensive quasi-simultaneous optical flux and polarization variability studies of OJ 287. OJ 287 showed large amplitude, ~3.0 mag flux variability,… ▽ More We study the optical flux and polarization variability of the binary black hole blazar OJ 287 using quasi-simultaneous observations from 2015 to 2023 carried out using telescopes in the USA, Japan, Russia, Crimea, and Bulgaria. This is one of the most extensive quasi-simultaneous optical flux and polarization variability studies of OJ 287. OJ 287 showed large amplitude, ~3.0 mag flux variability, large changes of ~37% in degree of polarization, and a large swing of ~215 degrees in the angle of the electric vector of polarization. During the period of observation, several flares in flux were detected. Those flares are correlated with a rapid increase in the degree of polarization and swings in electric vector of polarization angle. A peculiar behavior of anticorrelation between flux and polarization degree, accompanied by a nearly constant polarization angle, was detected from JD 2,458,156 to JD 2,458,292. We briefly discuss some explanations for the flux and polarization variations observed in OJ 287. △ Less

Submitted 4 November, 2023; originally announced November 2023.

Comments: 14 pages, 4 figures, 2 tables, Published in Astrophysical Journal Letters, 2023 November 1

Journal ref: Astrophysical Journal Letters, 957 (2023) L11 (11pp)

The Repeating Flaring Activity of Blazar AO 0235+164

Authors: Juan Escudero Pedrosa, Iván Agudo, Andrea Tramacere, Alan P. Marscher, Svetlana Jorstad, Z. R. Weaver, Carolina Casadio, Clemens Thum, Ioannis Myserlis, Antonio Fuentes, Efthalia Traianou, Jae-Young Kim, Joana Kramer, Rubén López-Coto, Filippo D'Ammando, M. Bernardos, Giacomo Bonnoli, Dmitriy A. Blinov, G. A. Borman, T. S. Grishina, V. A. Hagen-Thorn, E. N. Kopatskaya, E. G. Larionova, V. M. Larionov, L. V. Larionova , et al. (5 additional authors not shown)

Abstract: Context. Blazar AO 0235+164, located at redshift z = 0.94, has undergone several sharp multi-spectral-range flaring episodes during the last decades. In particular, the episodes peaking in 2008 and 2015, that received extensive multi-wavelength coverage, exhibited interesting behavior. Aims. We study the actual origin of these two observed flares by constraining the properties of the observed ph… ▽ More Context. Blazar AO 0235+164, located at redshift z = 0.94, has undergone several sharp multi-spectral-range flaring episodes during the last decades. In particular, the episodes peaking in 2008 and 2015, that received extensive multi-wavelength coverage, exhibited interesting behavior. Aims. We study the actual origin of these two observed flares by constraining the properties of the observed photo-polarimetric variability, those of the broad-band spectral energy-distribution and the observed time-evolution behavior of the source as seen by ultra-high resolution total-flux and polarimetric Very-long-baseline interferometry (VLBI) imaging. Methods. The analysis of VLBI images allows us to constrain kinematic and geometrical parameters of the 7 mm jet. We use the Discrete Correlation Function to compute the statistical correlation and the delays between emission at different spectral ranges. Multi-epoch modeling of the spectral energy distributions allows us to propose specific models of emission; in particular for the unusual spectral features observed in this source in the X-ray region of the spectrum during strong multi spectral-range flares. Results. We find that these X-ray spectral features can be explained by an emission component originating in a separate particle distribution than the one responsible for the two standard blazar bumps. This is in agreement with the results of our correlation analysis that do not find a strong correlation between the X-rays and the remaining spectral ranges. We find that both external Compton dominated and synchrotron self-Compton dominated models can explain the observed spectral energy distributions. However, synchrotron self-Compton models are strongly favored by the delays and geometrical parameters inferred from the observations. △ Less

Submitted 2 November, 2023; originally announced November 2023.

Observations of Low and Intermediate Spectral Peak Blazars with the Imaging X-ray Polarimetry Explorer

Authors: Herman L. Marshall, Ioannis Liodakis, Alan P. Marscher, Niccolo Di Lalla, Svetlana G. Jorstad, Dawoon E. Kim, Riccardo Middei, Michela Negro, Nicola Omodei, Abel L. Peirson, Matteo Perri, Simonetta Puccetti, Ivan Agudo, Giacomo Bonnoli, Andrei V. Berdyugin, Elisabetta Cavazzuti, Nicole Rodriguez Cavero, Immacolata Donnarumma, Laura Di Gesu, Jenni Jormanainen, Henric Krawczynski, Elina Lindfors, Frederic Marin, Francesco Massaro, Luigi Pacciani , et al. (133 additional authors not shown)

Abstract: We present X-ray polarimetry observations from the Imaging X-ray Polarimetry Explorer (IXPE) of three low spectral peak and one intermediate spectral peak blazars, namely 3C 273, 3C 279, 3C 454.3, and S5 0716+714. For none of these objects was IXPE able to detect X-ray polarization at the 3$σ$ level. However, we placed upper limits on the polarization degree at $\sim$10-30\%. The undetected polari… ▽ More We present X-ray polarimetry observations from the Imaging X-ray Polarimetry Explorer (IXPE) of three low spectral peak and one intermediate spectral peak blazars, namely 3C 273, 3C 279, 3C 454.3, and S5 0716+714. For none of these objects was IXPE able to detect X-ray polarization at the 3$σ$ level. However, we placed upper limits on the polarization degree at $\sim$10-30\%. The undetected polarizations favor models where the X-ray band is dominated by unpolarized photons upscattered by relativistic electrons in the jets of blazars, although hadronic models are not completely eliminated. We discuss the X-ray polarization upper limits in the context of our contemporaneous multiwavelength polarization campaigns. △ Less

Submitted 19 October, 2023; v1 submitted 17 October, 2023; originally announced October 2023.

Comments: 19 pages, 10 figures, 6 tables; submitted to the Astrophysical Journal

Extreme photometric and polarimetric variability of blazar S4 0954+65 at its maximum optical and $γ$-ray brightness levels

Authors: C. M. Raiteri, M. Villata, M. I. Carnerero, S. S. Savchenko, S. O. Kurtanidze, V. V. Vlasyuk, A. Marchini, K. Matsumoto, C. Lorey, M. D. Joner, K. Gazeas, D. Carosati, D. O. Mirzaqulov, J. A. Acosta Pulido, I. Agudo, R. Bachev, E. Benítez, G. A. Borman, P. Calcidese, W. P. Chen, G. Damljanovic, S. A. Ehgamberdiev, D. Elsässer, M. Feige, A. Frasca , et al. (42 additional authors not shown)

Abstract: In 2022 the BL Lac object S4 0954+65 underwent a major variability phase, reaching its historical maximum brightness in the optical and $γ$-ray bands. We present optical photometric and polarimetric data acquired by the Whole Earth Blazar Telescope (WEBT) Collaboration from 2022 April 6 to July 6. Many episodes of unprecedented fast variability were detected, implying an upper limit to the size of… ▽ More In 2022 the BL Lac object S4 0954+65 underwent a major variability phase, reaching its historical maximum brightness in the optical and $γ$-ray bands. We present optical photometric and polarimetric data acquired by the Whole Earth Blazar Telescope (WEBT) Collaboration from 2022 April 6 to July 6. Many episodes of unprecedented fast variability were detected, implying an upper limit to the size of the emitting region as low as $10^{-4}$ parsec. The WEBT data show rapid variability in both the degree and angle of polarization. We analyse different models to explain the polarization behaviour in the framework of a twisting jet model, which assumes that the long-term trend of the flux is produced by variations in the emitting region viewing angle. All the models can reproduce the average trend of the polarization degree, and can account for its general anticorrelation with the flux, but the dispersion of the data requires the presence of intrinsic mechanisms, such as turbulence, shocks, or magnetic reconnection. The WEBT optical data are compared to $γ$-ray data from the Fermi satellite. These are analysed with both fixed and adaptive binning procedures. We show that the strong correlation between optical and $γ$-ray data without measurable delay assumes different slopes in faint and high brightness states, and this is compatible with a scenario where in faint states we mainly see the imprint of the geometrical effects, while in bright states the synchrotron self-Compton process dominates. △ Less

Submitted 17 October, 2023; originally announced October 2023.

Comments: 12 pages, 13 figures, in press for MNRAS

Multi-year characterisation of the broad-band emission from the intermittent extreme BL Lac 1ES~2344+514

Authors: H. Abe, S. Abe, V. A. Acciari, I. Agudo, T. Aniello, S. Ansoldi, L. A. Antonelli, A. Arbet Engels, C. Arcaro, M. Artero, K. Asano, D. Baack, A. Babić, A. Baquero, U. Barres de Almeida, I. Batković, J. Baxter, J. Becerra González, E. Bernardini, J. Bernete, A. Berti, J. Besenrieder, C. Bigongiari, A. Biland, O. Blanch , et al. (210 additional authors not shown)

Abstract: The BL Lac 1ES 2344+514 is known for temporary extreme properties (e.g., a shift of the synchrotron SED peak energy $ν_{synch,p}$ above 1keV). While those extreme states were so far observed only during high flux levels, additional multi-year observing campaigns are required to achieve a coherent picture. Here, we report the longest investigation of the source from radio to VHE performed so far, f… ▽ More The BL Lac 1ES 2344+514 is known for temporary extreme properties (e.g., a shift of the synchrotron SED peak energy $ν_{synch,p}$ above 1keV). While those extreme states were so far observed only during high flux levels, additional multi-year observing campaigns are required to achieve a coherent picture. Here, we report the longest investigation of the source from radio to VHE performed so far, focusing on a systematic characterisation of the intermittent extreme states. While our results confirm that 1ES 2344+514 typically exhibits $ν_{synch,p}>$1keV during elevated flux periods, we also find periods where the extreme state coincides with low flux activity. A strong spectral variability thus happens in the quiescent state, and is likely caused by an increase of the electron acceleration efficiency without a change in the electron injection luminosity. We also report a strong X-ray flare (among the brightest for 1ES 2344+514) without a significant shift of $ν_{synch,p}$. During this particular flare, the X-ray spectrum is among the softest of the campaign. It unveils complexity in the spectral evolution, where the common harder-when-brighter trend observed in BL Lacs is violated. During a low and hard X-ray state, we find an excess of the UV flux with respect to an extrapolation of the X-ray spectrum to lower energies. This UV excess implies that at least two regions contribute significantly to the infrared/optical/ultraviolet/X-ray emission. Using the simultaneous MAGIC, XMM-Newton, NuSTAR, and AstroSat observations, we argue that a region possibly associated with the 10 GHz radio core may explain such an excess. Finally, we investigate a VHE flare, showing an absence of simultaneous variability in the 0.3-2keV band. Using a time-dependent leptonic modelling, we show that this behaviour, in contradiction to single-zone scenarios, can instead be explained by a two-component model. △ Less

Submitted 5 October, 2023; originally announced October 2023.

Comments: Accepted for publication in Astronomy & Astrophysics

Journal ref: A&A 682, A114 (2024)

X-ray Polarization Observations of BL Lacertae

Authors: Riccardo Middei, Ioannis Liodakis, Matteo Perri, Simonetta Puccetti, Elisabetta Cavazzuti, Laura Di Gesu, Steven R. Ehlert, Grzegorz Madejski, Alan P. Marscher, Herman L. Marshall, Fabio Muleri, Michela Negro, Svetlana G. Jorstad, Beatriz Agís-González, Iván Agudo, Giacomo Bonnoli, Maria I. Bernardos, Víctor Casanova, Maya García-Comas, César Husillos, Alessandro Marchini, Alfredo Sota, Pouya M. Kouch, George A. Borman, Evgenia N. Kopatskaya , et al. (121 additional authors not shown)

Abstract: Blazars are a class of jet-dominated active galactic nuclei with a typical double-humped spectral energy distribution. It is of common consensus the Synchrotron emission to be responsible for the low frequency peak, while the origin of the high frequency hump is still debated. The analysis of X-rays and their polarization can provide a valuable tool to understand the physical mechanisms responsibl… ▽ More Blazars are a class of jet-dominated active galactic nuclei with a typical double-humped spectral energy distribution. It is of common consensus the Synchrotron emission to be responsible for the low frequency peak, while the origin of the high frequency hump is still debated. The analysis of X-rays and their polarization can provide a valuable tool to understand the physical mechanisms responsible for the origin of high-energy emission of blazars. We report the first observations of BL Lacertae performed with the Imaging X-ray Polarimetry Explorer ({IXPE}), from which an upper limit to the polarization degree $Π_X<$12.6\% was found in the 2-8 keV band. We contemporaneously measured the polarization in radio, infrared, and optical wavelengths. Our multiwavelength polarization analysis disfavors a significant contribution of proton synchrotron radiation to the X-ray emission at these epochs. Instead, it supports a leptonic origin for the X-ray emission in BL Lac. △ Less

Submitted 24 November, 2022; originally announced November 2022.

Comments: 17 pages, 5 figures, accepted for publication in ApJL

Journal ref: 2023, ApJ Letters, 942, L10

Multi-messenger characterization of Mrk 501 during historically low X-ray and $γ$-ray activity

Authors: MAGIC collaboration, H. Abe, S. Abe, V. A. Acciari, I. Agudo, T. Aniello, S. Ansoldi, L. A. Antonelli, A. Arbet Engels, C. Arcaro, M. Artero, K. Asano, D. Baack, A. Babić, A. Baquero, U. Barres de Almeida, J. A. Barrio, I. Batković, J. Baxter, J. Becerra González, W. Bednarek, E. Bernardini, M. Bernardos, A. Berti, J. Besenrieder , et al. (300 additional authors not shown)

Abstract: We study the broadband emission of Mrk 501 using multi-wavelength observations from 2017 to 2020 performed with a multitude of instruments, involving, among others, MAGIC, Fermi-LAT, NuSTAR, Swift, GASP-WEBT, and OVRO. Mrk 501 showed an extremely low broadband activity, which may help to unravel its baseline emission. Nonetheless, significant flux variations are detected at all wavebands, with the… ▽ More We study the broadband emission of Mrk 501 using multi-wavelength observations from 2017 to 2020 performed with a multitude of instruments, involving, among others, MAGIC, Fermi-LAT, NuSTAR, Swift, GASP-WEBT, and OVRO. Mrk 501 showed an extremely low broadband activity, which may help to unravel its baseline emission. Nonetheless, significant flux variations are detected at all wavebands, with the highest occurring at X-rays and very-high-energy (VHE) $γ$-rays. A significant correlation ($>$3$σ$) between X-rays and VHE $γ$-rays is measured, supporting leptonic scenarios to explain the variable parts of the emission, also during low activity. This is further supported when we extend our data from 2008 to 2020, and identify, for the first time, significant correlations between Swift-XRT and Fermi-LAT. We additionally find correlations between high-energy $γ$-rays and radio, with the radio lagging by more than 100 days, placing the $γ$-ray emission zone upstream of the radio-bright regions in the jet. Furthermore, Mrk 501 showed a historically low activity in X-rays and VHE $γ$-rays from mid-2017 to mid-2019 with a stable VHE flux ($>$0.2 TeV) of 5% the emission of the Crab Nebula. The broadband spectral energy distribution (SED) of this 2-year-long low-state, the potential baseline emission of Mrk 501, can be characterized with one-zone leptonic models, and with (lepto)-hadronic models fulfilling neutrino flux constraints from IceCube. We explore the time evolution of the SED towards the low-state, revealing that the stable baseline emission may be ascribed to a standing shock, and the variable emission to an additional expanding or traveling shock. △ Less

Submitted 5 March, 2023; v1 submitted 5 October, 2022; originally announced October 2022.

Comments: 55 pages, 30 figures, 14 tables, accepted by APJS. Corresponding authors are L. Heckmann, D. Paneque, S. Gasparyan, M. Cerruti, and N. Sahakyan

Journal ref: ApJS 266 37 (2023)

Polarized Blazar X-rays imply particle acceleration in shocks

Authors: Ioannis Liodakis, Alan P. Marscher, Iván Agudo, Andrei V. Berdyugin, Maria I. Bernardos, Giacomo Bonnoli, George A. Borman, Carolina Casadio, Víctor Casanova, Elisabetta Cavazzuti, Nicole Rodriguez Cavero, Laura Di Gesu, Niccoló Di Lalla, Immacolata Donnarumma, Steven R. Ehlert, Manel Errando, Juan Escudero, Maya García-Comas, Beatriz Agís-González, César Husillos, Jenni Jormanainen, Svetlana G. Jorstad, Masato Kagitani, Evgenia N. Kopatskaya, Vadim Kravtsov , et al. (103 additional authors not shown)

Abstract: Most of the light from blazars, active galactic nuclei with jets of magnetized plasma that point nearly along the line of sight, is produced by high-energy particles, up to $\sim 1$ TeV. Although the jets are known to be ultimately powered by a supermassive black hole, how the particles are accelerated to such high energies has been an unanswered question. The process must be related to the magnet… ▽ More Most of the light from blazars, active galactic nuclei with jets of magnetized plasma that point nearly along the line of sight, is produced by high-energy particles, up to $\sim 1$ TeV. Although the jets are known to be ultimately powered by a supermassive black hole, how the particles are accelerated to such high energies has been an unanswered question. The process must be related to the magnetic field, which can be probed by observations of the polarization of light from the jets. Measurements of the radio to optical polarization - the only range available until now - probe extended regions of the jet containing particles that left the acceleration site days to years earlier (Jorstad et al., 2005; Marin et al., 2018; Blinov et al., 2021), and hence do not directly explore the acceleration mechanism, as could X-ray measurements. Here we report the detection of X-ray polarization from the blazar Markarian~501 (Mrk~501). We measure an X-ray linear polarization degree $Π_X \sim10\%$, a factor of $\sim2$ higher than the value at optical wavelengths, with a polarization angle parallel to the radio jet. This points to a shock front as the source of particle acceleration, and also implies that the plasma becomes increasingly turbulent with distance from the shock. △ Less

Submitted 3 September, 2023; v1 submitted 13 September, 2022; originally announced September 2022.

Comments: 34 pages, 4 figures, 4 tables

Journal ref: 2022, Nature, 611, 677-681

Emission Line Variability during a Nonthermal Outburst in the Gamma-Ray Bright Quasar 1156+295

Authors: Melissa K. Hallum, Svetlana G. Jorstad, Valeri M. Larionov, Alan P. Marscher, Manasvita Joshi, Zachary R. Weaver, Karen E. Williamson, Ivan Agudo, George A. Borman, Carolina Casadio, Antonio Fuentes, Tatiana S. Grishina, Evgenia N. Kopatskaya, Elena G. Larionova, Liyudmila V. Larionova, Daria A. Morozova, Anna A. Nikiforova, Sergey S. Savchenko, Ivan S. Troitsky, Yulia V. Troitskaya, Andrey A. Vasilyev

Abstract: We present multi-epoch optical spectra of the $γ$-ray bright blazar 1156+295 (4C +29.45, Ton 599) obtained with the 4.3~m Lowell Discovery Telescope. During a multi-wavelength outburst in late 2017, when the $γ$-ray flux increased to $2.5\times 10^{-6} \; \rm phot\; cm^{-2}\; s^{-1}$ and the quasar was first detected at energies $\geq100$ GeV, the flux of the Mg II $λ2798$ emission line changed, a… ▽ More We present multi-epoch optical spectra of the $γ$-ray bright blazar 1156+295 (4C +29.45, Ton 599) obtained with the 4.3~m Lowell Discovery Telescope. During a multi-wavelength outburst in late 2017, when the $γ$-ray flux increased to $2.5\times 10^{-6} \; \rm phot\; cm^{-2}\; s^{-1}$ and the quasar was first detected at energies $\geq100$ GeV, the flux of the Mg II $λ2798$ emission line changed, as did that of the Fe emission complex at shorter wavelengths. These emission line fluxes increased along with the highly polarized optical continuum flux, which is presumably synchrotron radiation from the relativistic jet, with a relative time delay of $\lesssim2$ weeks. This implies that the line-emitting clouds lie near the jet, which points almost directly toward the line of sight. The emission-line radiation from such clouds, which are located outside the canonical accretion-disk related broad-line region, may be a primary source of seed photons that are up-scattered to $γ$-ray energies by relativistic electrons in the jet. △ Less

Submitted 31 January, 2022; originally announced February 2022.

Comments: 25 pages, 12 figures

Journal ref: 2022 ApJ 926 180

Investigation of the correlation patterns and the Compton dominance variability of Mrk 421 in 2017

Authors: MAGIC Collaboration, V. A. Acciari, S. Ansoldi, L. A. Antonelli, A. Arbet Engels, M. Artero, K. Asano, A. Babić, A. Baquero, U. Barres de Almeida, J. A. Barrio, I. Batković, J. Becerra González, W. Bednarek, L. Bellizzi, E. Bernardini, M. Bernardos, A. Berti, J. Besenrieder, W. Bhattacharyya, C. Bigongiari, O. Blanch, Ž. Bošnjak, G. Busetto, R. Carosi , et al. (263 additional authors not shown)

Abstract: We present a detailed characterisation and theoretical interpretation of the broadband emission of the paradigmatic TeV blazar Mrk 421, with special focus on the multi-band flux correlations. The dataset has been collected through an extensive multiwavelength campaign organised between 2016 December and 2017 June. The instruments involved are MAGIC, FACT, Fermi-LAT, Swift, GASP-WEBT, OVRO, Medicin… ▽ More We present a detailed characterisation and theoretical interpretation of the broadband emission of the paradigmatic TeV blazar Mrk 421, with special focus on the multi-band flux correlations. The dataset has been collected through an extensive multiwavelength campaign organised between 2016 December and 2017 June. The instruments involved are MAGIC, FACT, Fermi-LAT, Swift, GASP-WEBT, OVRO, Medicina and Metsähovi. Additionally, four deep exposures (several hours long) with simultaneous MAGIC and NuSTAR observations allowed a precise measurement of the falling segments of the two spectral components. The very-high-energy (VHE; E > 100 GeV) gamma rays and X-rays are positively correlated at zero time lag, but the strength and characteristics of the correlation change substantially across the various energy bands probed. The VHE versus X-ray fluxes follow different patterns, partly due to substantial changes in the Compton dominance during a few days without a simultaneous increase in the X-ray flux (i.e. orphan gamma-ray activity). Studying the broadband spectral energy distribution (SED) during the days including NuSTAR observations, we show that these changes can be explained within a one-zone leptonic model with a blob that increases its size over time. Our multi-band correlation study also hints at an anti-correlation between UV/optical and X-ray at a significance higher than 3 sigmas. A VHE flare observed on 2017 February 4 shows gamma-ray variability on multi-hour timescales, with a factor 10 increase in the TeV flux but only a moderate increase in the keV flux. The related broadband SED is better described by a two-zone leptonic scenario rather than by a one-zone scenario. We find that the flare can be produced by the appearance of a compact second blob populated by high energetic electrons spanning a narrow range of Lorentz factors. △ Less

Submitted 10 June, 2021; originally announced June 2021.

Comments: 38 pages, 23 figures, accepted for publication in Astronomy and Astrophysics. Corresponding authors: Axel Arbet-Engels, David Paneque

Journal ref: A&A 655, A89 (2021)

Parameters of the type-IIP supernova SN 2012aw

Authors: A. A. Nikiforova, P. V. Baklanov, S. I. Blinnikov, D. A. Blinov, T. S. Grishina, Yu. V. Troitskaya, D. A. Morozova, E. N. Kopatskaya, E. G. Larionova, I. S. Troitsky

Abstract: We present the results the photometric observations of the Type IIP supernova SN 2012aw obtained for the time interval from 7 till 371 days after the explosion. Using the previously published values of the photospheric velocities we've computed the hydrodynamic model which simultaneously reproduced the photometry observations and velocity measurements. The model was calculated with the multi-energ… ▽ More We present the results the photometric observations of the Type IIP supernova SN 2012aw obtained for the time interval from 7 till 371 days after the explosion. Using the previously published values of the photospheric velocities we've computed the hydrodynamic model which simultaneously reproduced the photometry observations and velocity measurements. The model was calculated with the multi-energy group radiation hydrodynamics code STELLA. We found the parameters of the pre-supernova: radius $R = 500 R_\odot$, nickel mass $M(^{56}$Ni$)$ $\sim 0.06 M_\odot$, pre-supernova mass $25 M_\odot$, mass of ejected envelope $23.6 M_\odot$, explosion energy $E \sim 2 \times 10^{51}$ erg. The model progenitor mass $M=25 M_\odot$ significantly exceeds the upper limit mass $M=17 M_\odot$, obtained from analysis the pre-SNe observations. This result confirms once more that the 'Red Supergiant Problem' must be resolved by stellar evolution and supernova explosion theories in interaction with observations. △ Less

Submitted 12 April, 2021; v1 submitted 8 February, 2021; originally announced February 2021.

Comments: 7 pages, 8 figures, 3 tables. Accepted for publication in MNRAS

Short-Timescale Variability of the Blazar Mrk 421 from AstroSat and Simultaneous Multi-Wavelength Observations

Authors: Ritaban Chatterjee, Susmita Das, Archishman Khasnovis, Ritesh Ghosh, Neeraj Kumari, Sachindra Naik, V. M. Larionov, T. S. Grishina, E. N. Kopatskaya, E. G. Larionova, A. A. Nikiforova, D. A. Morozov, S. S. Savchenko, Yu. V. Troitskaya, I. S. Troitsky, A. A. Vasilyev

Abstract: We study the multi-wavelength variability of the blazar Mrk 421 at minutes to days timescales using simultaneous data at $γ$-rays from Fermi, 0.7-20 keV energies from AstroSat, and optical and near-infrared (NIR) wavelengths from ground-based observatories. We compute the shortest variability timescales at all of the above wavebands and find its value to be ~1.1 ks at the hard X-ray energies and i… ▽ More We study the multi-wavelength variability of the blazar Mrk 421 at minutes to days timescales using simultaneous data at $γ$-rays from Fermi, 0.7-20 keV energies from AstroSat, and optical and near-infrared (NIR) wavelengths from ground-based observatories. We compute the shortest variability timescales at all of the above wavebands and find its value to be ~1.1 ks at the hard X-ray energies and increasingly longer at soft X-rays, optical and NIR wavelengths as well as at the GeV energies. We estimate the value of the magnetic field to be 0.5 Gauss and the maximum Lorentz factor of the emitting electrons ~1.6 x $10^5$ assuming that synchrotron radiation cooling drives the shortest variability timescale. Blazars vary at a large range of timescales often from minutes to years. These results, as obtained here from the very short end of the range of variability timescales of blazars, are a confirmation of the leptonic scenario and in particular the synchrotron origin of the X-ray emission from Mrk 421 by relativistic electrons of Lorentz factor as high as $10^5$. This particular mode of confirmation has been possible using minutes to days timescale variability data obtained from AstroSat and simultaneous multi-wavelength observations. △ Less

Submitted 1 February, 2021; originally announced February 2021.

Comments: 8 pages, 2 figures, accepted for publication in the Journal of Astrophysics and Astronomy

Journal ref: J. Astrophys. Astr. (2021) 42:80

VHE gamma-ray detection of FSRQ QSO B1420+326 and modeling of its enhanced broadband state in 2020

Authors: V. A. Acciari, S. Ansoldi, L. A. Antonelli, A. Arbet Engels, M. Artero, K. Asano, D. Baack, A. Babić, A. Baquero, U. Barres de Almeida, J. A. Barrio, J. Becerra González, W. Bednarek, L. Bellizzi, E. Bernardini, M. Bernardos, A. Berti, J. Besenrieder, W. Bhattacharyya, C. Bigongiari, A. Biland, O. Blanch, G. Bonnoli, Ž. Bošnjak, G. Busetto , et al. (209 additional authors not shown)

Abstract: Context. QSO B1420+326 is a blazar classified as a Flat Spectrum Radio Quasar (FSRQ). In the beginning of 2020 it underwent an enhanced flux state. An extensive multiwavelength campaign allowed us to trace the evolution of the flare. Aims. We search for VHE gamma-ray emission from QSO B1420+326 during this flaring state. We aim to characterize and model the broadband emission of the source over di… ▽ More Context. QSO B1420+326 is a blazar classified as a Flat Spectrum Radio Quasar (FSRQ). In the beginning of 2020 it underwent an enhanced flux state. An extensive multiwavelength campaign allowed us to trace the evolution of the flare. Aims. We search for VHE gamma-ray emission from QSO B1420+326 during this flaring state. We aim to characterize and model the broadband emission of the source over different phases of the flare. Methods. The source was observed with a number of instruments in radio, near infrared, optical (including polarimetry and spectroscopy), ultra-violet, X-ray and gamma-ray bands. We use dedicated optical spectroscopy results to estimate the accretion disk and the dust torus luminosity. We perform spectral energy distribution modeling in the framework of combined Synchrotron-Self-Compton and External Compton scenario in which the electron energy distribution is partially determined from acceleration and cooling processes. Results. During the enhanced state the flux of both SED components drastically increased and the peaks were shifted to higher energies. Follow up observations with the MAGIC telescopes led to the detection of very-high-energy gamma-ray emission from this source, making it one of only a handful of FSRQs known in this energy range. Modeling allows us to constrain the evolution of the magnetic field and electron energy distribution in the emission region. The gamma-ray flare was accompanied by a rotation of the optical polarization vector during a low polarization state. Also, a new, superluminal radio knot contemporaneously appeared in the radio image of the jet. The optical spectroscopy shows a prominent FeII bump with flux evolving together with the continuum emission and a MgII line with varying equivalent width. △ Less

Submitted 21 December, 2020; originally announced December 2020.

Comments: 20 pages, 15 figures, 7 tables, accepted for publication in A&A

Journal ref: A&A 647, A163 (2021)

Two Flares with One Shock: the Interesting Case of 3C 454.3

Authors: I. Liodakis, D. Blinov, S. G. Jorstad, A. A. Arkharov, A. Di Paola, N. V. Efimova, T. S. Grishina, S. Kiehlmann, E. N. Kopatskaya, V. M. Larionov, L. V. Larionova, E. G. Larionova, A. P. Marscher, D. A. Morozova, A. A. Nikiforova, V. Pavlidou, E. Traianou, Yu. V. Troitskaya, I. S. Troitsky, M. Uemura, Z. R. Weaver

Abstract: The quasar 3C 454.3 is a blazar known for its rapid and violent outbursts seen across the electromagnetic spectrum. Using gamma-ray, X-ray, multi-band optical, and very long baseline interferometric data we investigate the nature of two such events that occurred in 2013 and 2014 accompanied by strong variations in optical polarization, including a 230 degree electric vector position angle (EVPA) r… ▽ More The quasar 3C 454.3 is a blazar known for its rapid and violent outbursts seen across the electromagnetic spectrum. Using gamma-ray, X-ray, multi-band optical, and very long baseline interferometric data we investigate the nature of two such events that occurred in 2013 and 2014 accompanied by strong variations in optical polarization, including a 230 degree electric vector position angle (EVPA) rotation. Our results suggest that a single disturbance was responsible for both flaring events. We interpret the disturbance as a shock propagating down the jet. Under this interpretation the 2013-flare originated most likely due to changes in the viewing angle caused by perhaps a bent or helical trajectory of the shock upstream of the radio core. The 2014-flare and optical polarization behavior are the result of the shock exiting the 43 GHz radio core, suggesting that shock crossings are one of the possible mechanisms for EVPA rotations. △ Less

Submitted 24 August, 2020; v1 submitted 19 August, 2020; originally announced August 2020.

Comments: 14 pages, 10 figures, 2 Tables, accepted for publication in The Astrophysical Journal

Multi-Wavelength Variability of BL Lacertae Measured with High Time Resolution

Authors: Zachary R. Weaver, K. E. Williamson, S. G. Jorstad, A. P. Marscher, V. M. Larionov, C. M. Raiteri, M. Villata, J. A. Acosta-Pulido, R. Bachev, G. V. Baida, T. J. Balonek, E. Benitez, G. A. Borman, V. Bozhilov, M. I. Carnerero, D. Carosati, W. P. Chen, G. Damljanovic, V. Dhiman, D. J. Dougherty, S. A. Ehgamberdiev, T. S. Grishina, A. C. Gupta, M. Hart, D. Hiriart , et al. (32 additional authors not shown)

Abstract: In an effort to locate the sites of emission at different frequencies and physical processes causing variability in blazar jets, we have obtained high time-resolution observations of BL Lacertae over a wide wavelength range: with the \emph{Transiting Exoplanet Survey Satellite} (TESS) at 6,000-10,000 Å with 2-minute cadence; with the Neil Gehrels \emph{Swift} satellite at optical, UV, and X-ray ba… ▽ More In an effort to locate the sites of emission at different frequencies and physical processes causing variability in blazar jets, we have obtained high time-resolution observations of BL Lacertae over a wide wavelength range: with the \emph{Transiting Exoplanet Survey Satellite} (TESS) at 6,000-10,000 Å with 2-minute cadence; with the Neil Gehrels \emph{Swift} satellite at optical, UV, and X-ray bands; with the Nuclear Spectroscopic Telescope Array at hard X-ray bands; with the \emph{Fermi} Large Area Telescope at $γ$-ray energies; and with the Whole Earth Blazar Telescope for measurement of the optical flux density and polarization. All light curves are correlated, with similar structure on timescales from hours to days. The shortest timescale of variability at optical frequencies observed with TESS is $\sim 0.5$ hr. The most common timescale is $13\pm1$~hr, comparable with the minimum timescale of X-ray variability, 14.5 hr. The multi-wavelength variability properties cannot be explained by a change solely in the Doppler factor of the emitting plasma. The polarization behavior implies that there are both ordered and turbulent components to the magnetic field in the jet. Correlation analysis indicates that the X-ray variations lag behind the $γ$-ray and optical light curves by up to $\sim 0.4$ days. The timescales of variability, cross-frequency lags, and polarization properties can be explained by turbulent plasma that is energized by a shock in the jet and subsequently loses energy to synchrotron and inverse Compton radiation in a magnetic field of strength $\sim3$ G △ Less

Submitted 15 July, 2020; originally announced July 2020.

Comments: 33 pages, 25 figures, 14 tables. Accepted to ApJ

Unravelling the complex behavior of Mrk 421 with simultaneous X-ray and VHE observations during an extreme flaring activity in April 2013

Authors: MAGIC collaboration, V. A. Acciari, S. Ansoldi, L. A. Antonelli, A. Arbet Engels, D. Baack, A. Babic, B. Banerjee, U. Barres de Almeida, J. A. Barrio, J. Becerra Gonzalez, W. Bednarek, L. Bellizzi, E. Bernardini, A. Berti, J. Besenrieder, W. Bhattacharyya, C. Bigongiari, A. Biland, O. Blanch, G. Bonnoli, Z. Bosnjak, G. Busetto, R. Carosi, G. Ceribella , et al. (215 additional authors not shown)

Abstract: We report on a multi-band variability and correlation study of the TeV blazar Mrk 421 during an exceptional flaring activity observed from 2013 April 11 to 2013 April 19. The study uses, among others, data from GASP-WEBT, Swift, NuSTAR, Fermi-LAT, VERITAS, and MAGIC. The large blazar activity, and the 43 hours of simultaneous NuSTAR and MAGIC/VERITAS observations, permitted variability studies on… ▽ More We report on a multi-band variability and correlation study of the TeV blazar Mrk 421 during an exceptional flaring activity observed from 2013 April 11 to 2013 April 19. The study uses, among others, data from GASP-WEBT, Swift, NuSTAR, Fermi-LAT, VERITAS, and MAGIC. The large blazar activity, and the 43 hours of simultaneous NuSTAR and MAGIC/VERITAS observations, permitted variability studies on 15 minute time bins, and over three X-ray bands (3-7 keV, 7-30 keV and 30-80 keV) and three very-high-energy (>0.1 TeV, hereafter VHE) gamma-ray bands (0.2-0.4 TeV, 0.4-0.8 TeV and >0.8 TeV). We detected substantial flux variations on multi-hour and sub-hour timescales in all the X-ray and VHE gamma-ray bands. The characteristics of the sub-hour flux variations are essentially energy-independent, while the multi-hour flux variations can have a strong dependence on the energy of the X-ray and the VHE gamma rays. The three VHE bands and the three X-ray bands are positively correlated with no time-lag, but the strength and the characteristics of the correlation changes substantially over time and across energy bands. Our findings favour multi-zone scenarios for explaining the achromatic/chromatic variability of the fast/slow components of the light curves, as well as the changes in the flux-flux correlation on day-long timescales. We interpret these results within a magnetic reconnection scenario, where the multi-hour flux variations are dominated by the combined emission from various plasmoids of different sizes and velocities, while the sub-hour flux variations are dominated by the emission from a single small plasmoid moving across the magnetic reconnection layer. △ Less

Submitted 23 January, 2020; originally announced January 2020.

Comments: 46 pages, 17 figures, 9 tables, accepted for publication in ApJ Supplements

Multiwavelength behaviour of the blazar 3C279: decade-long study from $γ$-ray to radio

Authors: V. M. Larionov, S. G. Jorstad, A. P. Marscher, M. Villata, C. M. Raiteri, P. S. Smith, I. Agudo, S. S. Savchenko, D. A. Morozova, J. A. Acosta-Pulido, M. F. Aller, H. D. Aller, T. S. Andreeva, A. A. Arkharov, R. Bachev, G. Bonnoli, G. A. Borman, V. Bozhilov, P. Calcidese, M. I. Carnerero, D. Carosati, C. Casadio, W. -P. Chen, G. Damljanovic, A. V. Dementyev , et al. (62 additional authors not shown)

Abstract: We report the results of decade-long (2008-2018) $γ$-ray to 1 GHz radio monitoring of the blazar 3C 279, including GASP/WEBT, $\it{Fermi}$ and $\it{Swift}$ data, as well as polarimetric and spectroscopic data. The X-ray and $γ$-ray light curves correlate well, with no delay > 3 hours, implying general co-spatiality of the emission regions. The $γ$-ray-optical flux-flux relation changes with activi… ▽ More We report the results of decade-long (2008-2018) $γ$-ray to 1 GHz radio monitoring of the blazar 3C 279, including GASP/WEBT, $\it{Fermi}$ and $\it{Swift}$ data, as well as polarimetric and spectroscopic data. The X-ray and $γ$-ray light curves correlate well, with no delay > 3 hours, implying general co-spatiality of the emission regions. The $γ$-ray-optical flux-flux relation changes with activity state, ranging from a linear to a more complex dependence. The behaviour of the Stokes parameters at optical and radio wavelengths, including 43 GHz VLBA images, supports either a predominantly helical magnetic field or motion of the radiating plasma along a spiral path. Apparent speeds of emission knots range from 10 to 37c, with the highest values requiring bulk Lorentz factors close to those needed to explain $γ$-ray variability on very short time scales. The Mg II emission line flux in the `blue' and `red' wings correlates with the optical synchrotron continuum flux density, possibly providing a variable source of seed photons for inverse Compton scattering. In the radio bands we find progressive delays of the most prominent light curve maxima with decreasing frequency, as expected from the frequency dependence of the $τ=1$ surface of synchrotron self-absorption. The global maximum in the 86 GHz light curve becomes less prominent at lower frequencies, while a local maximum, appearing in 2014, strengthens toward decreasing frequencies, becoming pronounced at $\sim5$ GHz. These tendencies suggest different Doppler boosting of stratified radio-emitting zones in the jet. △ Less

Submitted 17 January, 2020; originally announced January 2020.

Comments: 21 pages, 18 figures, accepted for publication in MNRAS

Journal ref: MNRAS 492 (2020) 3829

Quasi-periodic behaviour in the optical and γ-ray light curves of blazars 3C 66A and B2 1633+38

Authors: J. Otero-Santos, J. A. Acosta-Pulido, J. Becerra González, C. M. Raiteri, V. M. Larionov, P. Peñil, P. S. Smith, C. Ballester Niebla, G. A. Borman, M. I. Carnerero, N. Castro Segura, T. S. Grishina, E. N. Kopatskaya, E. G. Larionova, D. A. Morozova, A. A. Nikiforova, S. S. Savchenko, Yu. V. Troitskaya, I. S. Troitsky, A. A. Vasilyev, M. Villata

Abstract: We report on quasi-periodic variability found in two blazars included in the Steward Observatory Blazar Monitoring data sample: the BL Lac object 3C 66A and the Flat Spectrum Radio Quasar B2 1633+38. We collect optical photometric and polarimetric data in V and R bands of these sources from different observatories: St. Petersburg University, Crimean Astrophysical Observatory, WEBT-GASP, Catalina R… ▽ More We report on quasi-periodic variability found in two blazars included in the Steward Observatory Blazar Monitoring data sample: the BL Lac object 3C 66A and the Flat Spectrum Radio Quasar B2 1633+38. We collect optical photometric and polarimetric data in V and R bands of these sources from different observatories: St. Petersburg University, Crimean Astrophysical Observatory, WEBT-GASP, Catalina Real-Time Transient Survey, Steward Observatory, STELLA Robotic Observatory and Katzman Automatic Imaging Telescope. In addition, an analysis of the $γ$-ray light curves from $\textit{Fermi}$-LAT is included. Three methods are used to search for any periodic behaviour in the data: the Z-transform Discrete Correlation Function, the Lomb-Scargle periodogram and the Weighted Wavelet Z-transform. We find evidences of possible quasi-periodic variability in the optical photometric data of both sources with periods of $\sim$3 years for 3C 66A and $\sim$1.9 years for B2 1633+38, with significances between 3$σ$ and 5$σ$. Only B2 1633+38 shows evidence of this behaviour in the optical polarized data set at a confidence level of 2$σ$-4$σ$. This is the first reported evidence of quasi-periodic behaviour in the optical light curve of B2 1633+38. Also a hint of quasi-periodic behaviour is found in the $γ$-ray light curve of B2 1633+38 with a confidence level $\geqslant$2$σ$, while no periodicity is observed for 3C 66A in this energy range. We propose different jet emission models that could explain the quasi-periodic variability and the differences found between these two sources. △ Less

Submitted 5 February, 2020; v1 submitted 16 January, 2020; originally announced January 2020.

Comments: Accepted for publication in MNRAS

Multiband optical flux density and polarization microvariability study of optically bright blazars

Authors: Magdalena Pasierb, Arti Goyal, Michał Ostrowski, Łukasz Stawarz, Paul J. Wiita, Gopal-Krishna, Valeri M. Larionov, Daria A. Morozova, Ryosuke Itoh, Fahri Alicavus, Ahmet Erdem, Santosh Joshi, Staszek Zola, Georgy A. Borman, Tatiana S. Grishina, Evgenia N. Kopatskaya, Elena G. Larionova, Sergey S. Savchenko, Anna A. Nikiforova, Yulia V. Troitskaya, Ivan S. Troitsky, Hiroshi Akitaya, Miho Kawabata, Tatsuya Nakaoka

Abstract: We present the results of flux density, spectral index, and polarization intra-night monitoring studies of a sample of eight optically bright blazars, carried out by employing several small to moderate aperture (0.4\,m to 1.5\,m diameter) telescopes fitted with CCDs and polarimeters located in Europe, India, and Japan. The duty cycle of flux variability for the targets is found to be $\sim 45$ per… ▽ More We present the results of flux density, spectral index, and polarization intra-night monitoring studies of a sample of eight optically bright blazars, carried out by employing several small to moderate aperture (0.4\,m to 1.5\,m diameter) telescopes fitted with CCDs and polarimeters located in Europe, India, and Japan. The duty cycle of flux variability for the targets is found to be $\sim 45$ percent, similar to that reported in earlier studies. The computed two-point spectral indices are found to be between 0.65 to 1.87 for our sample, comprised of low- and intermediate frequency peaked blazars, with one exception; they are also found to be statistically variable for about half the instances where `confirmed' variability is detected in flux density. In the analysis of the spectral evolution of the targets on hourly timescale, a counter-clockwise loop (soft-lagging) is noted in the flux-spectral index plane on two occasions, and in one case a clear spectral flattening with the decreasing flux is observed. In our data set, we also observe a variety of flux-polarization degree variability patterns, including instances with a relatively straightforward anti-correlation, correlation, or counter-clockwise looping. These changes are typically reflected in the flux-polarization angle plane: the anti-correlation between the flux and polarization degree is accompanied by an anti-correlation between the polarization angle and flux, while the counter-clockwise flux-PD looping behaviour is accompanied by a clockwise looping in the flux-polarization angle representation. We discuss our findings in the framework of the internal shock scenario for blazar sources. △ Less

Submitted 11 December, 2019; originally announced December 2019.

Comments: MNRAS accepted

Investigating the multiwavelength behaviour of the flat spectrum radio quasar CTA 102 during 2013-2017

Authors: F. D'Ammando, C. M. Raiteri, M. Villata, J. A. Acosta-Pulido, I. Agudo, A. A. Arkharov, R. Bachev, G. V. Baida, E. Benitez, G. A. Borman, W. Boschin, V. Bozhilov, M. S. Butuzova, P. Calcidese, M. I. Carnerero, D. Carosati, C. Casadio, N. Castro-Segura, W. -P. Chen, G. Damljanovic, A. Di Paola, J. Echevarria, N. V. Efimova, Sh. A. Ehgamberdiev, C. Espinosa , et al. (72 additional authors not shown)

Abstract: We present a multiwavelength study of the flat-spectrum radio quasar CTA 102 during 2013-2017. We use radio-to-optical data obtained by the Whole Earth Blazar Telescope, 15 GHz data from the Owens Valley Radio Observatory, 91 and 103 GHz data from the Atacama Large Millimeter Array, near-infrared data from the Rapid Eye Monitor telescope, as well as data from the Swift (optical-UV and X-rays) and… ▽ More We present a multiwavelength study of the flat-spectrum radio quasar CTA 102 during 2013-2017. We use radio-to-optical data obtained by the Whole Earth Blazar Telescope, 15 GHz data from the Owens Valley Radio Observatory, 91 and 103 GHz data from the Atacama Large Millimeter Array, near-infrared data from the Rapid Eye Monitor telescope, as well as data from the Swift (optical-UV and X-rays) and Fermi ($γ$ rays) satellites to study flux and spectral variability and the correlation between flux changes at different wavelengths. Unprecedented $γ$-ray flaring activity was observed during 2016 November-2017 February, with four major outbursts. A peak flux of (2158 $\pm$ 63)$\times$10$^{-8}$ ph cm$^{-2}$ s$^{-1}$, corresponding to a luminosity of (2.2 $\pm$ 0.1)$\times$10$^{50}$ erg s$^{-1}$, was reached on 2016 December 28. These four $γ$-ray outbursts have corresponding events in the near-infrared, optical, and UV bands, with the peaks observed at the same time. A general agreement between X-ray and $γ$-ray activity is found. The $γ$-ray flux variations show a general, strong correlation with the optical ones with no time lag between the two bands and a comparable variability amplitude. This $γ$-ray/optical relationship is in agreement with the geometrical model that has successfully explained the low-energy flux and spectral behaviour, suggesting that the long-term flux variations are mainly due to changes in the Doppler factor produced by variations of the viewing angle of the emitting regions. The difference in behaviour between radio and higher energy emission would be ascribed to different viewing angles of the jet regions producing their emission. △ Less

Submitted 11 November, 2019; v1 submitted 8 October, 2019; originally announced October 2019.

Comments: 24 pages, 18 figures, and 4 tables. Accepted for publication in Monthly Notices of the Royal Astronomical Society

The June 2016 Optical and Gamma-Ray Outburst and Optical Micro-Variability of the Blazar 3C454.3

Authors: Zachary R. Weaver, Thomas J. Balonek, Svetlana G. Jorstad, Alan P. Marscher, Valeri M. Larionov, Paul S. Smith, Samantha J. Boni, George A. Borman, K. J. Chapman, Leah G. Jenks, Evgenia N. Kopatskaya, Elena G. Larionova, Daria A. Morozova, Anna A. Nikiforova, Alina Sabyr, Sergey S. Savchenko, Ryan W. Stahlin, Yulia V. Troitskaya, Ivan S. Troitsky, Saiyang Zhang

Abstract: The quasar 3C454.3 underwent a uniquely-structured multi-frequency outburst in June 2016. The blazar was observed in the optical $R$ band by several ground-based telescopes in photometric and polarimetric modes, at $γ$-ray frequencies by the \emph{Fermi}\ Large Area Telescope, and at 43 GHz with the Very Long Baseline Array. The maximum flux density was observed on 2016 June 24 at both optical and… ▽ More The quasar 3C454.3 underwent a uniquely-structured multi-frequency outburst in June 2016. The blazar was observed in the optical $R$ band by several ground-based telescopes in photometric and polarimetric modes, at $γ$-ray frequencies by the \emph{Fermi}\ Large Area Telescope, and at 43 GHz with the Very Long Baseline Array. The maximum flux density was observed on 2016 June 24 at both optical and $γ$-ray frequencies, reaching $S^\mathrm{max}_\mathrm{opt}=18.91\pm0.08$ mJy and $S_γ^\mathrm{max} =22.20\pm0.18\times10^{-6}$ ph cm$^{-2}$ s$^{-1}$, respectively. The June 2016 outburst possessed a precipitous decay at both $γ$-ray and optical frequencies, with the source decreasing in flux density by a factor of 4 over a 24-hour period in $R$ band. Intraday variability was observed throughout the outburst, with flux density changes between 1 and 5 mJy over the course of a night. The precipitous decay featured statistically significant quasi-periodic micro-variability oscillations with an amplitude of $\sim 2$-$3\%$ about the mean trend and a characteristic period of 36 minutes. The optical degree of polarization jumped from $\sim3\%$ to nearly 20\% during the outburst, while the position angle varied by $\sim120\degr$. A knot was ejected from the 43 GHz core on 2016 Feb 25, moving at an apparent speed $v_\mathrm{app}=20.3c\pm0.8c$. From the observed minimum timescale of variability $τ_\mathrm{opt}^\mathrm{min}\approx2$ hr and derived Doppler factor $δ=22.6$, we find a size of the emission region $r\lesssim2.6\times10^{15}$ cm. If the quasi-periodic micro-variability oscillations are caused by periodic variations of the Doppler factor of emission from a turbulent vortex, we derive a rotational speed of the vortex $\sim0.2c$. △ Less

Submitted 11 March, 2019; originally announced March 2019.

Comments: 19 pages, 13 figures, 3 tables, accepted to the Astrophysical Journal 2019 March 9

A Fast Very High Energy $γ$-ray Flare from BL Lacertae during a Period of Multiwavelength activity in June 2015

Authors: MAGIC Collaboration, V. A. Acciari, S. Ansoldi, L. A. Antonelli, A. Arbet Engels, D. Baack, A. Babić, B. Banerjee, P. Bangale, U. Barres de Almeida, J. A. Barrio, J. Becerra González, W. Bednarek, E. Bernardini, A. Berti, J. Besenrieder, W. Bhattacharyya, C. Bigongiari, A. Biland, O. Blanch, G. Bonnoli, R. Carosi, G. Ceribella, S. Cikota, S. M. Colak , et al. (141 additional authors not shown)

Abstract: The mechanisms producing fast variability of the $γ$-ray emission in active galactic nuclei are under debate. The MAGIC telescopes detected a fast very high energy (VHE, E$>100$ GeV) $γ$-ray flare from BL Lacertae on 2015 June 15. The flare had a maximum flux of $(1.5\pm 0.3)\times 10^{-10}$ photons cm$^{-2}$ s$^{-1}$ and halving time of $26\pm8$ minutes. The MAGIC observations were triggered by a… ▽ More The mechanisms producing fast variability of the $γ$-ray emission in active galactic nuclei are under debate. The MAGIC telescopes detected a fast very high energy (VHE, E$>100$ GeV) $γ$-ray flare from BL Lacertae on 2015 June 15. The flare had a maximum flux of $(1.5\pm 0.3)\times 10^{-10}$ photons cm$^{-2}$ s$^{-1}$ and halving time of $26\pm8$ minutes. The MAGIC observations were triggered by a high state in the optical and high energy (HE, E$>100$ MeV) $γ$-ray bands. In this paper we present the MAGIC VHE $γ$-ray data together with multiwavelength data from radio, optical, X-rays, and HE $γ$ rays from 2015 May 1 to July 31. Well-sampled multiwavelength data allow us to study the variability in detail and compare it to the other epochs when fast VHE $γ$-ray flares have been detected from this source. Interestingly, we find that the behaviour in radio, optical, X-rays and HE $γ$-rays is very similar to two other observed VHE $γ$-ray flares. In particular, also during this flare there was an indication of rotation of the optical polarization angle and of activity at the 43\,GHz core. These repeating patterns indicate a connection between the three events. We also test modelling of the spectral energy distribution, based on constraints from the light curves and VLBA observations, with two different geometrical setups of two-zone inverse Compton models. In addition we model the $γ$-ray data with the star-jet interaction model. We find that all of the tested emission models are compatible with the fast VHE $γ$-ray flare, but all have some tension with the multiwavelength observations. △ Less

Submitted 7 January, 2019; originally announced January 2019.

Comments: Accepted to Astronomy & Astrophysics

Journal ref: A&A 623, A175 (2019)

AGILE, Fermi, Swift, and GASP-WEBT multi-wavelength observations of the high-redshift blazar 4C $+$71.07 in outburst

Authors: S. Vercellone, P. Romano, G. Piano, V. Vittorini, I. Donnarumma, P. Munar-Adrover, C. M. Raiteri, M. Villata, F. Verrecchia, F. Lucarelli, C. Pittori, A. Bulgarelli, V. Fioretti, M. Tavani, J. A. Acosta-Pulido, I. Agudo, A. A. Arkharov, U. Bach, R. Bachev, G. A. Borman, M. S. Butuzova, M. I. Carnerero, C. Casadio, G. Damljanovic, F. D'Ammando , et al. (34 additional authors not shown)

Abstract: The flat-spectrum radio quasar 4C $+$71.07 is a high-redshift ($z=2.172$), $γ$-loud blazar whose optical emission is dominated by the thermal radiation from accretion disc. 4C $+$71.07 has been detected in outburst twice by the AGILE $γ$-ray satellite during the period end of October - mid November 2015, when it reached a $γ$-ray flux of the order of… ▽ More The flat-spectrum radio quasar 4C $+$71.07 is a high-redshift ($z=2.172$), $γ$-loud blazar whose optical emission is dominated by the thermal radiation from accretion disc. 4C $+$71.07 has been detected in outburst twice by the AGILE $γ$-ray satellite during the period end of October - mid November 2015, when it reached a $γ$-ray flux of the order of $F_{\rm E>100\,MeV} = (1.2 \pm 0.3)\times 10^{-6}$ photons cm$^{-2}$ s$^{-1}$ and $F_{\rm E>100\,MeV} = (3.1 \pm 0.6)\times 10^{-6}$ photons cm$^{-2}$ s$^{-1}$, respectively, allowing us to investigate the properties of the jet and of the emission region. We investigated its spectral energy distribution by means of almost simultaneous observations covering the cm, mm, near-infrared, optical, ultra-violet, X-ray and $γ$-ray energy bands obtained by the GASP-WEBT Consortium, the Swift and the AGILE and Fermi satellites. The spectral energy distribution of the second $γ$-ray flare (the one whose energy coverage is more dense) can be modelled by means of a one-zone leptonic model, yielding a total jet power of about $4\times10^{47}$ erg s$^{-1}$. During the most prominent $γ$-ray flaring period our model is consistent with a dissipation region within the broad-line region. Moreover, this class of high-redshift, large-mass black-hole flat-spectrum radio quasars might be good targets for future $γ$-ray satellites such as e-ASTROGAM. △ Less

Submitted 19 November, 2018; originally announced November 2018.

Comments: Accepted for publication in A&A. 9 pages, 4 Figures, 3 Tables

Journal ref: A&A 621, A82 (2019)

The extreme HBL behaviour of Markarian 501 during 2012

Authors: M. L. Ahnen, S. Ansoldi, L. A. Antonelli, C. Arcaro, A. Babić, B. Banerjee, P. Bangale, U. Barres de Almeida, J. A. Barrio, J. Becerra González, W. Bednarek, E. Bernardini, A. Berti, W. Bhattacharyya, O. Blanch, G. Bonnoli, R. Carosi, A. Carosi, A. Chatterjee, S. M. Colak, P. Colin, E. Colombo, J. L. Contreras, J. Cortina, S. Covino , et al. (254 additional authors not shown)

Abstract: A multiwavelength campaign was organized to take place between March and July of 2012. Excellent temporal coverage was obtained with more than 25 instruments, including the MAGIC, FACT and VERITAS Cherenkov telescopes, the instruments on board the Swift and Fermi spacecraft, and the telescopes operated by the GASP-WEBT collaboration. Mrk 501 showed a very high energy (VHE) gamma-ray flux above 0… ▽ More A multiwavelength campaign was organized to take place between March and July of 2012. Excellent temporal coverage was obtained with more than 25 instruments, including the MAGIC, FACT and VERITAS Cherenkov telescopes, the instruments on board the Swift and Fermi spacecraft, and the telescopes operated by the GASP-WEBT collaboration. Mrk 501 showed a very high energy (VHE) gamma-ray flux above 0.2 TeV of $\sim$0.5 times the Crab Nebula flux (CU) for most of the campaign. The highest activity occurred on 2012 June 9, when the VHE flux was $\sim$3 CU, and the peak of the high-energy spectral component was found to be at $\sim$2 TeV. This study reports very hard X-ray spectra, and the hardest VHE spectra measured to date for Mrk 501. The fractional variability was found to increase with energy, with the highest variability occurring at VHE, and a significant correlation between the X-ray and VHE bands. The unprecedentedly hard X-ray and VHE spectra measured imply that their low- and high-energy components peaked above 5 keV and 0.5 TeV, respectively, during a large fraction of the observing campaign, and hence that Mrk 501 behaved like an extreme high-frequency- peaked blazar (EHBL) throughout the 2012 observing season. This suggests that being an EHBL may not be a permanent characteristic of a blazar, but rather a state which may change over time. The one-zone synchrotron self-Compton (SSC) scenario can successfully describe the segments of the SED where most energy is emitted, with a significant correlation between the electron energy density and the VHE gamma-ray activity, suggesting that most of the variability may be explained by the injection of high-energy electrons. The one-zone SSC scenario used reproduces the behaviour seen between the measured X-ray and VHE gamma-ray fluxes, and predicts that the correlation becomes stronger with increasing energy of the X-rays. △ Less

Submitted 14 August, 2018; v1 submitted 13 August, 2018; originally announced August 2018.

Comments: 25 pages, 9 figures, accepted for publication in A&A. Corresponding authors: Gareth Hughes (gareth.hughes@cfa.harvard.edu), David Paneque (dpaneque@mppmu.mpg.de), Amit Shukla (amit.shukla@astro.uni-wuerzburg.de)

Journal ref: A&A 620, A181 (2018)

Multi-wavelength characterization of the blazar S5~0716+714 during an unprecedented outburst phase

Authors: MAGIC Collaboration, M. L. Ahnen, S. Ansoldi, L. A. Antonelli, C. Arcaro, D. Baack, A. Babić, B. Banerjee, P. Bangale, U. Barres de Almeida, J. A. Barrio, J. Becerra González, W. Bednarek, E. Bernardini, R. Ch. Berse, A. Berti, W. Bhattacharyya, A. Biland, O. Blanch, G. Bonnoli, R. Carosi, A. Carosi, G. Ceribella, A. Chatterjee, S. M. Colak , et al. (165 additional authors not shown)

Abstract: The BL Lac object S5~0716+714, a highly variable blazar, underwent an impressive outburst in January 2015 (Phase A), followed by minor activity in February (Phase B). The MAGIC observations were triggered by the optical flux observed in Phase A, corresponding to the brightest ever reported state of the source in the R-band. The comprehensive dataset collected is investigated in order to shed light… ▽ More The BL Lac object S5~0716+714, a highly variable blazar, underwent an impressive outburst in January 2015 (Phase A), followed by minor activity in February (Phase B). The MAGIC observations were triggered by the optical flux observed in Phase A, corresponding to the brightest ever reported state of the source in the R-band. The comprehensive dataset collected is investigated in order to shed light on the mechanism of the broadband emission. Multi-wavelength light curves have been studied together with the broadband Spectral Energy Distributions (SEDs). The data set collected spans from radio, optical photometry and polarimetry, X-ray, high-energy (HE, 0.1 GeV < E < 100 GeV) with \textit{Fermi}-LAT to the very-high-energy (VHE, E>100 GeV) with MAGIC. The flaring state of Phase A was detected in all the energy bands, providing for the first time a multi-wavelength sample of simultaneous data from the radio band to the VHE. In the constructed SED the \textit{Swift}-XRT+\textit{NuSTAR} data constrain the transition between the synchrotron and inverse Compton components very accurately, while the second peak is constrained from 0.1~GeV to 600~GeV by \textit{Fermi}+MAGIC data. The broadband SED cannot be described with a one-zone synchrotron self-Compton model as it severely underestimates the optical flux in order to reproduce the X-ray to $γ$-ray data. Instead we use a two-zone model. The EVPA shows an unprecedented fast rotation. An estimation of the redshift of the source by combined HE and VHE data provides a value of $z = 0.31 \pm 0.02_{stats} \pm 0.05_{sys}$, confirming the literature value. The data show the VHE emission originating in the entrance and exit of a superluminal knot in and out a recollimation shock in the inner jet. A shock-shock interaction in the jet seems responsible for the observed flares and EVPA swing. This scenario is also consistent with the SED modelling. △ Less

Submitted 1 July, 2018; originally announced July 2018.

Journal ref: Astronomy & Astrophysics 619, A45 (2018)

Exploring the connection between parsec-scale jet activity and broadband outbursts in 3C 279

Authors: B. Rani, S. G. Jorstad, A. P. Marscher, I. Agudo, K. V. Sokolovsky, V. M. Larionov, P. Smith, D. A. Mosunova, G. A. Borman, T. S. Grishina, E. N. Kopatskaya, A. A. Mokrushina, D. A. Morozova, S. S. Savchenko, Yu. V. Troitskaya, I. S. Troitsky, C. Thum, S. N. Molina, C. Casadio

Abstract: We use a combination of high-resolution very long baseline interferometry (VLBI) radio and multi-wavelength flux density and polarization observations to constrain the physics of the dissipation mechanism powering the broadband flares in 3C 279 during an episode of extreme flaring activity in 2013-2014. Six bright flares superimposed on a long-term outburst are detected at $γ$-ray energies. Four o… ▽ More We use a combination of high-resolution very long baseline interferometry (VLBI) radio and multi-wavelength flux density and polarization observations to constrain the physics of the dissipation mechanism powering the broadband flares in 3C 279 during an episode of extreme flaring activity in 2013-2014. Six bright flares superimposed on a long-term outburst are detected at $γ$-ray energies. Four of the flares have optical and radio counterparts. The two modes of flaring activity (faster flares sitting on top of a long term outburst) present at radio, optical, $γ$-ray frequencies are missing in X-rays. X-ray counterparts are only observed for two flares. The first three flares are accompanied by ejection of a new VLBI component (NC2) suggesting the 43 GHz VLBI core as the site of energy dissipation. Another new component, NC3, is ejected after the last three flares, which suggests that the emission is produced upstream from the core (closer to the black hole). The study therefore indicates multiple sites of energy dissipation in the source. An anti-correlation is detected between the optical percentage polarization (PP) and optical/$γ$-ray flux variations, while the PP has a positive correlation with optical/$γ$-rays spectral indices. Given that the mean polarization is inversely proportional to the number of cells in the emission region, the PP vs. optical/$γ$-ray anti-correlation could be due to more active cells during the outburst than at other times. In addition to the turbulent component, our analysis suggests the presence of a combined turbulent and ordered magnetic field, with the ordered component transverse to the jet axis. △ Less

Submitted 20 May, 2018; v1 submitted 12 May, 2018; originally announced May 2018.

Comments: ApJ (in press), 14 pages, 19 figures

The Bright $γ$-ray Flare of 3C 279 in June 2015: AGILE Detection and Multifrequency Follow-up Observations

Authors: C. Pittori, F. Lucarelli, F. Verrecchia, C. M. Raiteri, M. Villata, V. Vittorini, M. Tavani, S. Puccetti, M. Perri, I. Donnarumma, S. Vercellone, J. A. Acosta-Pulido, R. Bachev, E. Benitez, G. A. Borman, M. I. Carnerero, D. Carosati, W. P. Chen, Sh. A. Ehgamberdiev, A. Goded, T. S. Grishina, D. Hiriart, H. Y. Hsiao, S. G. Jorstad, G. N. Kimeridze , et al. (22 additional authors not shown)

Abstract: We report the AGILE detection and the results of the multifrequency follow-up observations of a bright $γ$-ray flare of the blazar 3C 279 in June 2015. We use AGILE-GRID and Fermi-LAT $γ$-ray data, together with Swift-XRT, Swift-UVOT, and ground-based GASP-WEBT optical observations, including polarization information, to study the source variability and the overall spectral energy distribution dur… ▽ More We report the AGILE detection and the results of the multifrequency follow-up observations of a bright $γ$-ray flare of the blazar 3C 279 in June 2015. We use AGILE-GRID and Fermi-LAT $γ$-ray data, together with Swift-XRT, Swift-UVOT, and ground-based GASP-WEBT optical observations, including polarization information, to study the source variability and the overall spectral energy distribution during the $γ$-ray flare. The $γ$-ray flaring data, compared with as yet unpublished simultaneous optical data which allow to set constraints on the big blue bump disk luminosity, show very high Compton dominance values of $\sim 100$, with a ratio of $γ$-ray to optical emission rising by a factor of three in a few hours. The multi-wavelength behavior of the source during the flare challenges one-zone leptonic theoretical models. The new observations during the June 2015 flare are also compared with already published data and non-simultaneous historical 3C 279 archival data. △ Less

Submitted 20 March, 2018; originally announced March 2018.

Comments: 13 pages, 5 figures,5 Tables. Accepted for publication in ApJ

The detection of the blazar S4 0954+65 at very-high-energy with the MAGIC telescopes during an exceptionally high optical state

Authors: MAGIC Collaboration, M. L. Ahnen, S. Ansoldi, L. A. Antonelli, C. Arcaro, D. Baack, A. Babić, B. Banerjee, P. Bangale, U. Barres de Almeida, J. A. Barrio, W. Bednarek, E. Bernardini, R. Ch. Berse, A. Berti, W. Bhattacharyya, A. Biland, O. Blanch, G. Bonnoli, R. Carosi, A. Carosi, G. Ceribella, A. Chatterjee, S. M. Colak, P. Colin , et al. (147 additional authors not shown)

Abstract: The very-high-energy (VHE, $\gtrsim 100$ GeV) $γ$-ray MAGIC observations of the blazar S4 0954+65, were triggered by an exceptionally high flux state of emission in the optical. This blazar has a disputed redshift of z=0.368 or z$\geqslant$0.45 and an uncertain classification among blazar subclasses. The exceptional source state described here makes for an excellent opportunity to understand physi… ▽ More The very-high-energy (VHE, $\gtrsim 100$ GeV) $γ$-ray MAGIC observations of the blazar S4 0954+65, were triggered by an exceptionally high flux state of emission in the optical. This blazar has a disputed redshift of z=0.368 or z$\geqslant$0.45 and an uncertain classification among blazar subclasses. The exceptional source state described here makes for an excellent opportunity to understand physical processes in the jet of S4 0954+65 and thus contribute to its classification. We investigate the multiwavelength (MWL) light curve and spectral energy distribution (SED) of the S4 0954+65 blazar during an enhanced state in February 2015 and put it in context with possible emission scenarios. We collect photometric data in radio, optical, X-ray, and $γ$ ray. We study both the optical polarization and the inner parsec-scale jet behavior with 43 GHz data. Observations with the MAGIC telescopes led to the first detection of S4 0954+65 at VHE. Simultaneous data with Fermi-LAT at high energy $γ$ ray\ (HE, 100 MeV < E < 100 GeV) also show a period of increased activity. Imaging at 43 GHz reveals the emergence of a new feature in the radio jet in coincidence with the VHE flare. Simultaneous monitoring of the optical polarization angle reveals a rotation of approximately 100$^\circ$. (...) The broadband spectrum can be modeled with an emission mechanism commonly invoked for flat spectrum radio quasars, i.e. inverse Compton scattering on an external soft photon field from the dust torus, also known as external Compton. The light curve and SED phenomenology is consistent with an interpretation of a blob propagating through a helical structured magnetic field and eventually crossing a standing shock in the jet, a scenario typically applied to flat spectrum radio quasars (FSRQs) and low-frequency peaked BL Lac objects (LBL). △ Less

Submitted 12 January, 2018; originally announced January 2018.

Comments: 15 pages, 7 figures, submitted to A&A

Journal ref: A&A 617, A30 (2018)

Multi-wavelength temporal and spectral variability of the blazar OJ 287 during and after the December 2015 flare: a major accretion disc contribution

Authors: P. Kushwaha, A. C. Gupta, P. J. Wiita, H. Gaur, E. M. de Gouveia Dal Pino, J. Bhagwan, O. M. Kurtanidze, V. M. Larionov, G. Damljanovic, M. Uemura, E. Semkov, A. Strigachev, R. Bachev, O. Vince, Minfeng Gu, Z. Zhang, T. Abe, A. Agarwal, G. A. Borman, J. H. Fan, T. S. Grishina, J. Hirochi, R. Itoh, M. Kawabata, E. N. Kopatskaya , et al. (11 additional authors not shown)

Abstract: We present a multi-wavelength spectral and temporal analysis of the blazar OJ 287 during its recent activity between December 2015 -- May 2016, showing strong variability in the near-infrared (NIR) to X-ray energies with detection at $γ$-ray energies as well. Most of the optical flux variations exhibit strong changes in polarization angle and degree. All the inter-band time lags are consistent wit… ▽ More We present a multi-wavelength spectral and temporal analysis of the blazar OJ 287 during its recent activity between December 2015 -- May 2016, showing strong variability in the near-infrared (NIR) to X-ray energies with detection at $γ$-ray energies as well. Most of the optical flux variations exhibit strong changes in polarization angle and degree. All the inter-band time lags are consistent with simultaneous emissions. Interestingly, on days with excellent data coverage in the NIR--UV bands, the spectral energy distributions (SEDs) show signatures of bumps in the visible--UV bands, never seen before in this source. The optical bump can be explained as accretion-disk emission associated with the primary black hole of mass $\sim \rm 1.8 \times10^{10} M_{\odot}$ while the little bump feature in the optical-UV appears consistent with line emission. Further, the broadband SEDs extracted during the first flare and during a quiescent period during this span show very different $γ$-ray spectra compared to previously observed flare or quiescent spectra. The probable thermal bump in the visible seems to have been clearly present since May 2013, as found by examining all available NIR-optical observations, and favors the binary super-massive black hole model. The simultaneous multi-wavelength variability and relatively weak $γ$-ray emission that shows a shift in the SED peak is consistent with $γ$-ray emission originating from inverse Compton scattering of photons from the line emission that apparently contributes to the little blue bump. △ Less

Submitted 14 September, 2017; originally announced September 2017.

Comments: 24 pages, 6 figures, 2 tables, MNRAS in press

Journal ref: 2018, MNRAS, 473, 1145

Multiband variability studies and novel broadband SED modeling of Mrk 501 in 2009

Authors: M. L. Ahnen, S. Ansoldi, L. A. Antonelli, P. Antoranz, A. Babic, B. Banerjee, P. Bangale, U. Barres de Almeida, J. A. Barrio, J. Becerra González, W. Bednarek, E. Bernardini, A. Berti, B. Biasuzzi, A. Biland, O. Blanch, S. Bonnefoy, G. Bonnoli, F. Borracci, T. Bretz, S. Buson, A. Carosi, A. Chatterjee, R. Clavero, P. Colin , et al. (268 additional authors not shown)

Abstract: We present an extensive study of the BL Lac object Mrk 501 based on a data set collected during the multi-instrument campaign spanning from 2009 March 15 to 2009 August 1 which includes, among other instruments, MAGIC, VERITAS, Whipple 10-m, Fermi-LAT, RXTE, Swift, GASP-WEBT and VLBA. We find an increase in the fractional variability with energy, while no significant interband correlations of flux… ▽ More We present an extensive study of the BL Lac object Mrk 501 based on a data set collected during the multi-instrument campaign spanning from 2009 March 15 to 2009 August 1 which includes, among other instruments, MAGIC, VERITAS, Whipple 10-m, Fermi-LAT, RXTE, Swift, GASP-WEBT and VLBA. We find an increase in the fractional variability with energy, while no significant interband correlations of flux changes are found in the acquired data set. The higher variability in the very high energy (>100 GeV, VHE) gamma-ray emission and the lack of correlation with the X-ray emission indicate that the highest-energy electrons that are responsible for the VHE gamma-rays do not make a dominant contribution to the ~1 keV emission. Alternatively, there could be a very variable component contributing to the VHE gamma-ray emission in addition to that coming from the synchrotron self-Compton (SSC) scenarios. The space of SSC model parameters is probed following a dedicated grid-scan strategy, allowing for a wide range of models to be tested and offering a study of the degeneracy of model-to-data agreement in the individual model parameters. We find that there is some degeneracy in both the one-zone and the two-zone SSC scenarios that were probed, with several combinations of model parameters yielding a similar model-to-data agreement, and some parameters better constrained than others. The SSC model grid-scan shows that the flaring activity around 2009 May 22 cannot be modeled adequately with a one-zone SSC scenario, while it can be suitably described within a two-independent-zone SSC scenario. The observation of an electric vector polarization angle rotation coincident with the gamma-ray flare from 2009 May 1 resembles those reported previously for low frequency peaked blazars, hence suggesting that there are many similarities in the flaring mechanisms of blazars with different jet properties. △ Less

Submitted 30 December, 2016; originally announced December 2016.

Comments: 33 pages, 15 figures, accepted for publication in A&A. Corresponding authors: Marlene Doert (marlene.doert@tu-dortmund.de) and David Paneque (dpaneque@mppmu.mpg.de)

Journal ref: A&A 603, A31 (2017)