Showing 1–50 of 127 results for author: Max-Moerbeck, W

A Multiwavelength Study of a Long-Duration VHE Flare from BL Lacertae with VERITAS

Authors: Atreya Acharyya, Avery Archer, Priyadarshini Bangale, Joshua Bartkoske, Wystan Benbow, James Buckley, Yu Chen, Jodi Christiansen, Alisha Chromey, Anne Duerr, Manel Errando, Miguel Escobar Godoy, Juan Escudero Pedrosa, Abe Falcone, Sydney Feldman, Qi Feng, Simon Filbert, Lucy Fortson, Amy Furniss, William Hanlon, Olivier Hervet, Claire Hinrichs, Jamie Holder, Zach Hughes, Thomas Humensky , et al. (50 additional authors not shown)

Abstract: We report the first observations of a long-duration very-high-energy (VHE; $E > 100$ GeV) flare from BL Lacertae (VER J2202+422), taken with the Very Energetic Radiation Imaging Telescope Array System (VERITAS). On October 15, 2022, the Fermi-Large Area Telescope (LAT) detected elevated GeV activity originating from this blazar. This triggered a multiwavelength campaign, which includes observation… ▽ More We report the first observations of a long-duration very-high-energy (VHE; $E > 100$ GeV) flare from BL Lacertae (VER J2202+422), taken with the Very Energetic Radiation Imaging Telescope Array System (VERITAS). On October 15, 2022, the Fermi-Large Area Telescope (LAT) detected elevated GeV activity originating from this blazar. This triggered a multiwavelength campaign, which includes observations from VERITAS, Swift, NuSTAR, and select optical and radio observatories. VERITAS observed the source for a total of $\sim 9.8$ hours between September 1, 2022 and December 1, 2022. An analysis of these data yields a $\sim 28 σ$ detection of the source. While previously observed VHE flares from BL Lacertae have lasted on time-scales of minutes to days, VERITAS continued to detect flaring activity from the source for over a month ($\sim 40$ days) after the original flaring activity was detected with Fermi-LAT. Broadband spectral modeling shows that a synchrotron self-Compton (SSC) model with an external inverse-Compton (EC) component is preferred over a one-zone SSC model. △ Less

Submitted 16 July, 2026; originally announced July 2026.

Comments: 19 pages, 8 figures. Accepted for publication in the Astrophysical Journal (ApJ)

Investigating Active Galactic Nuclei variability with the Cherenkov Telescope Array Observatory

Authors: G. Grolleron, J. Biteau, M. Cerruti, R. Grau, L. Gréaux, T. Hovatta, J. -P. Lenain, E. Lindfors, W. Max-Moerbeck, D. Miceli, A. Moralejo, K. Nilsson, E. Prandini, E. Pueschel, S. Kankkunen, J. Becerra Gonzalez, J. Finke, M. Joshi, P. Morris, M. Petropoulou, A. Sarkar, P. Romano, S. Vercellone, M. Zacharias

Abstract: Blazars, a type of active galactic nuclei (AGN) with relativistic jets pointed at the observer, exhibit flux variability across the electromagnetic spectrum due to particle acceleration in their jets. Power spectral density (PSD) studies show breaks at specific frequencies, particularly in X-rays, linked to the accretion regime and black hole mass. However, very-high-energy gamma-ray PSD breaks re… ▽ More Blazars, a type of active galactic nuclei (AGN) with relativistic jets pointed at the observer, exhibit flux variability across the electromagnetic spectrum due to particle acceleration in their jets. Power spectral density (PSD) studies show breaks at specific frequencies, particularly in X-rays, linked to the accretion regime and black hole mass. However, very-high-energy gamma-ray PSD breaks remain unexplored due to current instrument limitations. The Cherenkov Telescope Array Observatory (CTAO), with up to ten times greater sensitivity compared to current generation instruments, will allow precise PSD reconstruction and unprecedented study of blazar flares. These flares reveal key insights into particle acceleration, photon production, and jet properties. The AGN monitoring and flare programs in CTAO's Key Science Project aim to deepen our understanding of blazar emissions. △ Less

Submitted 9 January, 2026; originally announced January 2026.

Comments: Conference Gamma 2024

Compelling Evidence for a Harmonic in the Light Curve of the Supermassive Black Hole Binary Candidate PKS J1309+1154

Authors: A. C. S. Readhead, M. F. Aller, A. G. Sullivan, R. D. Blandford, P. Mróz, P. V. De la Parra, B. Molina, E. R. Most, M. L. Lister, A. Synani, H. Aller, M. C. Begelman, Y. Ding, M. J. Graham, F. Harrison, T. Hovatta, I. Liodakis, W. Max-Moerbeck, V. Pavlidou, T. J. Pearson, V. Ravi, R. A. Reeves, T. Surti, K. Tassis, S. E. Tremblay , et al. (1 additional authors not shown)

Abstract: We recently discovered a supermassive black hole binary (SMBHB) candidate, PKS J1309+1154, in the combined 46-yr University of Michigan Radio Astronomy Observatory (UMRAO) plus Owens Valley Radio Observatory (OVRO) blazar monitoring programs at 14.5/15 GHz. The light curve of PKS 1309+1154 exhibits a 17.9 year periodicity. We also reported a hint of a first harmonic with a 9 year periodicity in th… ▽ More We recently discovered a supermassive black hole binary (SMBHB) candidate, PKS J1309+1154, in the combined 46-yr University of Michigan Radio Astronomy Observatory (UMRAO) plus Owens Valley Radio Observatory (OVRO) blazar monitoring programs at 14.5/15 GHz. The light curve of PKS 1309+1154 exhibits a 17.9 year periodicity. We also reported a hint of a first harmonic with a 9 year periodicity in this object. Further analysis of the PKS J1309+1154 light curve provides compelling evidence that both the fundamental and the harmonic are real, confirming the existence of real periodicities in blazar light curves. This is the first case, to our knowledge, of watertight evidence for a fundamental and a harmonic periodicity in a blazar light curve. It makes PKS J1309+1154 a \textit{strong\/} supermassive black hole binary (SMBHB) candidate, and thus the third such candidate to be revealed through long-term radio monitoring, the other two being PKS J0805--0111 and PKS 2131--021, both discovered through the OVRO 40 m Telescope monitoring program. It is argued that hundreds of SMBHB candidates will be discovered by the Vera Rubin and Simons Observatories. Coherent searches for gravitational waves from a network of SMBHB candidates, starting immediately, are strongly motivated. △ Less

Submitted 12 November, 2025; originally announced November 2025.

A Search for Supermassive Black Hole Binary Candidates in 46-Year Radio Light Curves of 83 Blazars

Authors: B. Molina, P. Mróz, P. V. De la Parra, A. C. S. Readhead, T. Surti, M. F. Aller, J. D. Scargle, R. A. Reeves, H. Aller, M. C. Begelman, R. D. Blandford, Y. Ding, M. J. Graham, F. Harrison, T. Hovatta, I. Liodakis, M. L. Lister, W. Max-Moerbeck, V. Pavlidou, T. J. Pearson, V. Ravi, A. G. Sullivan, A. Synani, K. Tassis, S. E. Tremblay , et al. (1 additional authors not shown)

Abstract: The combined University of Michigan Radio Astronomy Observatory (UMRAO) and Owens Valley Radio Observatory (OVRO) blazar monitoring programs at 14.5/15 GHz provide uninterrupted light curves of $\sim~46-50$ yr duration for 83 blazars, selected from amongst the brightest and most rapidly flaring blazars north of declination $-20^\circ$. In a search for supermassive black hole binary (SMBHB) candida… ▽ More The combined University of Michigan Radio Astronomy Observatory (UMRAO) and Owens Valley Radio Observatory (OVRO) blazar monitoring programs at 14.5/15 GHz provide uninterrupted light curves of $\sim~46-50$ yr duration for 83 blazars, selected from amongst the brightest and most rapidly flaring blazars north of declination $-20^\circ$. In a search for supermassive black hole binary (SMBHB) candidates, we carried out tests for periodic variability using generalized Lomb-Scargle (GLS), weighted wavelet-Z (WWZ), and sine-wave fitting (SWF) analyses of this sample. We used simulations to test the effects of the power law spectrum of the power spectral density (PSD) on our findings, and show that the irregular sampling in the observed light curves has very little effect on the GLS spectra. Apparent periodicities and putative harmonics appear in all 83 of the GLS spectra of the blazars in our sample. We tested the reality of these apparent periodicities and harmonics with simulations, and found that in the overwhelming majority of cases they are due to the steep slope of the PSD, and should therefore be treated with great caution. We find one new SMBHB candidate: PKS 1309+1154, which exhibits a 17.9 year periodicity. The fraction of SMBHB candidates in our sample is $2.4_{-0.8}^{+3.2}\%$. △ Less

Submitted 27 October, 2025; originally announced October 2025.

Comments: 20 pages, 12 figures and 4 tables

Optical spectroscopy of blazars for the Cherenkov Telescope Array Observatory- IV

Authors: B. Rajput, P. Goldoni, W. Max-Moerbeck, E. Kasai, D. A. Williams, C. Boisson, S. Pita, M. Backes, U. Barres de Almeida, J. Becerra González, G. Cotter, F. D'Ammando, V. Fallah Ramazani, B. Hnatyk, O. Hervet, E. Lindfors, D. Mukhi-Nilo, M. Nikołajuk, M. Splettstoesser, B. Van Soelen

Abstract: Blazars, including BL Lacs and FSRQs, are the most luminous extragalactic γ-ray sources. They account for about 56% of the sources listed in the recent Fermi-LAT catalog (4FGL-DR4). The optical and UV spectra of BL Lacs are nearly featureless, making it difficult to precisely determine their redshifts. Consequently, nearly half of the γ-ray BL Lacs lack reliable redshift measurements. This poses a… ▽ More Blazars, including BL Lacs and FSRQs, are the most luminous extragalactic γ-ray sources. They account for about 56% of the sources listed in the recent Fermi-LAT catalog (4FGL-DR4). The optical and UV spectra of BL Lacs are nearly featureless, making it difficult to precisely determine their redshifts. Consequently, nearly half of the γ-ray BL Lacs lack reliable redshift measurements. This poses a major challenge, since redshift is crucial for studying the cosmic evolution of the blazar population and γ-ray propagation studies such as indirect evidence of EBL, placing constraints on IGMF and searches for LIV and ALPs. This paper is the fourth in a series dedicated to determining the redshift of a sample of blazars identified as key targets for future observations with the Cherenkov Telescope Array Observatory (CTAO). We performed Monte Carlo simulations to select γ-ray blazars detected by Fermi-LAT with hard spectra, that lack redshift measurements. These blazars are expected to be detectable by CTAO within 30 hours or less of exposure assuming an average flux state. In this fourth paper, we report the results of detailed spectroscopic observations of 29 blazars using the ESO/VLT, Keck II, and SALT telescopes. Our analysis involved a thorough search for spectral lines in the spectra of each blazar, and when features of the host galaxy were identified, we modeled its properties. We also compared the magnitudes of the targets during the observations to their long-term light curves. In the sample studied, 9 of 29 sources were observed with a high signal-to-noise ratio (S/N > 100), while the remaining 20 were observed with a moderate or low S/N. We successfully determined firm redshifts for 12 blazars, ranging from 0.1636 to 1.1427, and identified two lower limit redshifts at z > 1.0196 and z > 1.4454. The remaining 15 BL Lac objects exhibited featureless spectra. △ Less

Submitted 20 July, 2025; originally announced July 2025.

Comments: 18 pages, 5 figures, submitted to Astronomy and Astrophysics (A&A)

Journal ref: A&A 704, A190 (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

Characterization of Markarian 421 during its most violent year: Multiwavelength variability and correlations

Authors: 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, D. Baack, A. Babić, U. Barres de Almeida, J. A. Barrio, I. Batković, A. Bautista, J. Baxter, J. Becerra González, W. Bednarek, E. Bernardini, J. Bernete, A. Berti , et al. (190 additional authors not shown)

Abstract: Mrk 421 was in its most active state around early 2010, which led to the highest TeV gamma-ray flux ever recorded from any active galactic nuclei. We aim to characterize the multiwavelength behavior during this exceptional year for Mrk 421, and evaluate whether it is consistent with the picture derived with data from other less exceptional years. We investigated the period from November 5, 2009, (… ▽ More Mrk 421 was in its most active state around early 2010, which led to the highest TeV gamma-ray flux ever recorded from any active galactic nuclei. We aim to characterize the multiwavelength behavior during this exceptional year for Mrk 421, and evaluate whether it is consistent with the picture derived with data from other less exceptional years. We investigated the period from November 5, 2009, (MJD 55140) until July 3, 2010, (MJD 55380) with extensive coverage from very-high-energy (VHE; E$\,>\,$100$\,$GeV) gamma rays to radio with MAGIC, VERITAS, Fermi-LAT, RXTE, Swift, GASP-WEBT, VLBA, and a variety of additional optical and radio telescopes. We investigated the variability and correlation behavior among different energy bands in great detail. We find the strongest variability in X-rays and VHE gamma rays, and PSDs compatible with power-law functions. We observe strong correlations between X-rays and VHE gamma rays. We also report a marginally significant positive correlation between high-energy (HE; E$\,>\,$100$\,$MeV) gamma rays and the ultraviolet band. We detected marginally significant correlations between the HE and VHE gamma rays, and between HE gamma rays and the X-ray, that disappear when the large flare in February 2010 is excluded from the correlation study. The activity of Mrk 421 also yielded the first ejection of features in the VLBA images of the jet of Mrk 421. Yet the large uncertainties in the ejection times of these radio features prevent us from firmly associating them to the specific flares recorded during the campaign. We also show that the collected multi-instrument data are consistent with a scenario where the emission is dominated by two regions, a compact and extended zone, which could be considered as a simplified implementation of an energy-stratified jet as suggested by recent IXPE observations. △ Less

Submitted 7 January, 2025; originally announced January 2025.

Comments: Accepted for publication in Astronomy & Astrophysics. Corresponding authors: Felix Schmuckermaier, David Paneque, Axel Arbet Engels

Time-dependent modelling of short-term variability in the TeV-blazar VER J0521+211 during the major flare in 2020

Authors: MAGIC Collaboration, 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, M. Artero, K. Asano, D. Baack, A. Babić, U. Barres de Almeida, J. A. Barrio, I. Batković, A. Bautista, J. Baxter, J. Becerra González, W. Bednarek, E. Bernardini, J. Bernete , et al. (206 additional authors not shown)

Abstract: The BL Lacertae object VER J0521+211 underwent a notable flaring episode in February 2020. A short-term monitoring campaign, led by the MAGIC (Major Atmospheric Gamma Imaging Cherenkov) collaboration, covering a wide energy range from radio to very-high-energy (VHE, 100 GeV < E < 100 TeV) gamma rays was organised to study its evolution. These observations resulted in a consistent detection of the… ▽ More The BL Lacertae object VER J0521+211 underwent a notable flaring episode in February 2020. A short-term monitoring campaign, led by the MAGIC (Major Atmospheric Gamma Imaging Cherenkov) collaboration, covering a wide energy range from radio to very-high-energy (VHE, 100 GeV < E < 100 TeV) gamma rays was organised to study its evolution. These observations resulted in a consistent detection of the source over six consecutive nights in the VHE gamma-ray domain. Combining these nightly observations with an extensive set of multiwavelength data made modelling of the blazar's spectral energy distribution (SED) possible during the flare. This modelling was performed with a focus on two plausible emission mechanisms: i) a leptonic two-zone synchrotron-self-Compton scenario, and ii) a lepto-hadronic one-zone scenario. Both models effectively replicated the observed SED from radio to the VHE gamma-ray band. Furthermore, by introducing a set of evolving parameters, both models were successful in reproducing the evolution of the fluxes measured in different bands throughout the observing campaign. Notably, the lepto-hadronic model predicts enhanced photon and neutrino fluxes at ultra-high energies (E > 100 TeV). While the photon component, generated via decay of neutral pions, is not directly observable as it is subject to intense pair production (and therefore extinction) through interactions with the cosmic microwave background photons, neutrino detectors (e.g. IceCube) can probe the predicted neutrino component. Finally, the analysis of the gamma-ray spectra, as observed by MAGIC and the Fermi-LAT telescopes, yielded a conservative 95\% confidence upper limit of z \leq 0.244 for the redshift of this blazar. △ Less

Submitted 20 December, 2024; originally announced December 2024.

Comments: Accepted for publication in A&A

Journal ref: A&A 694, A308 (2025)

Multi-wavelength study of OT 081: broadband modelling of a transitional blazar

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, I. Batković, J. Baxter, E. Bernardini, M. Bernardos, J. Bernete, A. Berti, C. Bigongiari, A. Biland, O. Blanch , et al. (250 additional authors not shown)

Abstract: OT 081 is a well-known, luminous blazar that is remarkably variable in many energy bands. We present the first broadband study of the source which includes very-high-energy (VHE, $E>$100\,GeV) $γ$-ray data taken by the MAGIC and H.E.S.S. imaging Cherenkov telescopes. The discovery of VHE $γ$-ray emission happened during a high state of $γ$-ray activity in July 2016, observed by many instruments fr… ▽ More OT 081 is a well-known, luminous blazar that is remarkably variable in many energy bands. We present the first broadband study of the source which includes very-high-energy (VHE, $E>$100\,GeV) $γ$-ray data taken by the MAGIC and H.E.S.S. imaging Cherenkov telescopes. The discovery of VHE $γ$-ray emission happened during a high state of $γ$-ray activity in July 2016, observed by many instruments from radio to VHE $γ$-rays. We identify four states of activity of the source, one of which includes VHE $γ$-ray emission. Variability in the VHE domain is found on daily timescales. The intrinsic VHE spectrum can be described by a power-law with index $3.27\pm0.44_{\rm stat}\pm0.15_{\rm sys}$ (MAGIC) and $3.39\pm0.58_{\rm stat}\pm0.64_{\rm sys}$ (H.E.S.S.) in the energy range of 55--300\,GeV and 120--500\,GeV, respectively. The broadband emission cannot be sucessfully reproduced by a simple one-zone synchrotron self-Compton model. Instead, an additional external Compton component is required. We test a lepto-hadronic model that reproduces the dataset well and a proton-synchrotron dominated model that requires an extreme proton luminosity. Emission models that are able to successfully represent the data place the emitting region well outside of the Broad Line Region (BLR) to a location at which the radiative environment is dominated by the infrared thermal radiation field of the dusty torus. In the scenario described by this flaring activity, the source appears to be an FSRQ, in contrast with past categorizations. This suggests that the source can be considered to be a transitional blazar, intermediate between BL~Lac and FSRQ objects. △ Less

Submitted 12 November, 2024; v1 submitted 29 October, 2024; originally announced October 2024.

Comments: Accepted on MNRAS Corresponding authors: M. Manganaro, J. Becerra González, M. Seglar-Arroyo, D. A. Sanchez

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

Hidden by a star: the redshift and the offset broad line of the Flat Spectrum Radio Quasar PKS 0903-57

Authors: P. Goldoni, C. Boisson, S. Pita, F. D'Ammando, E. Kasai, W. Max-Moerbeck, M. Backes, G. Cotter

Abstract: Context: PKS 0903-57 is a little-studied gamma-ray blazar which has recently attracted considerable interest due to the strong flaring episodes observed since 2020 in HE (100 MeV < E < 100 GeV) and VHE (100 GeV < E < 10 TeV) gamma-rays. Its nature and properties are still not well determined. In particular, it is unclear whether PKS 0903-57 is a BL Lac or a Flat Spectrum Radio Quasar (FSRQ), while… ▽ More Context: PKS 0903-57 is a little-studied gamma-ray blazar which has recently attracted considerable interest due to the strong flaring episodes observed since 2020 in HE (100 MeV < E < 100 GeV) and VHE (100 GeV < E < 10 TeV) gamma-rays. Its nature and properties are still not well determined. In particular, it is unclear whether PKS 0903-57 is a BL Lac or a Flat Spectrum Radio Quasar (FSRQ), while its redshift estimation relies on a possibly misassociated low signal-to-noise spectrum. Aim: We aim to reliably measure the redshift of the blazar and to determine its spectral type and luminosity in the optical range. Methods: We performed spectroscopy of the optical counterpart of the blazar using the South African Large Telescope (SALT) and the Very Large Telescope (VLT) and monitored it photometrically with the Rapid Eye Mount (REM) telescope. Results: We firmly measured the redshift of the blazar as z= 0.2621 +/- 0.0006 thanks to the detection of five narrow optical lines. The detection of a symmetric broad Halpha line with Full Width at Half Maximum (FWHM) of 4020 +/- 30 km/s together with a jet-dominated continuum leads us to classify it as a FSRQ. Finally, we detected with high significance a redshift offset (about 1500 km/s) between the broad line and the host. This is the first time that such an offset is unequivocally detected in a VHE blazar, possibly pointing to a very peculiar accretion configuration, a merging system, or a recoiling Black Hole. △ Less

Submitted 9 October, 2024; originally announced October 2024.

Comments: Astronomy and Astrophysics Letters, Accepted

Journal ref: A&A 691, L5 (2024)

PKS~J0805$-$0111: A Second Owens Valley Radio Observatory Blazar Showing Highly Significant Sinusoidal Radio Variability -- The Tip of the Iceberg

Authors: P. V. de la Parra, S. Kiehlmann, P. Mroz, A. C. S. Readhead, A. Synani, M. C. Begelman, R. D. Blandford, Y. Ding, F. Harrison, I. Liodakis, W. Max-Moerbeck, V. Pavlidou, R. Reeves, M. Vallisneri, M. F. Aller, M. J. Graham, T. Hovatta, C. R. Lawrence, T. J. W. Lazio, A. A. Mahabal, B. Molina, S. O'Neill, T. J. Pearson, V. Ravi, K. Tassis , et al. (1 additional authors not shown)

Abstract: Owens Valley Radio Observatory (OVRO) observations of supermassive black hole binary (SMBHB) candidate PKS~2131$-$021 revealed, for the first time, six likely characteristics of the phenomenology exhibited by SMBHB in blazars, of which the most unexpected and critical is sinusoidal flux density variations. We have now identified a second blazar, PKS~J0805$-$0111, showing significant sinusoidal var… ▽ More Owens Valley Radio Observatory (OVRO) observations of supermassive black hole binary (SMBHB) candidate PKS~2131$-$021 revealed, for the first time, six likely characteristics of the phenomenology exhibited by SMBHB in blazars, of which the most unexpected and critical is sinusoidal flux density variations. We have now identified a second blazar, PKS~J0805$-$0111, showing significant sinusoidal variations, with an observed period that translates to $1.422 \pm 0.005$ yr in the rest frame of the $z = 1.388$ object. We generate $10^6$ simulated light curves to reproduce the radio variability characteristics of PKS~J0805$-$0111, and show that the global probability, considering the \textit{look-elsewhere effect}, indicates that the observed periodicity can be attributed to the red noise tail of the power spectral density, with a $p_0$ value of $7.8 \times 10^{-5}$ (i.e. 3.78$σ$). PKS J0805$-$0111 displays all six characteristics observed in PKS 2131$-$021. Taking into account the well-defined OVRO sample size, the false positive probability $\sim 0.22$, but the rare behavior makes this a strong SMBHB candidate. The discovery of a second SMBHB candidate exhibiting these rare characteristics reveals that PKS~2131$-$021 is not a unique, isolated case. With these two strong cases we are clearly seeing only the tip of the iceberg. We estimate that the number of SMBHB candidates amongst blazars $\sim$ 1 in 100. △ Less

Submitted 5 August, 2024; originally announced August 2024.

PKS 2131-021 -- Discovery of Strong Coherent Sinusoidal Variations from Radio to Optical Frequencies: Compelling Evidence for a Blazar Supermassive Black Hole Binary

Authors: S. Kiehlmann, P. V. de la Parra, A. G. Sullivan, A. Synani, I. Liodakis, P. Mróz, S. K. Næss, A. C. S. Readhead, M. C. Begelman, R. D. Blandford, K. Chatziioannou, Y. Ding, M. J. Graham, F. Harrison, D. C. Homan, T. Hovatta, S. R. Kulkarni, M. L. Lister, R. Maiolino, W. Max-Moerbeck, B. Molina, C. P. O'Dea, V. Pavlidou, T. J. Pearson, M. F. Aller , et al. (9 additional authors not shown)

Abstract: Haystack and Owens Valley Radio Observatory (OVRO) observations recently revealed strong, intermittent, sinusoidal total flux-density variations that maintained coherence between 1975 and 2021 in the blazar PKS 2131$-$021 ($z=1.283$). This was interpreted as possible evidence of a supermassive black hole binary (SMBHB). Extended observations through 2023 show coherence over 47.9 years, with an obs… ▽ More Haystack and Owens Valley Radio Observatory (OVRO) observations recently revealed strong, intermittent, sinusoidal total flux-density variations that maintained coherence between 1975 and 2021 in the blazar PKS 2131$-$021 ($z=1.283$). This was interpreted as possible evidence of a supermassive black hole binary (SMBHB). Extended observations through 2023 show coherence over 47.9 years, with an observed period $P_\textrm{15 GHz}=(1739.8 \pm 17.4)$ days}. We reject, with $p$-value = $2.09 \times 10^{-7}$, the hypothesis that the variations are due to random fluctuations in the red noise tail of the power spectral density. There is clearly a physical phenomenon in PKS 2131$-$021 producing coherent sinusoidal flux density variations. We find the coherent sinusoidal intensity variations extend from below 2.7 GHz to optical frequencies, from which we derive an observed period $P_\textrm{optical}=(1764 \pm 36)$ days. Across this broad frequency range there is a smoothly-varying monotonic phase shift in the sinusoidal variations with frequency. Hints of periodic variations are also observed at $γ$-ray energies. The importance of well-vetted SMBHB candidates to searches for gravitational waves is pointed out. We estimate the fraction of blazars that are SMBHB candidates to be > 1 in 100. Thus monitoring programs covering tens of thousands of blazars could discover hundreds of SMBHB candidates. △ Less

Submitted 11 April, 2025; v1 submitted 12 July, 2024; originally announced July 2024.

Comments: 30 pages, 20 figures, 5 tables. Accepted for publication in ApJ

Association of the IceCube neutrinos with blazars in the CGRaBS sample

Authors: Pouya M. Kouch, Elina Lindfors, Talvikki Hovatta, Ioannis Liodakis, Karri I. I. Koljonen, Kari Nilsson, Sebastian Kiehlmann, Walter Max-Moerbeck, Anthony C. S. Readhead, Rodrigo A. Reeves, Timothy J. Pearson, Jenni Jormanainen, Vandad Fallah Ramazani, Matthew J. Graham

Abstract: The origin of high-energy (HE) astrophysical neutrinos has remained an elusive hot topic in the field of HE astrophysics for the past decade. Apart from a handful of individual associations, the vast majority of HE neutrinos arise from unknown sources. While there are theoretically-motivated candidate populations, such as blazars -- a subclass of AGN with jets pointed towards our line-of-sight --… ▽ More The origin of high-energy (HE) astrophysical neutrinos has remained an elusive hot topic in the field of HE astrophysics for the past decade. Apart from a handful of individual associations, the vast majority of HE neutrinos arise from unknown sources. While there are theoretically-motivated candidate populations, such as blazars -- a subclass of AGN with jets pointed towards our line-of-sight -- they have not yet been convincingly linked to HE neutrino production. Here, we perform a spatio-temporal association analysis between a sample of blazars (from CGRaBS catalog) in the radio and optical bands and the most up-to-date IceCube HE neutrino catalog. We find that if the IceCube error regions are enlarged by 1$^\circ$ in quadrature, to account for unknown systematic errors at maximal level, a spatio-temporal correlation between the multiwavelength light curves of the CGRaBS blazars and the IceCube HE neutrinos is hinted at least at a 2.17$σ$ significance level. On the other hand, when the IceCube error regions are taken as their published values, we do not find any significant correlations. A discrepancy in the blazar-neutrino correlation strengths, when using such minimal and enlarged error region scenarios, was also obtained in a recent study by the IceCube collaboration. In our study, this difference arises because several flaring blazars -- coinciding with a neutrino arrival time -- happen to narrowly miss the published 90\%-likelihood error region of the nearest neutrino event. For all of the associations driving our most significant correlations, the flaring blazar is much less than 1$^\circ$ away from the published error regions. Therefore, our results indicate that the question of the blazar-neutrino connection is highly sensitive to the reconstruction of the neutrino error regions, whose reliability is expected to improve with the next generation of neutrino observatories. △ Less

Submitted 9 July, 2024; originally announced July 2024.

Comments: Accepted for publication in A&A, 24 pages, 10 figures, 2 tables, 3 electronic tables

Journal ref: A&A 690, A111 (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

Insights into the broad-band emission of the TeV blazar Mrk 501 during the first X-ray polarization measurements

Authors: S. Abe, J. Abhir, V. A. Acciari, A. Aguasca-Cabot, I. Agudo, T. Aniello, S. Ansoldi, L. A. Antonelli, A. Arbet Engels, C. Arcaro, K. Asano, A. Babić, A. Baquero, U. Barres de Almeida, J. A. Barrio, I. Batković, A. Bautista, J. Baxter, J. Becerra González, W. Bednarek, E. Bernardini, M. Bernardos, J. Bernete, A. Berti, J. Besenrieder , et al. (239 additional authors not shown)

Abstract: We present the first multi-wavelength study of Mrk 501 including very-high-energy (VHE) gamma-ray observations simultaneous to X-ray polarization measurements from the Imaging X-ray Polarimetry Explorer (IXPE). We use radio-to-VHE data from a multi-wavelength campaign organized between 2022-03-01 and 2022-07-19. The observations were performed by MAGIC, Fermi-LAT, NuSTAR, Swift (XRT and UVOT), and… ▽ More We present the first multi-wavelength study of Mrk 501 including very-high-energy (VHE) gamma-ray observations simultaneous to X-ray polarization measurements from the Imaging X-ray Polarimetry Explorer (IXPE). We use radio-to-VHE data from a multi-wavelength campaign organized between 2022-03-01 and 2022-07-19. The observations were performed by MAGIC, Fermi-LAT, NuSTAR, Swift (XRT and UVOT), and several instruments covering the optical and radio bands. During the IXPE pointings, the VHE state is close to the average behavior with a 0.2-1 TeV flux of 20%-50% the emission of the Crab Nebula. Despite the average VHE activity, an extreme X-ray behavior is measured for the first two IXPE pointings in March 2022 with a synchrotron peak frequency >1 keV. For the third IXPE pointing in July 2022, the synchrotron peak shifts towards lower energies and the optical/X-ray polarization degrees drop. The X-ray polarization is systematically higher than at lower energies, suggesting an energy-stratification of the jet. While during the IXPE epochs the polarization angle in the X-ray, optical and radio bands align well, we find a clear discrepancy in the optical and radio polarization angles in the middle of the campaign. We model the broad-band spectra simultaneous to the IXPE pointings assuming a compact zone dominating in the X-rays and VHE, and an extended zone stretching further downstream the jet dominating the emission at lower energies. NuSTAR data allow us to precisely constrain the synchrotron peak and therefore the underlying electron distribution. The change between the different states observed in the three IXPE pointings can be explained by a change of magnetization and/or emission region size, which directly connects the shift of the synchrotron peak to lower energies with the drop in polarization degree. △ Less

Submitted 1 September, 2025; v1 submitted 16 January, 2024; originally announced January 2024.

Comments: A&A, 685, A117 (2024). 19 pages, 9 figures. Corresponding authors: Lea Heckmann, Axel Arbet Engels, David Paneque

Optical spectroscopy of blazars for the Cherenkov Telescope Array -- III

Authors: F. D'Ammando, P. Goldoni, W. Max-Moerbeck, J. Becerra Gonzalez, E. Kasai, D. A. Williams, N. Alvarez-Crespo, M. Backes, U. Barres de Almeida, C. Boisson, G. Cotter, V. Fallah Ramazani, O. Hervet, E. Lindfors, D. Mukhi-Nilo, S. Pita, M. Splettstoesser, B. van Soelen

Abstract: Due to their almost featureless optical/UV spectra, it is challenging to measure the redshifts of BL Lacs. As a result, about 50% of gamma-ray BL Lacs lack a firm measurement of this property, which is fundamental for population studies, indirect estimates of the EBL, and fundamental physics probes. This paper is the third in a series of papers aimed at determining the redshift of a sample of blaz… ▽ More Due to their almost featureless optical/UV spectra, it is challenging to measure the redshifts of BL Lacs. As a result, about 50% of gamma-ray BL Lacs lack a firm measurement of this property, which is fundamental for population studies, indirect estimates of the EBL, and fundamental physics probes. This paper is the third in a series of papers aimed at determining the redshift of a sample of blazars selected as prime targets for future observations with the next generation, ground-based VHE gamma-ray astronomy observatory, Cherenkov Telescope Array Observatory (CTAO). The accurate determination of the redshift of these objects is an important aid in source selection and planning of future CTAO observations. The selected targets were expected to be detectable with CTAO in observations of 30 hours or less. We performed deep spectroscopic observations of 41 of these blazars using the Keck II, Lick, SALT, GTC, and ESO/VLT telescopes. We carefully searched for spectral lines in the spectra and whenever features of the host galaxy were detected, we attempted to model the properties of the host galaxy. The magnitudes of the targets at the time of the observations were also compared to their long-term light curves. Spectra from 24 objects display spectral features or a high S/N. From these, 12 spectroscopic redshifts were determined, ranging from 0.2223 to 0.7018. Furthermore, 1 tentative redshift (0.6622) and 2 redshift lower limits at z > 0.6185 and z > 0.6347 were obtained. The other 9 BL Lacs showed featureless spectra, despite the high S/N (> 100) observations. Our comparisons with long-term optical light curves tentatively suggest that redshift measurements are more straightforward during an optical low state of the AGN. Overall, we have determined 37 redshifts and 6 spectroscopic lower limits as part of our programme thus far. △ Less

Submitted 15 January, 2024; originally announced January 2024.

Comments: Accepted for publication in Astronomy and Astrophysics. 17 pages, 4 Figures, 10 Tables

Investigation of the correlation between optical and $γ$-ray flux variation in the blazar Ton 599

Authors: Bhoomika Rajput, Amit Kumar Mandal, Ashwani Pandey, C. S. Stalin, Walter Max-Moerbeck, Blesson Mathew

Abstract: The correlation between optical and $γ$-ray flux variations in blazars reveals a complex behaviour. In this study, we present our analysis of the connection between changes in optical and $γ$-ray emissions in the blazar Ton 599 over a span of approximately 15 years, from August 2008 to March 2023. Ton 599 reached its highest flux state across the entire electromagnetic spectrum during the second w… ▽ More The correlation between optical and $γ$-ray flux variations in blazars reveals a complex behaviour. In this study, we present our analysis of the connection between changes in optical and $γ$-ray emissions in the blazar Ton 599 over a span of approximately 15 years, from August 2008 to March 2023. Ton 599 reached its highest flux state across the entire electromagnetic spectrum during the second week of January 2023. To investigate the connection between changes in optical and $γ$-ray flux, we have designated five specific time periods, labeled as epochs A, B, C, D, and E. During periods B, C, D, and E, the source exhibited optical flares, while it was in its quiescent state during period A. The $γ$-ray counterparts to these optical flares are present during periods B, C, and E, however during period D, the $γ$-ray counterpart is either weak or absent. We conducted a broadband spectral energy distribution (SED) fitting by employing a one-zone leptonic emission model for these epochs. The SED analysis unveiled that the optical-UV emission primarily emanated from the accretion disk in quiescent period A, whereas synchrotron radiation from the jet dominated during periods B, C, D, and E. Diverse correlated patterns in the variations of optical and $γ$-ray emissions, like correlated optical and $γ$-ray flares, could be accounted for by changes in factors such as the magnetic field, bulk Lorentz factor, and electron density. On the other hand, an orphan optical flare could result from increased magnetic field and bulk Lorentz factor. △ Less

Submitted 22 December, 2023; originally announced December 2023.

Comments: 16 pages, 8 figures, Accepted for publishing in the MNRAS

Multi-wavelength observations of the lensed quasar PKS 1830$-$211 during the 2019 $γ$-ray flare

Authors: S. Vercellone, I. Donnarumma, C. Pittori, F. Capitanio, A. De Rosa, L. Di Gesu, S. Kiehlmann, M. N. Iacolina, P. A. Pellizzoni, E. Egron, L. Pacciani, G. Piano, S. Puccetti, S. Righini, G. Valente, F. Verrecchia, V. Vittorini, M. Tavani, E. Brocato, A. W. Chen, T. Hovatta, A. Melis, W. Max-Moerbeck, D. Perrodin, M. Pilia , et al. (10 additional authors not shown)

Abstract: PKS 1830$-$211 is a $γ$-ray emitting, high-redshift (z $= 2.507 \pm 0.002$), lensed flat-spectrum radio quasar. During the period mid-February to mid-April 2019, this source underwent a series of strong $γ$-ray flares that were detected by both AGILE-GRID and Fermi-LAT, reaching a maximum $γ$-ray flux of $F_{\rm E>100 MeV}\approx 2.3\times10^{-5}$ ph cm$^{-2}$ s$^{-1}$. Here we report on a coordin… ▽ More PKS 1830$-$211 is a $γ$-ray emitting, high-redshift (z $= 2.507 \pm 0.002$), lensed flat-spectrum radio quasar. During the period mid-February to mid-April 2019, this source underwent a series of strong $γ$-ray flares that were detected by both AGILE-GRID and Fermi-LAT, reaching a maximum $γ$-ray flux of $F_{\rm E>100 MeV}\approx 2.3\times10^{-5}$ ph cm$^{-2}$ s$^{-1}$. Here we report on a coordinated campaign from both on-ground (Medicina, OVRO, REM, SRT) and orbiting facilities (AGILE, Fermi, INTEGRAL, NuSTAR, Swift, Chandra), with the aim of investigating the multi-wavelength properties of PKS 1830$-$211 through nearly simultaneous observations presented here for the first time. We find a possible break in the radio spectra in different epochs above 15 GHz, and a clear maximum of the 15 GHz data approximately 110 days after the $γ$-ray main activity periods. The spectral energy distribution shows a very pronounced Compton dominance (> 200) which challenges the canonical one-component emission model. Therefore we propose that the cooled electrons of the first component are re-accelerated to a second component by, e.g., kink or tearing instability during the $γ$-ray flaring periods. We also note that PKS 1830$-$211 could be a promising candidate for future observations with both Compton satellites (e.g., e-ASTROGAM) and Cherenkov arrays (CTAO) which will help, thanks to their improved sensitivity, in extending the data availability in energy bands currently uncovered. △ Less

Submitted 13 November, 2023; originally announced November 2023.

Comments: Accepted for publication in MNRAS. 16 pages, 18 figures, 12 tables

Chasing Gravitational Waves with the Cherenkov Telescope Array

Authors: Jarred Gershon Green, Alessandro Carosi, Lara Nava, Barbara Patricelli, Fabian Schüssler, Monica Seglar-Arroyo, Cta Consortium, :, Kazuki Abe, Shotaro Abe, Atreya Acharyya, Remi Adam, Arnau Aguasca-Cabot, Ivan Agudo, Jorge Alfaro, Nuria Alvarez-Crespo, Rafael Alves Batista, Jean-Philippe Amans, Elena Amato, Filippo Ambrosino, Ekrem Oguzhan Angüner, Lucio Angelo Antonelli, Carla Aramo, Cornelia Arcaro, Luisa Arrabito , et al. (545 additional authors not shown)

Abstract: The detection of gravitational waves from a binary neutron star merger by Advanced LIGO and Advanced Virgo (GW170817), along with the discovery of the electromagnetic counterparts of this gravitational wave event, ushered in a new era of multimessenger astronomy, providing the first direct evidence that BNS mergers are progenitors of short gamma-ray bursts (GRBs). Such events may also produce very… ▽ More The detection of gravitational waves from a binary neutron star merger by Advanced LIGO and Advanced Virgo (GW170817), along with the discovery of the electromagnetic counterparts of this gravitational wave event, ushered in a new era of multimessenger astronomy, providing the first direct evidence that BNS mergers are progenitors of short gamma-ray bursts (GRBs). Such events may also produce very-high-energy (VHE, > 100GeV) photons which have yet to be detected in coincidence with a gravitational wave signal. The Cherenkov Telescope Array (CTA) is a next-generation VHE observatory which aims to be indispensable in this search, with an unparalleled sensitivity and ability to slew anywhere on the sky within a few tens of seconds. New observing modes and follow-up strategies are being developed for CTA to rapidly cover localization areas of gravitational wave events that are typically larger than the CTA field of view. This work will evaluate and provide estimations on the expected number of of gravitational wave events that will be observable with CTA, considering both on- and off-axis emission. In addition, we will present and discuss the prospects of potential follow-up strategies with CTA. △ Less

Submitted 5 February, 2024; v1 submitted 11 October, 2023; originally announced October 2023.

Comments: Presented at the 38th International Cosmic Ray Conference (ICRC 2023), 2023 (arXiv:2309.08219)

Report number: CTA-ICRC/2023/30

Variability studies of active galactic nuclei from the long-term monitoring program with the Cherenkov Telescope Array

Authors: Guillaume Grolleron, Josefa Becerra González, Jonathan Biteau, Matteo Cerruti, Roger Grau, Lucas Gréaux, Talvikki Hovatta, Jean-Philippe Lenain, Elina Lindfors, Walter Max-Moerbeck, Davide Miceli, Abelardo Moralejo, Kari Nilsson, Elisa Pueschel, Arkadipta Sarkar, Sofia Suutarinen

Abstract: Blazars are active galactic nuclei (AGN) with a relativistic jet oriented toward the observer. This jet is composed of accelerated particles which can display emission over the entire electromagnetic spectrum. Spectral variability has been observed on short- and long-time scales in AGN, with a power spectral density (PSD) that can show a break at frequencies below the well-known red-noise process.… ▽ More Blazars are active galactic nuclei (AGN) with a relativistic jet oriented toward the observer. This jet is composed of accelerated particles which can display emission over the entire electromagnetic spectrum. Spectral variability has been observed on short- and long-time scales in AGN, with a power spectral density (PSD) that can show a break at frequencies below the well-known red-noise process. This break frequency in the PSD has been observed in X-rays to scale with the accretion regime and the mass of the central black hole. It is expected that a break could also be seen in the very-high-energy gamma rays, but constraining the shape of the PSD in these wavelengths has not been possible with the current instruments. The Cherenkov Telescope Array (CTA) will be more sensitive by a factor of five to ten depending on energy than the current generation of imaging atmospheric Cherenkov telescopes, therefore it will be possible with CTA to reconstruct the PSD with a high accuracy, bringing new information about AGN variability. In this work, we focus on the AGN long-term monitoring program planned with CTA. The program is proposed to begin with early-start observing campaigns with CTA precursors. This would allow us to probe longer time scales on the AGN PSD. △ Less

Submitted 21 September, 2023; originally announced September 2023.

Comments: Presented at the 38th International Cosmic Ray Conference (ICRC 2023), 2023 (arXiv:submit/5125530)

Report number: CTA-ICRC/2023/19

Milliarcsecond Core Size Dependence of the Radio Variability of Blazars

Authors: Po Chih Hsu, Jun Yi Koay, Satoki Matsushita, Chorng-Yuan Hwang, Talvikki Hovatta, Sebastian Kiehlmann, Anthony Readhead, Walter Max-Moerbeck, Rodrigo Reeves

Abstract: Studying the long-term radio variability (timescales of months to years) of blazars enables us to gain a better understanding of the physical structure of these objects on sub-parsec scales, and the physics of super massive black holes. In this study, we focus on the radio variability of 1157 blazars observed at 15 GHz through the Owens Valley Radio Observatory (OVRO) Blazar Monitoring Program. We… ▽ More Studying the long-term radio variability (timescales of months to years) of blazars enables us to gain a better understanding of the physical structure of these objects on sub-parsec scales, and the physics of super massive black holes. In this study, we focus on the radio variability of 1157 blazars observed at 15 GHz through the Owens Valley Radio Observatory (OVRO) Blazar Monitoring Program. We investigate the dependence of the variability amplitudes and timescales, characterized based on model fitting to the structure functions, on the milliarcsecond core sizes measured by Very Long Baseline Interferometry. We find that the most compact sources at milliarcsecond scales exhibit larger variability amplitudes and shorter variability timescales than more extended sources. Additionally, for sources with measured redshifts and Doppler boosting factors, the correlation between linear core sizes against variability amplitudes and intrinsic timescales are also significant. The observed relationship between variability timescales and core sizes is expected, based on light travel-time arguments. This variability vs core size relation extends beyond the core sizes measured at 15 GHz; we see significant correlation between the 15 GHz variability amplitudes (as well as timescales) and core sizes measured at other frequencies, which can be attributed to a frequency-source size relationship arising from the intrinsic jet structure. At low frequencies of 1 GHz where the core sizes are dominated by interstellar scattering, we find that the variability amplitudes have significant correlation with the 1 GHz intrinsic core angular sizes, once the scatter broadening effects are deconvoluted from the intrinsic core sizes. △ Less

Submitted 20 August, 2023; v1 submitted 18 August, 2023; originally announced August 2023.

Comments: 17 pages, 12 figures

VERITAS discovery of very high energy gamma-ray emission from S3 1227+25 and multiwavelength observations

Authors: Atreya Acharyya, Colin Adams, Avery Archer, Priyadarshini Bangale, Wystan Benbow, Aryeh Brill, Jodi Christiansen, Alisha Chromey, Manel Errando, Abe Falcone, Qi Feng, John Finley, Gregory Foote, Lucy Fortson, Amy Furniss, Greg Gallagher, William Hanlon, David Hanna, Olivier Hervet, Claire Hinrichs, John Hoang, Jamie Holder, Weidong Jin, Madalyn Johnson, Philip Kaaret , et al. (46 additional authors not shown)

Abstract: We report the detection of very high energy gamma-ray emission from the blazar S3 1227+25 (VER J1230+253) with the Very Energetic Radiation Imaging Telescope Array System (VERITAS). VERITAS observations of the source were triggered by the detection of a hard-spectrum GeV flare on May 15, 2015 with the Fermi-Large Area Telescope (LAT). A combined five-hour VERITAS exposure on May 16th and May 18th… ▽ More We report the detection of very high energy gamma-ray emission from the blazar S3 1227+25 (VER J1230+253) with the Very Energetic Radiation Imaging Telescope Array System (VERITAS). VERITAS observations of the source were triggered by the detection of a hard-spectrum GeV flare on May 15, 2015 with the Fermi-Large Area Telescope (LAT). A combined five-hour VERITAS exposure on May 16th and May 18th resulted in a strong 13$σ$ detection with a differential photon spectral index, $Γ$ = 3.8 $\pm$ 0.4, and a flux level at 9% of the Crab Nebula above 120 GeV. This also triggered target of opportunity observations with Swift, optical photometry, polarimetry and radio measurements, also presented in this work, in addition to the VERITAS and Fermi-LAT data. A temporal analysis of the gamma-ray flux during this period finds evidence of a shortest variability timescale of $τ_{obs}$ = 6.2 $\pm$ 0.9 hours, indicating emission from compact regions within the jet, and the combined gamma-ray spectrum shows no strong evidence of a spectral cut-off. An investigation into correlations between the multiwavelength observations found evidence of optical and gamma-ray correlations, suggesting a single-zone model of emission. Finally, the multiwavelength spectral energy distribution is well described by a simple one-zone leptonic synchrotron self-Compton radiation model. △ Less

Submitted 4 May, 2023; originally announced May 2023.

Comments: 18 pages, 6 figures. Accepted for publication in the Astrophysical Journal (ApJ)

Probing AGN variability with the Cherenkov Telescope Array

Authors: F. Cangemi, T. Hovatta, E. Lindfors, M. Cerruti, J. Becerra-Gonzalez, J. Biteau, C. Boisson, M. Böttcher, E. de Gouveia Dal Pino, D. Dorner, G. Grolleron, J. -P. Lenain, M. Manganaro, W. Max-Moerbeck, P. Morris, K. Nilsson, L. Passos Reis, P. Romano, O. Sergijenko, F. Tavecchio, S. Vercellone, S. Wagner, M. Zacharias

Abstract: Relativistic jets launched by Active Galactic Nuclei are among the most powerful particle accelerators in the Universe. The emission over the entire electromagnetic spectrum of these relativistic jets can be extremely variable with scales of variability from less than few minutes up to several years. These variability patterns, which can be very complex, contain information about the acceleration… ▽ More Relativistic jets launched by Active Galactic Nuclei are among the most powerful particle accelerators in the Universe. The emission over the entire electromagnetic spectrum of these relativistic jets can be extremely variable with scales of variability from less than few minutes up to several years. These variability patterns, which can be very complex, contain information about the acceleration processes of the particles and the area(s) of emission. Thanks to its sensitivity, five-to twenty-times better than the current generation of Imaging Atmospheric Cherenkov Telescopes depending on energy, the Cherenkov Telescope Array will be able to follow the emission from these objects with a very accurate time sampling and over a wide spectral coverage from 20 GeV to > 20 TeV and thus reveal the nature of the acceleration processes at work in these objects. We will show the first results of our lightcurve simulations and long-term behavior of AGN as will be observed by CTA, based on state-of-art particle acceleration models. △ Less

Submitted 27 April, 2023; originally announced April 2023.

Comments: Proceedings for Gamma 2022, 8 pages, 6 figures

Long-term multi-wavelength study of 1ES 0647+250

Authors: MAGIC Collaboration, V. A. Acciari, 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. Becerra González, W. Bednarek, E. Bernardini, M. Bernardos, A. Berti, J. Besenrieder, W. Bhattacharyya, C. Bigongiari, A. Biland, O. Blanch , et al. (195 additional authors not shown)

Abstract: The BL Lac object 1ES 0647+250 is one of the few distant $γ$-ray emitting blazars detected at very high energies (VHE, $\gtrsim$100 GeV) during a non-flaring state. It was detected with the MAGIC telescopes during its low activity in the years 2009-2011, as well as during three flaring activities in the years 2014, 2019 and 2020, with the highest VHE flux in the latter epoch. An extensive multi-in… ▽ More The BL Lac object 1ES 0647+250 is one of the few distant $γ$-ray emitting blazars detected at very high energies (VHE, $\gtrsim$100 GeV) during a non-flaring state. It was detected with the MAGIC telescopes during its low activity in the years 2009-2011, as well as during three flaring activities in the years 2014, 2019 and 2020, with the highest VHE flux in the latter epoch. An extensive multi-instrument data set was collected within several coordinated observing campaigns throughout these years. We aim to characterise the long-term multi-band flux variability of 1ES 0647+250, as well as its broadband spectral energy distribution (SED) during four distinct activity states selected in four different epochs, in order to constrain the physical parameters of the blazar emission region under certain assumptions. We evaluate the variability and correlation of the emission in the different energy bands with the fractional variability and the Z-transformed Discrete Correlation Function, as well as its spectral evolution in X-rays and $γ$ rays. Owing to the controversy in the redshift measurements of 1ES 0647+250 reported in the literature, we also estimate its distance in an indirect manner through the comparison of the GeV and TeV spectra from simultaneous observations with Fermi-LAT and MAGIC during the strongest flaring activity detected to date. Moreover, we interpret the SEDs from the four distinct activity states within the framework of one-component and two-component leptonic models, proposing specific scenarios that are able to reproduce the available multi-instrument data. △ Less

Submitted 23 November, 2022; originally announced November 2022.

Comments: 20 pages, 7 figures. Accepted in A&A. Corresponding authors: Jorge Otero-Santos; Daniel Morcuende; Vandad Fallah Ramazani; Daniela Dorner; David Paneque (mailto: contact.magic@mpp.mpg.de)

Journal ref: A&A 670, A49 (2023)

Optical Spectroscopy of Blazars for the Cherenkov Telescope Array -- II

Authors: E. Kasai, P. Goldoni, S. Pita, D. A. Williams, W. Max-Moerbeck, O. Hervet, G. Cotter, M. Backes, C. Boisson, J. Becerra González, U. Barres de Almeida, F. D'Ammando, V. Fallah Ramazani, E. Lindfors

Abstract: Active galactic nuclei (AGNs) make up about 35 per cent of the more than 250 sources detected in very-high-energy (VHE) gamma rays to date with Imaging Atmospheric Cherenkov Telescopes. Apart from four nearby radio galaxies and two AGNs of unknown type, all known VHE AGNs are blazars. Knowledge of the cosmological redshift of gamma-ray blazars is key to enabling the study of their intrinsic emissi… ▽ More Active galactic nuclei (AGNs) make up about 35 per cent of the more than 250 sources detected in very-high-energy (VHE) gamma rays to date with Imaging Atmospheric Cherenkov Telescopes. Apart from four nearby radio galaxies and two AGNs of unknown type, all known VHE AGNs are blazars. Knowledge of the cosmological redshift of gamma-ray blazars is key to enabling the study of their intrinsic emission properties, as the interaction between gamma rays and the extragalactic background light (EBL) results in a spectral softening. Therefore, the redshift determination exercise is crucial to indirectly placing tight constraints on the EBL density and to studying blazar population evolution across cosmic time. Due to the powerful relativistic jets in blazars, most of their host galaxies' spectral features are outshined, and dedicated high signal-to-noise spectroscopic observations are required. Deep medium- to high-resolution spectroscopy of 33 gamma-ray blazar optical counterparts was performed with the European Southern Observatory New Technology Telescope, Keck II telescope, Shane 3-meter telescope and the Southern African Large Telescope. From the sample, spectra from 25 objects display spectral features or are featureless and have high signal-to-noise. The other eight objects have low quality featureless spectra. We systematically searched for absorption and emission features and estimated, when possible, the fractional host galaxy flux in the measured total flux. Our measurements yielded 14 firm spectroscopic redshifts, ranging from 0.0838 to 0.8125, one tentative redshift, and two lower limits: one at z > 0.382 and the other at z > 0.629. △ Less

Submitted 9 November, 2022; originally announced November 2022.

Comments: 19 pages, 5 figures This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society (https://doi.org/10.1093/mnras/stac3167) Published by Oxford University Press on behalf of the Royal Astronomical Society

The hunt for extraterrestrial high-energy neutrino counterparts

Authors: I. Liodakis, T. Hovatta, V. Pavlidou, A. C. S. Readhead, R. D. Blandford, S. Kiehlmann, E. Lindfors, W. Max-Moerbeck, T. J. Pearson, M. Petropoulou

Abstract: The origin of Petaelectronvolt (PeV) astrophysical neutrinos is fundamental to our understanding of the high-energy Universe. Apart from the technical challenges of operating detectors deep below ice, oceans, and lakes, the phenomenological challenges are even greater than those of gravitational waves; the sources are unknown, hard to predict, and we lack clear signatures. Neutrino astronomy there… ▽ More The origin of Petaelectronvolt (PeV) astrophysical neutrinos is fundamental to our understanding of the high-energy Universe. Apart from the technical challenges of operating detectors deep below ice, oceans, and lakes, the phenomenological challenges are even greater than those of gravitational waves; the sources are unknown, hard to predict, and we lack clear signatures. Neutrino astronomy therefore represents the greatest challenge faced by the astronomy and physics communities thus far. The possible neutrino sources range from accretion disks and tidal disruption events, to relativistic jets and galaxy clusters with blazar TXS~0506+056 the most compelling association thus far. Since that association, immense effort has been put into proving or disproving that jets are indeed neutrino emitters, but to no avail. By generating simulated neutrino counterpart samples, we explore the potential of detecting a significant correlation of neutrinos with jets from active galactic nuclei. We find that, given the existing challenges, even our best experiments could not have produced a $>3σ$ result. Larger programs over the next few years will be able to detect a significant correlation only if the brightest radio sources, rather than all jetted active galactic nuclei, are neutrino emitters. We discuss the necessary strategies required to steer future efforts into successful experiments. △ Less

Submitted 15 August, 2022; originally announced August 2022.

Comments: 8 pages, 1 figure, 1 table, accepted for publication in A&A

Journal ref: A&A 666, A36 (2022)

Assigning degrees of stochasticity to blazar light curves in the radio band using complex networks

Authors: Belén Acosta-Tripailao, Walter Max-Moerbeck, Denisse Pastén, Pablo S. Moya

Abstract: {We focus on characterizing the high-energy emission mechanisms of blazars by analyzing the variability in the radio band of the light curves of more than a thousand sources. We are interested in assigning complexity parameters to these sources, modeling the time series of the light curves with the method of the Horizontal Visibility Graph (HVG), which allows us to obtain properties from degree di… ▽ More {We focus on characterizing the high-energy emission mechanisms of blazars by analyzing the variability in the radio band of the light curves of more than a thousand sources. We are interested in assigning complexity parameters to these sources, modeling the time series of the light curves with the method of the Horizontal Visibility Graph (HVG), which allows us to obtain properties from degree distributions, such as a characteristic exponent to describe its stochasticity and the Kullback-Leibler Divergence (KLD), presenting a new perspective to the methods commonly used to study Active Galactic Nuclei (AGN). We contrast these parameters with the excess variance, an astronomical measurement of variability in light curves, at the same time we use the spectral classification of the sources. While it is not possible to find significant correlations with the excess variance, the degree distributions extracted from the network are detecting differences related to the spectral classification of blazars. These differences suggest a chaotic behavior in the time series for the BL Lac sources and a correlated stochastic behavior in the time series for the FSRQ sources. Our results show that complex networks may be a valuable alternative tool to study AGNs according to the variability of their energy output. △ Less

Submitted 9 May, 2022; originally announced May 2022.

Comments: 12 pages, 8 figures, 2 tables. Accepted for publication in Entropy

Investigating the blazar TXS 0506+056 through sharp multi-wavelength eyes during 2017-2019

Authors: MAGIC Collaboration, V. A. Acciari, T. Aniello, 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, I. Batković, J. Becerra González, W. Bednarek, E. Bernardini, M. Bernardos, A. Berti, J. Besenrieder, W. Bhattacharyya, C. Bigongiari, A. Biland, O. Blanch, H. Bökenkamp , et al. (192 additional authors not shown)

Abstract: The blazar TXS 0506+056 got into the spotlight of the astrophysical community in September 2017, when a high-energy neutrino detected by IceCube (IceCube-170922A) was associated at the 3 $σ$ level to a $γ$-ray flare from this source. This multi-messenger photon-neutrino association remains, as per today, the most significant one ever observed. TXS 0506+056 was a poorly studied object before the Ic… ▽ More The blazar TXS 0506+056 got into the spotlight of the astrophysical community in September 2017, when a high-energy neutrino detected by IceCube (IceCube-170922A) was associated at the 3 $σ$ level to a $γ$-ray flare from this source. This multi-messenger photon-neutrino association remains, as per today, the most significant one ever observed. TXS 0506+056 was a poorly studied object before the IceCube-170922A event. To better characterize its broad-band emission, we organized a multi-wavelength campaign lasting 16 months (November 2017 to February 2019), covering the radio-band (Metsähovi, OVRO), the optical/UV (ASAS-SN, KVA, REM, Swift/UVOT), the X-rays (Swift/XRT, NuSTAR), the high-energy $γ$ rays (Fermi/LAT) and the very-high-energy (VHE) $γ$ rays (MAGIC). In $γ$ rays, the behaviour of the source was significantly different from the 2017 one: MAGIC observations show the presence of flaring activity during December 2018, while the source only shows an excess at the 4$σ$ level during the rest of the campaign (74 hours of accumulated exposure); Fermi/LAT observations show several short (days-to-week timescale) flares, different from the long-term brightening of 2017. No significant flares are detected at lower energies. The radio light curve shows an increasing flux trend, not seen in other wavelengths. We model the multi-wavelength spectral energy distributions in a lepto-hadronic scenario, in which the hadronic emission emerges as Bethe-Heitler and pion-decay cascade in the X-rays and VHE $γ$ rays. According to the model presented here, the December 2018 $γ$-ray flare was connected to a neutrino emission that was too brief and not bright enough to be detected by current neutrino instruments. △ Less

Submitted 1 May, 2022; v1 submitted 5 February, 2022; originally announced February 2022.

Comments: 18 pages, 6 figures; V2 to match published version, typo fixed

New Tests of Millilensing in the Blazar PKS 1413+135

Authors: A. L. Peirson, I. Liodakis, A. C. S. Readhead, M. L. Lister, E. S. Perlman, M. F. Aller, R. D. Blandford, K. J. B. Grainge, D. A. Green, M. A. Gurwell, M. W. Hodges, T. Hovatta, S. Kiehlmann, A. Lähteenmäki, W. Max-Moerbeck, T. Mcaloone, S. O'Neill, V. Pavlidou, T. J. Pearson, V. Ravi, R. A. Reeves, P. F. Scott, G. B. Taylor, D. J. Titterington, M. Tornikoski , et al. (4 additional authors not shown)

Abstract: Symmetric Achromatic Variability (SAV) is a rare form of radio variability in blazars that has been attributed to gravitational millilensing by a ~$10^2 - 10^5$ $M_\odot$ mass condensate. Four SAVs have been identified between 1980 and 2020 in the long-term radio monitoring data of the blazar PKS 1413+135. We show that all four can be fitted with the same, unchanging, gravitational lens model. If… ▽ More Symmetric Achromatic Variability (SAV) is a rare form of radio variability in blazars that has been attributed to gravitational millilensing by a ~$10^2 - 10^5$ $M_\odot$ mass condensate. Four SAVs have been identified between 1980 and 2020 in the long-term radio monitoring data of the blazar PKS 1413+135. We show that all four can be fitted with the same, unchanging, gravitational lens model. If SAV is due to gravitational millilensing, PKS 1413+135 provides a unique system for studying active galactic nuclei with unprecedented microarcsecond resolution, as well as for studying the nature of the millilens itself. We discuss two possible candidates for the putative millilens: a giant molecular cloud hosted in the intervening edge-on spiral galaxy, and an undetected dwarf galaxy with a massive black hole. We find a significant dependence of SAV crossing time on frequency, which could indicate a fast shock moving in a slower underlying flow. We also find tentative evidence for a 989-day periodicity in the SAVs, which, if real, makes possible the prediction of future SAVs: the next three windows for possible SAVs begin in August 2022, May 2025, and February 2028. △ Less

Submitted 8 January, 2022; v1 submitted 4 January, 2022; originally announced January 2022.

Comments: Accepted to ApJ

Multiwavelength variability power spectrum analysis of the blazars 3C 279 and PKS 1510-089 on multiple timescales

Authors: Arti Goyal, Marian Soida, Lukasz Stawarz, Paul J. Wiita, Kari Nilsson, Svetlana Jorstad, Alan P. Marscher, Margo F. Aller, Hugh D. Aller, Anne Lahteenmaki, Talvikki Hovatta, Staszek Zola, Krzysztof Nalewajko, Merja Tornikoski, Joni Tammi, Mark Hodges, Sebastian Kiehlmann, Anthony C. S. Readhead, Walter Max-Moerbeck, Elina Lindfors, Vandad Fallah Ramazani, D. E. Reichart, D. B. Caton, Janeth Valverde, Deirdre Horan , et al. (2 additional authors not shown)

Abstract: We present the results of variability power spectral density (PSD) analysis using multiwavelength radio to GeV\,$γ$-ray light curves covering decades/years to days/minutes timescales for the blazars 3C 279 and PKS 1510-089. The PSDs are modeled as single power-laws, and the best-fit spectral shape is derived using the `power spectral response' method. With more than ten years of data obtained with… ▽ More We present the results of variability power spectral density (PSD) analysis using multiwavelength radio to GeV\,$γ$-ray light curves covering decades/years to days/minutes timescales for the blazars 3C 279 and PKS 1510-089. The PSDs are modeled as single power-laws, and the best-fit spectral shape is derived using the `power spectral response' method. With more than ten years of data obtained with weekly/daily sampling intervals, most of the PSDs cover ~2-4 decades in temporal frequency; moreover, in the optical band, the PSDs cover ~6 decades for 3C 279 due to the availability of intranight light curves. Our main results are the following: (1) on timescales ranging from decades to days, the synchrotron and the inverse Compton spectral components, in general, exhibit red-noise (slope ~2) and flicker-noise (slope ~1) type variability, respectively; (2) the slopes of $γ$-ray variability PSDs obtained using a 3-hr integration bin and a 3-weeks total duration exhibit a range between ~1.4 and ~2.0 (mean slope = 1.60$\pm$0.70), consistent within errors with the slope on longer timescales; (3) comparisons of fractional variability indicate more power on timescales $\leq$100\, days at $γ$-ray frequencies as compared to longer wavelengths, in general (except between $γ$-ray and optical frequencies for PKS 1510$-$089); (4) the normalization of intranight optical PSDs for 3C\,279 appears to be a simple extrapolation from longer timescales, indicating a continuous (single) process driving the variability at optical wavelengths; (5) the emission at optical/infrared wavelengths may involve a combination of disk and jet processes for PKS\,1510$-$089. △ Less

Submitted 20 January, 2022; v1 submitted 8 December, 2021; originally announced December 2021.

Comments: ApJ accepted. Comments are welcome

Multiwavelength study of the gravitationally lensed blazar QSO B0218+357 between 2016 and 2020

Authors: MAGIC Collaboration, 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, I. Batković, 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 , et al. (186 additional authors not shown)

Abstract: We report multiwavelength observations of the gravitationally lensed blazar QSO B0218+357 in 2016-2020. Optical, X-ray and GeV flares were detected. The contemporaneous MAGIC observations do not show significant very-high-energy (VHE, >= 100 GeV) gamma-ray emission. The lack of enhancement in radio emission measured by OVRO indicates the multi-zone nature of the emission from this object. We const… ▽ More We report multiwavelength observations of the gravitationally lensed blazar QSO B0218+357 in 2016-2020. Optical, X-ray and GeV flares were detected. The contemporaneous MAGIC observations do not show significant very-high-energy (VHE, >= 100 GeV) gamma-ray emission. The lack of enhancement in radio emission measured by OVRO indicates the multi-zone nature of the emission from this object. We constrain the VHE duty cycle of the source to be < 16 2014-like flares per year (95% confidence). For the first time for this source, a broadband low-state SED is constructed with a deep exposure up to the VHE range. A flux upper limit on the low-state VHE gamma-ray emission of an order of magnitude below that of the 2014 flare is determined. The X-ray data are used to fit the column density of (8.10 +- 0.93 stat ) x 10^21 cm^-2 of the dust in the lensing galaxy. VLBI observations show a clear radio core and jet components in both lensed images, yet no significant movement of the components is seen. The radio measurements are used to model the source-lens-observer geometry and determine the magnifications and time delays for both components. The quiescent emission is modeled with the high-energy bump explained as a combination of synchrotron-self-Compton and external Compton emission from a region located outside of the broad line region. The bulk of the low-energy emission is explained as originating from a tens-of-parsecs scale jet. △ Less

Submitted 25 November, 2021; originally announced November 2021.

Comments: 21 pages, 12 figures, 5 tables, accepted for publication in MNRAS

The Unanticipated Phenomenology of the Blazar PKS~2131$-$021: A Unique Super-Massive Black Hole Binary Candidate

Authors: S. O'Neill, S. Kiehlmann, A. C. S. Readhead, M. F. Aller, R. D. Blandford, I. Liodakis, M. L. Lister, P. Mróz, C. P. O'Dea, T. J. Pearson, V. Ravi, M. Vallisneri, K. A. Cleary, M. J. Graham, K. J. B. Grainge, M. W. Hodges, T. Hovatta, A. Lähteenmäki, J. W. Lamb, T. J. W. Lazio, W. Max-Moerbeck, V. Pavlidou, T. A. Prince, R. A. Reeves, M. Tornikoski , et al. (2 additional authors not shown)

Abstract: Most large galaxies host supermassive black holes in their nuclei and are subject to mergers, which can produce a supermassive black hole binary (SMBHB), and hence periodic signatures due to orbital motion. We report unique periodic radio flux density variations in the blazar PKS~2131$-$021, which strongly suggest an SMBHB with an orbital separation of $\sim 0.001-0.01$ pc. Our 45.1-year radio lig… ▽ More Most large galaxies host supermassive black holes in their nuclei and are subject to mergers, which can produce a supermassive black hole binary (SMBHB), and hence periodic signatures due to orbital motion. We report unique periodic radio flux density variations in the blazar PKS~2131$-$021, which strongly suggest an SMBHB with an orbital separation of $\sim 0.001-0.01$ pc. Our 45.1-year radio light curve shows two epochs of strong sinusoidal variation with the same period and phase to within $<2\%$ and $\sim 10\%$, respectively, straddling a 20-year period when this variation was absent. Our simulated light curves accurately reproduce the ``red noise'' of this object, and Lomb-Scargle, weighted wavelet Z-transform, and least-squares sine wave analyses demonstrate conclusively, at the $4.6σ$ significance level, that the periodicity in this object is not due to random fluctuations in flux density. The observed period translates to $2.082\pm 0.003$ years in the rest frame at the $z=1.285$ redshift of PKS~2131$-$021. The periodic variation in PKS~2131$-$021 is remarkably sinusoidal. We present a model in which orbital motion, combined with the strong Doppler boosting of the approaching relativistic jet, produces a sine-wave modulation in the flux density which easily fits the observations. Given the rapidly-developing field of gravitational wave experiments with pulsar timing arrays, closer counterparts to PKS~2131$-$021 and searches using the techniques we have developed are strongly motivated. These results constitute a compelling demonstration that the phenomenology, not the theory, must provide the lead in this field. △ Less

Submitted 24 January, 2022; v1 submitted 3 November, 2021; originally announced November 2021.

Comments: 24 pages, 13 figure, 3 Tables, accepted for publication in APJL

Southern African Large Telescope Spectroscopy of BL Lacs for the CTA project

Authors: E. Kasai, P. Goldoni, M. Backes, G. Cotter, S. Pita, C. Boisson, D. A. Williams, F D'Ammando, E. Lindfors, U. Barres de Almeida, W. Max-Moerbeck, V. Navarro-Aranguiz, J. Becerra-Gonzalez, O. Hervet, J. -P. Lenain, H. Sol, S. Wagner

Abstract: In the last two decades, very-high-energy gamma-ray astronomy has reached maturity: over 200 sources have been detected, both Galactic and extragalactic, by ground-based experiments. At present, Active Galactic Nuclei (AGN) make up about 40% of the more than 200 sources detected at very high energies with ground-based telescopes, the majority of which are blazars, i.e. their jets are closely align… ▽ More In the last two decades, very-high-energy gamma-ray astronomy has reached maturity: over 200 sources have been detected, both Galactic and extragalactic, by ground-based experiments. At present, Active Galactic Nuclei (AGN) make up about 40% of the more than 200 sources detected at very high energies with ground-based telescopes, the majority of which are blazars, i.e. their jets are closely aligned with the line of sight to Earth and three quarters of which are classified as high-frequency peaked BL Lac objects. One challenge to studies of the cosmological evolution of BL Lacs is the difficulty of obtaining redshifts from their nearly featureless, continuum- dominated spectra. It is expected that a significant fraction of the AGN to be detected with the future Cherenkov Telescope Array (CTA) observatory will have no spectroscopic redshifts, compromising the reliability of BL Lac population studies, particularly of their cosmic evolution. We started an effort in 2019 to measure the redshifts of a large fraction of the AGN that are likely to be detected with CTA, using the Southern African Large Telescope (SALT). In this contribution, we present two results from an on-going SALT program focused on the determination of BL Lac object redshifts that will be relevant for the CTA observatory. △ Less

Submitted 10 August, 2021; originally announced August 2021.

Comments: 15 pages, 4 figures, 37th International Cosmic Ray Conference

Journal ref: Proceedings of Science, PoS(ICRC2021)881

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)

Optical spectroscopy of Blazars for the Cherenkov Telescope Array

Authors: P. Goldoni, S. Pita, C. Boisson, W. Max-Moerbeck, E. Kasai, D. A. Williams, F. D'Ammando, V. Navarro-Aranguiz, M. Backes, U. Barres de Almeida, J. Becerra-Gonzalez, G. Cotter, O. Hervet, J. -P. Lenain, E. Lindfors, H. Sol, S. Wagner

Abstract: Context. Blazars are the most numerous class of High Energy (HE; E about 50 MeV - few 100 GeV) and Very High Energy (VHE; E about 100 GeV - 10 TeV) gamma-ray emitters. As of today, a measured spectroscopic redshift is available for only about 50% of gamma-ray BL Lacs, mainly due to the difficulty of measuring reliable redshifts from their nearly featureless, continuum-dominated optical spectra. Th… ▽ More Context. Blazars are the most numerous class of High Energy (HE; E about 50 MeV - few 100 GeV) and Very High Energy (VHE; E about 100 GeV - 10 TeV) gamma-ray emitters. As of today, a measured spectroscopic redshift is available for only about 50% of gamma-ray BL Lacs, mainly due to the difficulty of measuring reliable redshifts from their nearly featureless, continuum-dominated optical spectra. The knowledge of the redshift is fundamental for understanding the emission from blazars, for population studies and also for indirect studies of the extragalactic background light and searches for Lorentz invariance violation and axion-like particles using blazars. Aims. This paper is the first of a series of papers which aim to measure the redshift of a sample of blazars likely to be detected with the upcoming Cherenkov Telescope Array (CTA), a ground based gamma-ray observatory. Methods. Monte Carlo simulations were performed to select those hard spectrum gamma-ray blazars detected with the Fermi-LAT telescope still lacking redshift measurements but likely to be detected by CTA in 30 hours of observing time or less. Optical observing campaigns involving deep imaging and spectroscopic observations were organised to efficiently constrain their redshifts. We performed deep medium to high resolution spectroscopy of nineteen blazar optical counterparts using the ESI spectrograph at Keck, the RSS spectrograph at the SALT telescope, and the EFOSC2 spectrograph at the ESO NTT. We searched systematically for spectral features and, when possible, we estimated the contribution of the host galaxy to the total flux. Results. We measured eleven firm spectroscopic redshifts with values ranging from 0.1116 to 0.482. one tentative redshift, three redshift lower limits including one at z > 0.449 and another at z > 0.868. There were four objects found to have featureless spectra. △ Less

Submitted 22 April, 2021; v1 submitted 9 December, 2020; originally announced December 2020.

Comments: Accepted by Astronomy & Astrophysics

Journal ref: A&A 650, A106 (2021)

The Relativistic Jet Orientation and Host Galaxy of the Peculiar Blazar PKS 1413+135

Authors: A. C. S. Readhead, V. Ravi, I. Liodakis, M. L. Lister, V. Singh, M. F. Aller, R. D. Blandford, I. W. A. Browne, V. Gorjian, K. J. B. Grainge, M. A. Gurwell, M. W. Hodges, T. Hovatta, S. Kiehlmann, A. Lähteenmäki, T. McAloone, W. Max-Moerbeck, V. Pavlidou, T. J. Pearson, A. L. Peirson, E. S. Perlman, R. A. Reeves, B. T. Soifer, G. B. Taylor, M. Tornikoski , et al. (4 additional authors not shown)

Abstract: PKS 1413+135 is one of the most peculiar blazars known. Its strange properties led to the hypothesis almost four decades ago that it is gravitationally lensed by a mass concentration associated with an intervening galaxy. It exhibits symmetric achromatic variability, a rare form of variability that has been attributed to gravitational milli-lensing. It has been classified as a BL Lac object, and i… ▽ More PKS 1413+135 is one of the most peculiar blazars known. Its strange properties led to the hypothesis almost four decades ago that it is gravitationally lensed by a mass concentration associated with an intervening galaxy. It exhibits symmetric achromatic variability, a rare form of variability that has been attributed to gravitational milli-lensing. It has been classified as a BL Lac object, and is one of the rare objects in this class with a visible counterjet. BL Lac objects have jet axes aligned close to the line of sight. It has also been classified as a compact symmetric object, which have jet axes not aligned close to the line of sight. Intensive efforts to understand this blazar have hitherto failed to resolve even the questions of the orientation of the relativistic jet, and the host galaxy. Answering these two questions is important as they challenge our understanding of jets in active galactic nuclei and the classification schemes we use to describe them. We show that the jet axis is aligned close to the line of sight and PKS 1413+135 is almost certainly not located in the apparent host galaxy, but is a background object in the redshift range $0.247 < z < 0.5$. The intervening spiral galaxy at $z = 0.247$ provides a natural host for the putative lens responsible for symmetric achromatic variability and is shown to be a Seyfert 2 galaxy. We also show that, as for the radio emission, a "multizone" model is needed to account for the high-energy emission. △ Less

Submitted 7 December, 2020; originally announced December 2020.

Comments: Accepted for publication in The Astrophysical Journal. 28 pages, 15 figures

Association of IceCube neutrinos with radio sources observed at Owens Valley and Metsähovi Radio Observatories

Authors: T. Hovatta, E. Lindfors, S. Kiehlmann, W. Max-Moerbeck, M. Hodges, I. Liodakis, A. Lahteenmaki, T. J. Pearson, A. C. S. Readhead, R. A. Reeves, S. Suutarinen, J. Tammi, M. Tornikoski

Abstract: Identifying the most likely sources for high-energy neutrino emission has been one of the main topics in high-energy astrophysics ever since the first observation of high-energy neutrinos by the IceCube Neutrino Observatory. Active galactic nuclei with relativistic jets, also known as blazars, have been considered to be one of the main candidates because of their ability to accelerate particles to… ▽ More Identifying the most likely sources for high-energy neutrino emission has been one of the main topics in high-energy astrophysics ever since the first observation of high-energy neutrinos by the IceCube Neutrino Observatory. Active galactic nuclei with relativistic jets, also known as blazars, have been considered to be one of the main candidates because of their ability to accelerate particles to high energies. We study the connection between radio emission and IceCube neutrino events using data from the Owens Valley Radio Observatory (OVRO) and Metsähovi Radio Observatory blazar monitoring programs. We identify sources in our radio monitoring sample that are positionally consistent with IceCube high-energy neutrino events. We estimate their mean flux density and variability amplitudes around the neutrino arrival time, and compare these with values from random samples to establish the significance of our results. We find radio source associations within our samples with 15 high-energy neutrino events detected by IceCube. Nearly half of the associated sources are not detected in the $γ$-ray energies, but their radio variability properties and Doppler boosting factors are similar to the $γ$-ray detected objects in our sample, meaning that they could still be potential neutrino emitters. We find that the number of strongly flaring objects in our statistically complete OVRO samples is unlikely to be a random coincidence (at $2σ$ level). Based on our results, we conclude that although it is clear that not all neutrino events are associated with strong radio flaring blazars, observations of large-amplitude radio flares in a blazar at the same time as a neutrino event are unlikely to be a random coincidence. △ Less

Submitted 15 April, 2021; v1 submitted 22 September, 2020; originally announced September 2020.

Comments: Accepted for publication in A&A (18 pages, 6 figures). Accepted version (v2) includes changes to methodology and associations, but the main conclusions are unaffected. V3 minor fix to numbers in Sect. 3.3.1. and some language edits

Journal ref: A&A 650, A83 (2021)

An intermittent extreme BL Lac: MWL study of 1ES 2344+514 in an enhanced state

Authors: MAGIC Collaboration, V. A. Acciari, S. Ansoldi, L. A. Antonelli, A. Arbet Engels, D. Baack, A. Babić, B. Banerjee, U. Barres de Almeida, J. A. Barrio, J. Becerra González, W. Bednarek, L. Bellizzi, E. Bernardini, A. Berti, J. Besenrieder, W. Bhattacharyya, C. Bigongiari, O. Blanch, G. Bonnoli, Ž. Bošnjak, G. Busetto, R. Carosi, G. Ceribella, M. Cerruti , et al. (196 additional authors not shown)

Abstract: Extreme High-frequency BL~Lacs (EHBL) feature their synchrotron peak of the broadband spectral energy distribution (SED) at $ν_{\rm s} \geq $10$^{17}$\,Hz. The BL~Lac object 1ES~2344+514 was included in the EHBL family because of its impressive shift of the synchrotron peak in 1996. During the following years, the source appeared to be in a low state without showing any extreme behaviours. In Augu… ▽ More Extreme High-frequency BL~Lacs (EHBL) feature their synchrotron peak of the broadband spectral energy distribution (SED) at $ν_{\rm s} \geq $10$^{17}$\,Hz. The BL~Lac object 1ES~2344+514 was included in the EHBL family because of its impressive shift of the synchrotron peak in 1996. During the following years, the source appeared to be in a low state without showing any extreme behaviours. In August 2016, 1ES~2344+514 was detected with the ground-based $γ$-ray telescope FACT during a high $γ$-ray state, triggering multi-wavelength (MWL) observations. We studied the MWL light curves of 1ES~2344+514 during the 2016 flaring state, using data from radio to VHE $γ$ rays taken with OVRO, KAIT, KVA, NOT, some telescopes of the GASP-WEBT collaboration at the Teide, Crimean, and St. Petersburg observatories, \textit{Swift}-UVOT, \textit{Swift}-XRT, \textit{Fermi}-LAT, FACT and MAGIC. With simultaneous observations of the flare, we built the broadband SED and studied it in the framework of a leptonic and an hadronic model. The VHE $γ$-ray observations show a flux level of 55\% of the Crab Nebula flux above 300\,GeV, similar to the historical maximum of 1995. The combination of MAGIC and \textit{Fermi}-LAT spectra provides an unprecedented characterization of the inverse-Compton peak for this object during a flaring episode. The $Γ$ index of the intrinsic spectrum in the VHE $γ$-ray band is $2.04\pm0.12_{\rm stat}\pm0.15_{\rm sys}$. We find the source in an extreme state with a shift of the position of the synchrotron peak to frequencies above or equal to $10^{18}$\,Hz △ Less

Submitted 1 August, 2020; v1 submitted 11 June, 2020; originally announced June 2020.

Comments: Accepted in MNRAS. Corresponding authors: M. Manganaro, A. Arbet Engels, D. Dorner

Journal ref: MNRAS 496, 3912 (2020)

Testing two-component models on very-high-energy gamma-ray emitting BL Lac objects

Authors: MAGIC Collaboration, V. A. Acciari, S. Ansoldi, L. A. Antonelli, A. Arbet Engels, D. Baack, A. Babić, B. Banerjee, U. Barres de Almeida, J. A. Barrio, J. Becerra González, W. Bednarek, L. Bellizzi, E. Bernardini, A. Berti, J. Besenrieder, W. Bhattacharyya, C. Bigongiari, A. Biland, O. Blanch, G. Bonnoli, Ž. Bošnjak, G. Busetto, R. Carosi, G. Ceribella , et al. (179 additional authors not shown)

Abstract: Context. It has become evident that one-zone synchrotron self-Compton models are not always adequate for very-high-energy (VHE) gamma-ray emitting blazars. While two-component models are performing better, they are difficult to constrain due to the large number of free parameters. Aims. In this work, we make a first attempt to take into account the observational constraints from Very Long Baseline… ▽ More Context. It has become evident that one-zone synchrotron self-Compton models are not always adequate for very-high-energy (VHE) gamma-ray emitting blazars. While two-component models are performing better, they are difficult to constrain due to the large number of free parameters. Aims. In this work, we make a first attempt to take into account the observational constraints from Very Long Baseline Interferometry (VLBI) data, long-term light curves (radio, optical, and X-rays) and optical polarisation to limit the parameter space for a two-component model and test if it can still reproduce the observed spectral energy distribution (SED) of the blazars. Methods. We selected five TeV BL Lac objects based on the availability of VHE gamma-ray and optical polarisation data. We collected constraints for the jet parameters from VLBI observations. We evaluated the contributions of the two components to the optical flux by means of decomposition of long-term radio and optical light curves as well as modeling of the optical polarisation variability of the objects. We selected eight epochs for these five objects, based on the variability observed at VHE gamma rays, for which we constructed the SEDs that we then modeled with a two-component model. Results. We found parameter sets which can reproduce the broadband SED of the sources in the framework of two-component models considering all available observational constraints from VLBI observations. Moreover, the constraints obtained from the long-term behavior of the sources in the lower energy bands could be used to determine the region where the emission in each band originates. Finally, we attempted to use optical polarisation data to shed new light on the behavior of the two components in the optical band. Our observationally constrained two zone model allows explanation of the entire SED from radio to VHE with two co-located emission regions. △ Less

Submitted 8 June, 2020; originally announced June 2020.

Journal ref: A&A 640, A132 (2020)

A decade of multi-wavelength observations of the TeV blazar 1ES 1215+303: Extreme shift of the synchrotron peak frequency and long-term optical-gamma-ray flux increase

Authors: Janeth Valverde, Deirdre Horan, Denis Bernard, Stephen Fegan, A. U. Abeysekara, A. Archer, W. Benbow, R. Bird, A. Brill, R. Brose, M. Buchovecky, J. H. Buckley, J. L. Christiansen, W. Cui, A. Falcone, Q. Feng, J. P. Finley, L. Fortson, A. Furniss, A. Gent, G. H. Gillanders, C. Giuri, O. Gueta, D. Hanna, T. Hassan , et al. (64 additional authors not shown)

Abstract: Blazars are known for their variability on a wide range of timescales at all wavelengths. Most studies of TeV gamma-ray blazars focus on short timescales, especially during flares. With a decade of observations from the Fermi-LAT and VERITAS, we present an extensive study of the long-term multi-wavelength radio-to-gamma-ray flux-density variability, with the addition of a couple of short-time radi… ▽ More Blazars are known for their variability on a wide range of timescales at all wavelengths. Most studies of TeV gamma-ray blazars focus on short timescales, especially during flares. With a decade of observations from the Fermi-LAT and VERITAS, we present an extensive study of the long-term multi-wavelength radio-to-gamma-ray flux-density variability, with the addition of a couple of short-time radio-structure and optical polarization observations of the blazar 1ES 1215+303 (z=0.130), with a focus on its gamma-ray emission from 100 MeV to 30 TeV. Multiple strong GeV gamma-ray flares, a long-term increase in the gamma-ray and optical flux baseline and a linear correlation between these two bands are observed over the ten-year period. Typical HBL behaviors are identified in the radio morphology and broadband spectrum of the source. Three stationary features in the innermost jet are resolved by VLBA at 43.1, 22.2, and 15.3 GHz. We employ a two-component synchrotron self-Compton model to describe different flux states of the source, including the epoch during which an extreme shift in energy of the synchrotron peak frequency from infrared to soft X-rays is observed. △ Less

Submitted 12 February, 2020; v1 submitted 10 February, 2020; originally announced February 2020.

Comments: Accepted for publication in the Astrophysical Journal

Journal ref: ApJ 891 (2020) 170

The Great Markarian 421 Flare of February 2010: Multiwavelength variability and correlation studies

Authors: A. U. Abeysekara, W. Benbow, R. Bird, A. Brill, R. Brose, M. Buchovecky, J. H. Buckley, J. L. Christiansen, A. J. Chromey, M. K. Daniel, J. Dumm, A. Falcone, Q. Feng, J. P. Finley, L. Fortson, A. Furniss, N. Galante, A. Gent, G. H. Gillanders, C. Giuri, O. Gueta, T. Hassan, O. Hervet, J. Holder, G. Hughes , et al. (234 additional authors not shown)

Abstract: We report on variability and correlation studies using multiwavelength observations of the blazar Mrk 421 during the month of February, 2010 when an extraordinary flare reaching a level of $\sim$27~Crab Units above 1~TeV was measured in very-high-energy (VHE) $γ$-rays with the VERITAS observatory. This is the highest flux state for Mrk 421 ever observed in VHE $γ$-rays. Data are analyzed from a co… ▽ More We report on variability and correlation studies using multiwavelength observations of the blazar Mrk 421 during the month of February, 2010 when an extraordinary flare reaching a level of $\sim$27~Crab Units above 1~TeV was measured in very-high-energy (VHE) $γ$-rays with the VERITAS observatory. This is the highest flux state for Mrk 421 ever observed in VHE $γ$-rays. Data are analyzed from a coordinated campaign across multiple instruments including VHE $γ$-ray (VERITAS, MAGIC), high-energy (HE) $γ$-ray (Fermi-LAT), X-ray (Swift}, RXTE, MAXI), optical (including the GASP-WEBT collaboration and polarization data) and radio (Metsähovi, OVRO, UMRAO). Light curves are produced spanning multiple days before and after the peak of the VHE flare, including over several flare `decline' epochs. The main flare statistics allow 2-minute time bins to be constructed in both the VHE and optical bands enabling a cross-correlation analysis that shows evidence for an optical lag of $\sim$25-55 minutes, the first time-lagged correlation between these bands reported on such short timescales. Limits on the Doppler factor ($δ\gtrsim 33$) and the size of the emission region ($ δ^{-1}R_B \lesssim 3.8\times 10^{13}\,\,\mbox{cm}$) are obtained from the fast variability observed by VERITAS during the main flare. Analysis of 10-minute-binned VHE and X-ray data over the decline epochs shows an extraordinary range of behavior in the flux-flux relationship: from linear to quadratic to lack of correlation to anti-correlation. Taken together, these detailed observations of an unprecedented flare seen in Mrk 421 are difficult to explain by the classic single-zone synchrotron self-Compton model. △ Less

Submitted 10 February, 2020; originally announced February 2020.

Comments: 41 pages including 3 appendices, 13 figures; version accepted to Astrophysical Journal

Journal ref: The Astrophysical Journal, Volume 890, Number 2 (2020)

Patterns in the multi-wavelength behavior of candidate neutrino blazars

Authors: A. Franckowiak, S. Garrappa, V. Paliya, B. Shappee, R. Stein, N. L. Strotjohann, M. Kowalski, S. Buson, S. Kiehlmann, W. Max-Moerbeck, R. Angioni

Abstract: Motivated by the identification of the blazar TXS 0506+056 as the first promising high-energy neutrino counterpart candidate, we search for additional neutrino blazars candidates among the Fermi-LAT detected blazars. We investigate the multi-wavelength behavior from radio to GeV gamma rays of blazars found to be in spatial coincidence with single high-energy neutrinos and lower-energy neutrino f… ▽ More Motivated by the identification of the blazar TXS 0506+056 as the first promising high-energy neutrino counterpart candidate, we search for additional neutrino blazars candidates among the Fermi-LAT detected blazars. We investigate the multi-wavelength behavior from radio to GeV gamma rays of blazars found to be in spatial coincidence with single high-energy neutrinos and lower-energy neutrino flare candidates. In addition, we compare the average gamma-ray emission of the potential neutrino-emitting sources to the entire sample of gamma-ray blazars. We find that neutrino-emitting blazar candidates are statistically compatible with both hypothesis of a linear correlation and of no correlation between neutrino and gamma-ray energy flux. △ Less

Submitted 27 March, 2020; v1 submitted 28 January, 2020; originally announced January 2020.

Comments: accepted for publication by ApJ

Journal ref: ApJ, 893, 162 (2020)

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

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 presence of interstellar scintillation in the 15 GHz interday variability of 1158 OVRO-monitored blazars

Authors: J. Y. Koay, D. L. Jauncey, T. Hovatta, S. Kiehlmann, H. E. Bignall, W. Max-Moerbeck, T. J. Pearson, A. C. S. Readhead, R. Reeves, C. Reynolds, H. Vedantham

Abstract: We have conducted the first systematic search for interday variability in a large sample of extragalactic radio sources at 15 GHz. From the sample of 1158 radio-selected blazars monitored over a $\sim$10 year span by the Owens Valley Radio Observatory 40-m telescope, we identified 20 sources exhibiting significant flux density variations on 4-day timescales. The sky distribution of the variable so… ▽ More We have conducted the first systematic search for interday variability in a large sample of extragalactic radio sources at 15 GHz. From the sample of 1158 radio-selected blazars monitored over a $\sim$10 year span by the Owens Valley Radio Observatory 40-m telescope, we identified 20 sources exhibiting significant flux density variations on 4-day timescales. The sky distribution of the variable sources is strongly dependent on the line-of-sight Galactic H$α$ intensities from the Wisconsin H$α$ Mapper Survey, demonstrating the contribution of interstellar scintillation (ISS) to their interday variability. 21% of sources observed through sight-lines with H$α$ intensities larger than 10 rayleighs exhibit significant ISS persistent over the $\sim$10 year period. The fraction of scintillators is potentially larger when considering less significant variables missed by our selection criteria, due to ISS intermittency. This study demonstrates that ISS is still important at 15 GHz, particularly through strongly scattered sight-lines of the Galaxy. Of the 20 most significant variables, 11 are observed through the Orion-Eridanus superbubble, photoionized by hot stars of the Orion OB1 association. The high-energy neutrino source TXS0506$+$056 is observed through this region, so ISS must be considered in any interpretation of its short-term radio variability. J0616$-$1041 appears to exhibit large $\sim$20% interday flux density variations, comparable in magnitude to that of the very rare class of extreme, intrahour scintillators that includes PKS0405$-$385, J1819$+$3845 and PKS1257$-$326; this needs to be confirmed by higher cadence follow-up observations. △ Less

Submitted 4 September, 2019; originally announced September 2019.

Comments: 17 pages, 12 figures, accepted for publication in MNRAS

MOJAVE. XVII. Jet Kinematics and Parent Population Properties of Relativistically Beamed Radio-Loud Blazars

Authors: M. L. Lister, D. C. Homan, T. Hovatta, K. I. Kellermann, S. Kiehlmann, Y. Y. Kovalev, W. Max-Moerbeck, A. B. Pushkarev, A. C. S. Readhead, E. Ros, T. Savolainen

Abstract: We present results from a parsec-scale jet kinematics study of 409 bright radio-loud AGNs based on 15 GHz VLBA data obtained between 1994 August 31 and 2016 December 26 as part of the 2cm VLBA survey and MOJAVE programs. We tracked 1744 individual bright features in 382 jets over at least five epochs. A majority (59%) of the best-sampled jet features showed evidence of accelerated motion at the >3… ▽ More We present results from a parsec-scale jet kinematics study of 409 bright radio-loud AGNs based on 15 GHz VLBA data obtained between 1994 August 31 and 2016 December 26 as part of the 2cm VLBA survey and MOJAVE programs. We tracked 1744 individual bright features in 382 jets over at least five epochs. A majority (59%) of the best-sampled jet features showed evidence of accelerated motion at the >3sigma level. Although most features within a jet typically have speeds within ~40% of a characteristic median value, we identified 55 features in 42 jets that had unusually slow pattern speeds, nearly all of which lie within 4 pc (100 pc de-projected) of the core feature. Our results combined with other speeds from the literature indicate a strong correlation between apparent jet speed and synchrotron peak frequency, with the highest jet speeds being found only in low-peaked AGNs. Using Monte Carlo simulations, we find best fit parent population parameters for a complete sample of 174 quasars above 1.5 Jy at 15 GHz. Acceptable fits are found with a jet population that has a simple unbeamed power law luminosity function incorporating pure luminosity evolution, and a power law Lorentz factor distribution ranging from 1.25 to 50 with slope -1.4 +- 0.2. The parent jets of the brightest radio quasars have a space density of 261 +- 19 Gpc$^{-3}$ and unbeamed 15 GHz luminosities above ~$10^{24.5}$ W/Hz, consistent with FR II class radio galaxies. △ Less

Submitted 25 February, 2019; originally announced February 2019.

Comments: 19 pages, 13 figures, 7 tables; accepted to the Astrophysical Journal; full electronic tables are available from the preprint source

Journal ref: ApJ 874 (2019) 43

Constraining the limiting brightness temperature and Doppler factors for the largest sample of radio bright blazars

Authors: I. Liodakis, T. Hovatta, D. Huppenkothen, S. Kiehlmann, W. Max-Moerbeck, A. C. S. Readhead

Abstract: Relativistic effects dominate the emission of blazar jets complicating our understanding of their intrinsic properties. Although many methods have been proposed to account for them, the variability Doppler factor method has been shown to describe the blazar populations best. We use a Bayesian hierarchical code called {\it Magnetron} to model the light curves of 1029 sources observed by the Owens V… ▽ More Relativistic effects dominate the emission of blazar jets complicating our understanding of their intrinsic properties. Although many methods have been proposed to account for them, the variability Doppler factor method has been shown to describe the blazar populations best. We use a Bayesian hierarchical code called {\it Magnetron} to model the light curves of 1029 sources observed by the Owens Valley Radio Observatory's 40-m telescope as a series of flares with an exponential rise and decay, and estimate their variability brightness temperature. Our analysis allows us to place the most stringent constraints on the equipartition brightness temperature i.e., the maximum achieved intrinsic brightness temperature in beamed sources which we found to be $\rm \langle T_{eq}\rangle=2.78\times10^{11}K\pm26\%$. Using our findings we estimated the variability Doppler factor for the largest sample of blazars increasing the number of available estimates in the literature by almost an order of magnitude. Our results clearly show that $γ$-ray loud sources have faster and higher amplitude flares than $γ$-ray quiet sources. As a consequence they show higher variability brightness temperatures and thus are more relativistically beamed, with all of the above suggesting a strong connection between the radio flaring properties of the jet and $γ$-ray emission. △ Less

Submitted 21 September, 2018; originally announced September 2018.

Comments: 14 pages, 8 figures, accepted for publication in APJ

Multiwavelength Cross-Correlations and Flaring Activity in Bright Blazars

Authors: I. Liodakis, R. W. Romani, A. V. Filippenko, S. Kiehlmann, W. Max-Moerbeck, A. C. S. Readhead, W. Zheng

Abstract: Blazars are known for their energetic multiwavelength flares from radio wavelengths to high-energy $γ$-rays. In this work, we study radio, optical, and $γ$-ray light curves of 145 bright blazars spanning up to 8~yr, to probe the flaring activity and interband correlations. Of these, 105 show $>1σ$ correlations between one or more wavebands, 26 of which have a $>3σ$ correlation in at least one wave… ▽ More Blazars are known for their energetic multiwavelength flares from radio wavelengths to high-energy $γ$-rays. In this work, we study radio, optical, and $γ$-ray light curves of 145 bright blazars spanning up to 8~yr, to probe the flaring activity and interband correlations. Of these, 105 show $>1σ$ correlations between one or more wavebands, 26 of which have a $>3σ$ correlation in at least one wavelength pair, as measured by the discrete correlation function. The most common and strongest correlations are found between the optical and $γ$-ray bands, with fluctuations simultaneous within our $\sim 30$~d resolution. The radio response is usually substantially delayed with respect to the other wavelengths with median time lags of $\sim 100$--160~d. A systematic flare identification via Bayesian block analysis provides us with a first uniform sample of flares in the three bands, allowing us to characterise the relative rates of multiband and "orphan" flares. Multiband flares tend to have higher amplitudes than "orphan" flares. △ Less

Submitted 16 August, 2018; originally announced August 2018.

Comments: 13 pages, 7 figures, accepted for publication in MNRAS

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)