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The gamma-ray emission from Radio Galaxies and their contribution to the Isotropic Gamma-Ray Background
Authors:
A. Circiello,
A. McDaniel,
M. Di Mauro,
C. Karwin,
N. Khatiya,
M. Ajello,
F. Donato,
D. Hartmann,
A. Strong
Abstract:
We evaluate the contribution to the Isotropic Gamma-Ray Background (IGRB) coming from Radio Galaxies (RGs), the subclass of radio-loud Active Galactic Nuclei (AGN) with the highest misalignment from the line of sight (l.o.s.). Since only a small number of RGs are detected in gamma rays compared to the largest known radio population, the correlation between radio and gamma-ray emission serves as a…
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We evaluate the contribution to the Isotropic Gamma-Ray Background (IGRB) coming from Radio Galaxies (RGs), the subclass of radio-loud Active Galactic Nuclei (AGN) with the highest misalignment from the line of sight (l.o.s.). Since only a small number of RGs are detected in gamma rays compared to the largest known radio population, the correlation between radio and gamma-ray emission serves as a crucial tool to characterize the gamma-ray properties of these sources. We analyse the population of RGs using two samples. The first sample contains 26 sources individually detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope at gamma rays. The second sample contains 210 RGs for which the gamma-ray emission is not significantly detected by the LAT. We use a stacking analysis to characterize the average properties of the gamma-ray emission of the two samples, separately at first and then combined. We then evaluate the correlation between their gamma-ray emission and the emission from their radio core at 5 GHz, and we use it to determine their contribution to the IGRB. Due to the limited number of RGs detected at the gamma-rays, information on the gamma-ray luminosity function is limited. The correlation between the gamma-ray emission and the emission of the radio core allows us to characterize it starting from the luminosity function of the radio cores, which is modeled with greater accuracy due to the larger number of sources detected at these frequencies. We find that the diffuse emission as extrapolated from the properties of the subthreshold RGs is lower than the one inferred from detected RGs, showing that the contribution of the population of RGs to the IGRB is lower than the previous estimates and it is around the 30% level of the IGRB intensity.
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Submitted 7 October, 2025;
originally announced October 2025.
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Transient QPOs of Fermi-LAT blazars with Linearly Multiplicative Oscillations
Authors:
P. Penil,
J. Otero-Santos,
A. Circiello,
A. Banerjee,
S. Buson,
A. Rico,
M. Ajello,
S. Adhikari
Abstract:
We present a study on the detection and characterization of transient quasi-periodic oscillations (QPOs) in the $γ$-ray emission of blazars 4C +31.03, MG1 J123931+0443, and PKS 1622$-$253. Using light curves derived from \textit{Fermi} Large Area Telescope data, we investigate oscillatory patterns characterized by periodic multiplicative amplitudes that vary linearly over time. By segmenting the l…
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We present a study on the detection and characterization of transient quasi-periodic oscillations (QPOs) in the $γ$-ray emission of blazars 4C +31.03, MG1 J123931+0443, and PKS 1622$-$253. Using light curves derived from \textit{Fermi} Large Area Telescope data, we investigate oscillatory patterns characterized by periodic multiplicative amplitudes that vary linearly over time. By segmenting the light curves into increasing and decreasing trends, we analyze each segment independently, allowing for precise measurements of both the periodicity and long-term variations. To interpret these QPOs, we explore various theoretical scenarios that could explain their origin and underlying physical mechanisms. The variability observed in 4C~+31.03 is more consistent with a stochastic process, whereas the periods estimated for MG1~J123931+0443 and PKS~1622$-$253 align with the precessional dynamics expected from binary supermassive black hole systems. However, the current results remain tentative and do not allow for a definitive conclusion.
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Submitted 10 December, 2025; v1 submitted 18 July, 2025;
originally announced July 2025.
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Discovering High-$z$ BL Lacs Using Swift/UVOT and SARA Observations with the Dropout Technique
Authors:
Y. Sheng,
K. Imam,
A. Kaur,
M. Ajello,
A. Domínguez,
A. Rau,
S. B. Cenko,
J. Greiner,
D. H. Hartmann,
A. Circiello,
I. Cox,
S. Joffre,
A. Mcdaniel,
G. Rajguru,
R. Silver,
N. Torres-Albá,
A. Webber
Abstract:
Measuring spectroscopic redshifts for BL Lacertae (BL Lac) objects, a class of blazar, is challenging because their optical spectrum lacks, or has weak, emission lines ( equivalent width $\leqslant5Å$). In this situation, alternative techniques are necessary for the estimation of distances to these sources. In this paper, we estimate the redshift by the photometric dropout technique for a sample o…
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Measuring spectroscopic redshifts for BL Lacertae (BL Lac) objects, a class of blazar, is challenging because their optical spectrum lacks, or has weak, emission lines ( equivalent width $\leqslant5Å$). In this situation, alternative techniques are necessary for the estimation of distances to these sources. In this paper, we estimate the redshift by the photometric dropout technique for a sample of 64 blazars (59 BL Lacs and five blazar candidates of uncertain type). Two telescopes are utilized to observe the sample. The Ultraviolet/Optical Telescope (UVOT) on board {\it Swift} ({\it Swift}/UVOT) observes sources in $uvw2,\ uvm2,\ uvw1,\ u,\ b,\ v$ filters, while the ground-based telescopes SARA-CT/RM observed sources in $g',\ r,' \ i',\ z'$ filters. We fit the photometric data with the LePHARE package and report four new high-$z$ ($z>1.3$) BL Lacs at $2.03^{+0.07}_{-0.05}$, $1.84^{+0.10}_{-0.03}$, $2.04^{+0.16}_{-0.14}$, and $2.93^{+0.01}_{-0.04}$ as well as upper limits for 50 sources. This work increased the number of high-$z$ BL Lacs found by this method up to 23. The high-$z$ sources are discussed in the context of the cosmic gamma-ray horizon, blazar sequence, Fermi blazar divide, and masquerading BL Lacs.
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Submitted 15 November, 2025; v1 submitted 24 September, 2024;
originally announced September 2024.
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Evaluating the Potential to Constrain Dark Matter Annihilation with Fermi-LAT Observations of Ultra-Faint Compact Stellar Systems
Authors:
A. Circiello,
A. McDaniel,
A. Drlica-Wagner,
C. Karwin,
M. Ajello,
M. Di Mauro,
M. Sánchez-Conde
Abstract:
Recent results from numerical simulations and models of galaxy formation suggest that recently discovered ultra-faint compact stellar systems (UFCSs) in the halo of the Milky Way (MW) may be some of the smallest and faintest galaxies. If this is the case, these systems would be attractive targets for indirect searches of weakly interacting massive particle (WIMP) dark matter (DM) annihilation due…
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Recent results from numerical simulations and models of galaxy formation suggest that recently discovered ultra-faint compact stellar systems (UFCSs) in the halo of the Milky Way (MW) may be some of the smallest and faintest galaxies. If this is the case, these systems would be attractive targets for indirect searches of weakly interacting massive particle (WIMP) dark matter (DM) annihilation due to their relative proximity and high expected DM content. In this study, we analyze 14.3 years of gamma-ray data collected by the Fermi-LAT coincident with 26 UFCSs. No significant excess gamma-ray emission is detected, and we present gamma-ray flux upper limits for these systems. Assuming that the UFCSs are dark-matter-dominated galaxies consistent with being among the faintest and least massive MW dwarf spheroidal (dSphs) satellite galaxies, we derive the projected sensitivity for a dark matter annihilation signal. We find that observations of UFCSs have the potential to yield some of the most powerful constraints on DM annihilation, with sensitivity comparable to observations of known dSphs and the Galactic center. This result emphasizes the importance of precise kinematic studies of UFCSs to empirically determine their DM content.
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Submitted 12 December, 2024; v1 submitted 1 April, 2024;
originally announced April 2024.
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Searching for very-high-energy electromagnetic counterparts to gravitational-wave events with the Cherenkov Telescope Array
Authors:
Barbara Patricelli,
Alessandro Carosi,
Lara Nava,
Monica Seglar-Arroyo,
Fabian Schüssler,
Antonio Stamerra,
Andrea Adelfio,
Halim Ashkar,
Andrea Bulgarelli,
Tristano Di Girolamo,
Ambra Di Piano,
Thomas Gasparetto,
Jarred Green,
Francesco Longo,
Ivan Agudo,
Alessio Berti,
Elisabetta Bissaldi,
Giancarlo Cella,
Antonio Circiello,
Stefano Covino,
Giancarlo Ghirlanda,
Brian Humensky,
Susumu Inoue,
Julien Lefaucheur,
Miroslav Filipovic
, et al. (5 additional authors not shown)
Abstract:
The detection of electromagnetic (EM) emission following the gravitational wave (GW) event GW170817 opened the era of multi-messenger astronomy with GWs and provided the first direct evidence that at least a fraction of binary neutron star (BNS) mergers are progenitors of short Gamma-Ray Bursts (GRBs). GRBs are also expected to emit very-high energy (VHE, > 100 GeV) photons, as proven by the recen…
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The detection of electromagnetic (EM) emission following the gravitational wave (GW) event GW170817 opened the era of multi-messenger astronomy with GWs and provided the first direct evidence that at least a fraction of binary neutron star (BNS) mergers are progenitors of short Gamma-Ray Bursts (GRBs). GRBs are also expected to emit very-high energy (VHE, > 100 GeV) photons, as proven by the recent MAGIC and H.E.S.S. observations. One of the challenges for future multi-messenger observations will be the detection of such VHE emission from GRBs in association with GWs. In the next years, the Cherenkov Telescope Array (CTA) will be a key instrument for the EM follow-up of GW events in the VHE range, owing to its unprecedented sensitivity, rapid response, and capability to monitor a large sky area via scan-mode operation. We present the CTA GW follow-up program, with a focus on the searches for short GRBs possibly associated with BNS mergers. We investigate the possible observational strategies and we outline the prospects for the detection of VHE EM counterparts to transient GW events.
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Submitted 2 August, 2021;
originally announced August 2021.