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1. Variability and Spectral Characteristics of Three Flaring Gamma-Ray Quasars Observed by VERITAS and Fermi-LAT

   https://doi.org/10.3847/1538-4357/ac32bd  

   Adams, C. B. ; Batshoun, J. ; Benbow, W. ; Brill, A. ; Buckley, J. H. ; Capasso, M. ; Cavins, B. ; Christiansen, J. L. ; Coppi, P. ; Errando, M. ; et al ( January 2022 , The Astrophysical Journal)

   Flat-spectrum radio quasars (FSRQs) are the most luminous blazars at GeV energies but only rarely emit detectable fluxes of TeV gamma rays, typically during bright GeV flares. We explore the gamma-ray variability and spectral characteristics of three FSRQs that have been observed at GeV and TeV energies by Fermi-LAT and VERITAS, making use of almost 100 hr of VERITAS observations spread over 10 yr: 3C 279, PKS 1222+216, and Ton 599. We explain the GeV flux distributions of the sources in terms of a model derived from a stochastic differential equation describing fluctuations in the magnetic field in the accretion disk and estimate the timescales of magnetic flux accumulation and stochastic instabilities in their accretion disks. We identify distinct flares using a procedure based on Bayesian blocks and analyze their daily and subdaily variability and gamma-ray energy spectra. Using observations from VERITAS, as well as Fermi, Swift, and the Steward Observatory, we model the broadband spectral energy distributions of PKS 1222+216 and Ton 599 during very high energy (VHE)–detected flares in 2014 and 2017, respectively, strongly constraining the jet Doppler factors and gamma-ray emission region locations during these events. Finally, we place theoretical constraints on the potential production ofmore »PeV-scale neutrinos during these VHE flares.

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2. Multiwavelength Observations of the Blazar VER J0521+211 during an Elevated TeV Gamma-Ray State

   https://doi.org/10.3847/1538-4357/ac6dd9  

   Adams, C. B. ; Batista, P. ; Benbow, W. ; Brill, A. ; Brose, R. ; Buckley, J. H. ; Capasso, M. ; Christiansen, J. L. ; Errando, M. ; Feng, Q. ; et al ( June 2022 , The Astrophysical Journal)

   Abstract We report on a long-lasting, elevated gamma-ray flux state from VER J0521+211 observed by VERITAS, MAGIC, and Fermi-LAT in 2013 and 2014. The peak integral flux above 200 GeV measured with the nightly binned light curve is (8.8 ± 0.4) × 10 −7 photons m −2 s −1 , or ∼37% of the Crab Nebula flux. Multiwavelength observations from X-ray, UV, and optical instruments are also presented. A moderate correlation between the X-ray and TeV gamma-ray fluxes was observed, and the X-ray spectrum appeared harder when the flux was higher. Using the gamma-ray spectrum and four models of the extragalactic background light (EBL), a conservative 95% confidence upper limit on the redshift of the source was found to be z ≤ 0.31. Unlike the gamma-ray and X-ray bands, the optical flux did not increase significantly during the studied period compared to the archival low-state flux. The spectral variability from optical to X-ray bands suggests that the synchrotron peak of the spectral energy distribution (SED) may become broader during flaring states, which can be adequately described with a one-zone synchrotron self-Compton model varying the high-energy end of the underlying particle spectrum. The synchrotron peak frequency of the SED and themore »radio morphology of the jet from the MOJAVE program are consistent with the source being an intermediate-frequency-peaked BL Lac object.« less
3. X-ray hot spots in the eastern ear of the supernova remnant W 50 and the microquasar SS 433 system

   https://doi.org/10.1093/pasj/psac011  

   Hayakawa, Ryota ; Yamada, Shinya ; Suda, Hirotaka ; Ichinohe, Yuto ; Higurashi, Ryota ; Sakemi, Haruka ; Machida, Mami ; Ohmura, Takumi ; Katsuda, Satoru ; Uchiyama, Hideki ; et al ( March 2022 , Publications of the Astronomical Society of Japan)

   We examined the X-ray and radio spatial structure at the eastern ear of the W 50/SS 433 system to clarify a characteristic feature of the termination region of the SS 433 jet, and found that a hot spot ahead of the filament structure, which is considered to be a terminal shock of the SS 433 eastern jet, is clearly different from a single point source. The detailed spatial structure of the X-ray emission is finely resolved by Chandra observations, showing that there are two sources. By comparing the point-spread function of Chandra with the radial profiles of the two sources, the northern one is clearly more extended than a point source while the other seems marginally extended. Since there are no point sources nearby, the northern hot spot is likely a localized diffuse source. The northern hot spot spatially corresponds to the peak of the radio emission. Its spatial correlation is confirmed by an X-ray image using XMM-Newton. The X-ray spectra of the two sources are reproduced by a single absorbed power-law but the column density of the northern part is larger by a factor of ∼3. When a radiation model comprising synchrotron emission and inverse Compton emission is applied to the spectralmore »energy distribution of the northern hot spot, the emission from this spot can be explained by the radiation from an electron population accelerated up to 30 TeV in a magnetic field strength of B ≲ 50 μG. This model also agrees with the radio and X-ray data, as well as the upper limit of gamma-ray emission obtained by the Fermi satellite.

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4. New jet feature in the parsec-scale jet of the blazar OJ 287 connected to the 2017 teraelectronvolt flaring activity

   https://doi.org/10.1051/0004-6361/202142948  

   Lico, R. ; Casadio, C. ; Jorstad, S. G. ; Gómez, J. L. ; Marscher, A. P. ; Traianou, E. ; Kim, J.-Y. ; Zhao, G.-Y. ; Fuentes, A. ; Cho, I. ; et al ( February 2022 , Astronomy & Astrophysics)

   Context. In February 2017 the blazar OJ 287, one of the best super-massive binary-black-hole-system candidates, was detected for the first time at very high energies (VHEs; E  > 100 GeV) with the ground-based γ -ray observatory VERITAS. Aims. Very high energy γ rays are thought to be produced in the near vicinity of the central engine in active galactic nuclei. For this reason, and with the main goal of providing useful information for the characterization of the physical mechanisms connected with the observed teraelectronvolt flaring event, we investigate the parsec-scale source properties by means of high-resolution very long baseline interferometry observations. Methods. We use 86 GHz Global Millimeter-VLBI Array (GMVA) observations from 2015 to 2017 and combine them with additional multiwavelength radio observations at different frequencies from other monitoring programs. We investigate the source structure by modeling the brightness distribution with two-dimensional Gaussian components in the visibility plane. Results. In the GMVA epoch following the source VHE activity, we find a new jet feature (labeled K) at ∼0.2 mas from the core region and located in between two quasi-stationary components (labeled S1 and S2). Multiple periods of enhanced activity are detected at different radio frequencies before and during the VHE flaringmore »state. Conclusions. Based on the findings of this work, we identify as a possible trigger for the VHE flaring emission during the early months of 2017 the passage of a new jet feature through a recollimation shock (represented by the model-fit component S1) in a region of the jet located at a de-projected distance of ∼10 pc from the radio core.« less
5. Leptonic modelling of Ton 599 in flare and quiescent states

   https://doi.org/10.1093/mnras/stz3490  

   Patel, S. R. ; Chitnis, V. R. ( December 2019 , Monthly Notices of the Royal Astronomical Society)

   The flat-spectrum radio quasar Ton 599 attained its highest ever γ-ray flux state during the first week of 2017 November. Observations of the source by the Swift satellite during this period made it possible to generate a simultaneous high flux state broad-band spectral energy distribution (SED). The high flux state activity of Ton 599 is modelled in this work for the first time. We modelled one high flux state and one quiescent state of the source in order to characterize the evolution of SEDs covering the entire dynamic range of γ-ray flux observed by Fermi-LAT. An attempt was made to model the 2017 November state of the source using an external Compton (EC) model in the leptonic scenario. We reproduce the broad-band flaring state SED using a two-component leptonic emission model. We considered one component as an EC+synchrotron self-Compton (SSC) component and the other as pure SSC, lying further down in the jet. The EC+SSC component was located outside the broad-line region (BLR). It mainly reproduces the GeV emission by an EC process with a dusty torus (DT) photon field providing seed photons. We reproduce the broad-band emission from Ton 599 satisfactorily during its peculiar flaring state with a leptonic two-component model.more »Besides this, we compare the model parameters of a quiescent-state SED with the available average state model parameters in the literature.

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