Abstract
Abstract 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 of PeV-scale neutrinos during these VHE flares.
Highlights
Blazars are a class of active galactic nuclei (AGN) with jets oriented nearly along our line of sight
While Flat spectrum radio quasars (FSRQs) make up only 8 of the 79 AGN that have been detected in the TeV band to date1, they are more commonly detected at GeV energies, comprising 650 of 2863 AGN detected by the Large Area Telescope (LAT) on board the Fermi GammaRay space telescope (Fermi -LAT) (Ajello et al 2020) and dominating the blazar population detected by the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray Observatory (Mukherjee 2001)
We present an analysis of the gamma-ray and multiwavelength emission of three bright, variable FSRQs observed by Fermi -LAT and Very Energetic Radiation Imaging Telescope Array System (VERITAS): 3C 279, PKS 1222+216, and Ton 599, making use of almost 100 hours of VERITAS observations of these sources
Summary
Blazars are a class of active galactic nuclei (AGN) with jets oriented nearly along our line of sight. While short variability timescales observed in blazars suggest that the emission may be connected to processes in the central engine or accretion disk, the ability of very-high-energy (VHE; 100 GeV) emission to escape the AGN implies an origin further out in the jet, where absorption is reduced (Abeysekara et al 2015). The pγ process may co-occur with leptonic gamma-ray emission Under this scenario, FSRQs may be sources of relativistic neutrinos at PeV or even EeV energies We investigate strong gammaray flares from three FSRQs at intermediate redshifts These three sources were continuously monitored by Fermi -LAT (Section 2.2) during the ten-year period from 2008 to 2018, and observed during periods of high gamma-ray activity by the Very Energetic Radiation Imaging Telescope Array System (VERITAS; Section 2.1).
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