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

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.

Variability Signatures of a Burst Process in Flaring Gamma-Ray Blazars

“…We set a fairly high autocorrelation parameter of f = 0.95, consistent with the absence of a resolvable spectral break in the gamma-ray PSD of 3C 279 as determined both by CARMA modeling (Ryan et al 2019) and by the power spectral response method (Goyal et al 2021). With these parameters, α obs ≈ 2.3, similar to the best-fit values of λ + 1 = 1.7 and 2.0 reported, respectively, by Tavecchio et al (2020) and Adams et al (2022).…”

“…2 Tavecchio et al (2020) showed that this PDF provides a satisfactory representation of the gamma-ray flux distributions of six bright FSRQs observed by Fermi-LAT using the light curves analyzed by Meyer et al (2019). Subsequently, Adams et al (2022) demonstrated that this model provides a better fit than a lognormal PDF to the Fermi-LAT flux distributions of three bright FSRQs, 3C 279, PKS 1222+216, and Ton 599.…”

“…To generate a representative light curve for an FSRQ, we estimated r and  from the observations reported by Adams et al (2022) of flares of 3C 279, giving r ≈ 24/ 54 bursts day −1 ≈ 5 × 10 −6 s −1 and 0.85 10 erg cm 3 2…”

“…One example of the flaring phenomenon comes from the bright FSRQ 3C 279, which has undergone several such flares (e.g., Ackermann et al 2016;Adams et al 2022). In particular, Adams et al (2022) analyzed its subdaily variability during 10 bright flaring periods lasting between 1 and 11 days, for a total of 54 days of observations.…”

“…One example of the flaring phenomenon comes from the bright FSRQ 3C 279, which has undergone several such flares (e.g., Ackermann et al 2016;Adams et al 2022). In particular, Adams et al (2022) analyzed its subdaily variability during 10 bright flaring periods lasting between 1 and 11 days, for a total of 54 days of observations. From modeling using exponential profiles, each flare was resolved into between one and four separate components, for a total of 24 distinct components that had rise and decay times ranging from days to less than 1 hr.…”