Characterizing the Gamma-Ray Variability of the Brightest Flat Spectrum Radio Quasars Observed with the Fermi LAT

Abstract

Almost 10 yr of γ-ray observations with the Fermi Large Area Telescope (LAT) have revealed extreme γ-ray outbursts from flat spectrum radio quasars (FSRQs), temporarily making these objects the brightest γ-ray emitters in the sky. Yet, the location and mechanisms of the γ-ray emission remain elusive. We characterize long-term γ-ray variability and the brightest γ-ray flares of six FSRQs. Consecutively zooming in on the brightest flares, which we identify in an objective way through Bayesian blocks and a hill-climbing algorithm, we find variability on subhour time scales and as short as minutes for two sources in our sample (3C 279, CTA 102) and weak evidence for variability at time scales less than the Fermi satellite's orbit of 95 minutes for PKS 1510-089 and 3C 454.3. This suggests extremely compact emission regions in the jet. We do not find any signs for γ-ray absorption in the broad-line region (BLR), which indicates that γ-rays are produced at distances greater than hundreds of gravitational radii from the central black hole. This is further supported by a cross-correlation analysis between γ-ray and radio/millimeter light curves, which is consistent with γ-ray production at the same location as the millimeter core for 3C 273, CTA 102, and 3C 454.3. The inferred locations of the γ-ray production zones are still consistent with the observed decay times of the brightest flares if the decay is caused by external Compton scattering with BLR photons. However, the minute-scale variability is challenging to explain in such scenarios.

Constraints on the emission region of 3C 279 during strong flares in 2014 and 2015 through VHE γ-ray observations with H.E.S.S.

“…The Lyα luminosity is the highest in the model (a factor of 12 higher than L(Hβ)) and is therefore responsible for most of the absorption. The values for R Lyα and the Lyα luminosity are broadly consistent with typical values obtained from reverberation mapping (Kaspi et al 2007;Bentz et al 2009;Meyer et al 2019) The resulting optical depths, τ γγ (r, E), for both geometries and different distances r of the emission region from the central black hole are shown as a function of the γ-ray energy in Fig. 5.…”

Blazar variability power spectra from radio up to TeV photon energies: Mrk 421 and PKS 2155−304

“…Since the aim of the present study is to derive shapes of PSDs, the best-fit PSD parameters are determined using Monte Carlo (MC) simulations, following the 'power spectral response' (PSRESP) method, introduced by Uttley et al (2002) and followed by many others, including Chatterjee et al Meyer et al (2019). We refer the readers to these studies for a detailed discussion about this approach while here we only briefly recall the main features.…”

“…Each simulated light curve was then rebinned to have the same sampling pattern as that of the observed light curve. Finally, the measurement errors in the simulated flux values were incorporated by adding a Gaussian random variable with mean 0 and standard deviation equal to the mean error of the measurement uncertainties on the observed flux values (Meyer et al 2019). In such a manner, 1,000 light curves are simulated in the β range 0.2 to 3.0, with a step of 0.1 for each observed light curve.…”

Investigating the multiwavelength behaviour of the flat spectrum radio quasar CTA 102 during 2013–2017

“…The search for variability on very short time-scale in γ -rays is beyond the scope of this paper. Rapid variability on time-scale of minutes was observed in 2016 December, with a peak flux of ∼3.5 × 10 −5 ph cm −2 s −1 (Gasparyan et al 2018;Shukla et al 2018;Meyer, Scargle & Blandford 2019).…”