Localising the gamma-ray emission region of flaring Flat Spectrum Radio Quasars

Abstract

Observations by the Large Area Telescope detector on-board the Fermi Gamma-ray Space Telescope are used to examine the 0.1 < E_gamma < 300 GeV gamma-ray emission characteristics of flat spectrum radio quasars. Specifically, the gamma-ray emission from 3C 454.3 and 3C 279 are analysed in detail, in order to put constraints on the location of the emission region. The variability in the spectral shape is explored, whether evidence of a spectral cutoff can be found and whether or not an energy-dependence of the emitting electron cooling exists. The significance of VHE emission is also quantified.

In May - July 2014, 3C 454.3 exhibited strong flaring behaviour. Observations with the Fermi-LAT captured the gamma-ray flux increasing fivefold during this period, with two distinct peaks in emission. The peak daily binned flux climbed to F = 1.3 ± 0.1 x 10^-5 ph cm^-2 s^-1 on MJD 56823. Gamma-ray intrinsic flux doubling timescales as small as tau_int = 0.68 ± 0.01 h at a significance of >5 sigma are found, providing evidence of a compact emission region. Significant E_gamma,emitted > 35 GeV and E_gamma,emitted > 50 GeV emission are also observed. The location of the emission region can be constrained to r > 1.3 x R_BLR^out, a location outside the broad-line region. The spectral variation of 3C 454.3 also suggests that these flares may be originating further downstream of the supermassive black hole than the emission before and after the flares.

3C 279 flared spectacularly in June 2015, becoming brighter than ever previously recorded by Fermi. The peak daily binned flux reached F = (2.5 ± 0.1) x 10^-5 ph cm^-2 s^-1 on MJD 56823. Interestingly, the smallest intrinsic flux doubling timescale is tau_int = 1.38 ± 0.16 h, and no sub-hour flux doubling timescales are found. Significant E_gamma,emitted > 35 GeV and E_gamma,emitted > 50 GeV emission are observed during the flare, alongside a significant spectral hardening. Using photon-photon opacity constraints, the location of the emission region must lie at least r > 2.5 x R_BLR from the SMBH, the mid-point of the broad-line region. As with 3C 454.3, the spectral variation across the period of interest hints that a multi-zonal model may be applicable to the gamma-ray emission.