Localising the gamma-ray emission region during the June 2014 outburst of 3C 454.3

Abstract

In May - July 2014, the flat spectrum radio quasar 3C 454.3 exhibited strong flaring behaviour. Observations with the Large Area Telescope detector on-board the Fermi Gamma-ray Space Telescope captured the $\gamma$-ray flux at energies 0.1 $\le E_{\gamma }\le$ 300 GeV increasing fivefold during this period, with two distinct peaks in emission. The $\gamma$-ray emission is analysed in detail, in order to study the emission characteristics and put constraints on the location of the emission region. We explore variability in the spectral shape of 3C 454.3, search for evidence of a spectral cutoff, quantify the significance of VHE emission and investigate whether or not an energy-dependence of the emitting electron cooling exists. $\gamma$-ray intrinsic doubling timescales as small as ${\tau }_{int}=0.68$ $\pm$ 0.01 h at a significance of > 5$\sigma$ are found, providing evidence of a compact emission region. Significant $E_{\gamma ,emitted}\ge$ 35 GeV and $E_{\gamma ,emitted}\ge$ 50 GeV emission is also observed. The location of the emission region can be constrained to $r\ge 1.3$ $\times$ $R_{BLR}^{out}$, 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.

Accepted for publication in MNRAS, 12 pages, 8 figures, 3 tables

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