Fermi-LAT Observations of the 2014 May-July outburst from 3C 454.3

TL;DR

This paper reports on the detailed gamma-ray observations of the 2014 outburst from quasar 3C 454.3 by Fermi-LAT, revealing a complex multi-stage light curve, high flux levels, and rapid variability, with implications for the emission region's location.

Contribution

It presents the first detection of a high-flux, hard-spectrum gamma-ray outburst from 3C 454.3 with Fermi-LAT, including detailed temporal and spectral analysis.

Findings

Peak flux reached 17.6 x 10^-6 ph cm^-2 s^-1 on June 15

Detected photons above 20 GeV, including one at 45 GeV

Outburst exhibited a three-stage light-curve pattern

Abstract

A prominent outburst of the flat spectrum radio quasar 3C~454.3 was observed in 2014 June with the \emph{Fermi} Large Area Telescope. This outburst was characterized by a three-stage light-curve pattern---plateau, flare and post-flare---that occurred from 2014 May to July, in a similar pattern as observed during the exceptional outburst in 2010 November. The highest flux of the outburst reported in this paper occurred during 2014 June 7--29, showing a multiple-peak structure in the light-curves. The average flux in these 22 days was found to be $F[E > 100~\mathrm{MeV}] = (7.2 \pm 0.2) \times 10^{-6}$~ph~cm$^{-2}$~s$^{-1}$, with a spectral index, for a simple power law, of $\Gamma = 2.04 \pm 0.01$. That made this outburst the first $\gamma$-ray high state of 3C~454.3 ever to be detected by \emph{Fermi} with such a hard spectrum over several days. The highest flux was recorded on 2014 June 15, in a 3 hr bin, at MJD 56823.5625, at a level of $F[E > 100~\mathrm{MeV}] = (17.6 \pm 1.9) \times 10^{-6}$~ph~cm$^{-2}$~s$^{-1}$. The rise time of one of the short subflares was found to be $T_r= 1200 \pm 700$~s at MJD = 56827, when the flux increased from 4 to 12 $\times 10^{-6}$~ph~cm$^{-2}$~s$^{-1}$. Several photons above 20 GeV were collected during this outburst, including one at 45 GeV on MJD 56827, constraining the $\gamma$-ray emission region to be located close to the outer boundary of the broad-line region, leading to fast flux variability.