A prominent outburst of the flat spectrum radio quasar 3C 454.3 was observed in 2014 June with the 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 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 .