Fermi Gamma-ray Space Telescope Observations of the Gamma-ray Outburst from 3C454.3 in November 2010

The flat-spectrum radio quasar 3C454.3 underwent an extraordinary 5 day γ-ray outburst in 2010 November when the daily flux measured with the Fermi Large Area Telescope (LAT) at photon energies E > 100 MeV reached (66 ± 2) × 10<SUP>-6</SUP> photons cm<SUP>-2</SUP> s<SUP>-1</SUP>. This is a factor of three higher than its previous maximum flux recorded in 2009 December and >~ 5 times brighter than the Vela pulsar, which is normally the brightest source in the γ-ray sky. The 3 hr peak flux was (85 ± 5)×10<SUP>-6</SUP> photons cm<SUP>-2</SUP> s<SUP>-1</SUP>, corresponding to an apparent isotropic luminosity of (2.1 ± 0.2)×10<SUP>50</SUP> erg s<SUP>-1</SUP>, the highest ever recorded for a blazar. In this Letter, we investigate the features of this exceptional event in the γ-ray band of the Fermi-LAT. In contrast to previous flares of the same source observed with the Fermi-LAT, clear spectral changes are observed during the flare.