We present the gamma-ray data of the extraordinary flaring activity above 100?MeV from the flat spectrum radio quasar 3C 454.3 detected by AGILE during the month of 2009 December. 3C 454.3, which has been among the most active blazars of the FSRQ type since 2007, has been detected in the gamma-ray range with a progressively rising flux since 2009 November 10. The gamma-ray flux reached a value comparable with that of the Vela pulsar on 2009 December 2. Remarkably, between 2009 December 2 and 3, the source more than doubled its gamma-ray emission and became the brightest gamma-ray source in the sky with a peak flux of F γ,p = (2000 ± 400) × 10–8 ph cm–2 s–1 for a 1 day integration above 100?MeV. The gamma-ray intensity decreased in the following days with the source flux remaining at large values near F γ (1000 ± 200) × 10–8 ph cm–2 s–1 for more than a week. This exceptional gamma-ray flare dissipated among the largest ever detected intrinsic radiated power in gamma-rays above 100?MeV (L γ,source,peak 3 × 1046 erg s–1, for a relativistic Doppler factor of δ 30). The total isotropic irradiated energy of the month-long episode in the range 100?MeV-3?GeV is E γ,iso 1056 erg. We report the intensity and spectral evolution of the gamma-ray emission across the flaring episode. We briefly discuss the important theoretical implications of our detection.
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