Author(s): Ackermann M., Ajello M., Baldini L., Ballet J., Barbiellini G., Bastieri D., Bechtol K., Bellazzini R., Berenji B., D. Blandford R., Bonamente E., Bruel P., M. Casandjian J., Cohen-Tanugi J., Corbel S., Dumora D., Escande L., J. Fegan S., Giebels B., A. Grenier I., Guiriec Sylvain, Knodlseder J., Lott B., Mehault J., Nuss E., Pelassa V., Piron F., Sanchez D., Tibaldo L., Vilchez N.
(Article) Published:
The Astrophysical Journal / The Astrophysical Journal Letters, vol. 721 p.1383-1396 (2010)
Links openAccess full text : 
Ref HAL: in2p3-00530295_v1
DOI: 10.1088/0004-637X/721/2/1383
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112 citations
Abstract:
The flat spectrum radio quasar 3C 454.3 underwent an extraordinary outburst in 2009 December when it became the brightest γ-ray source in the sky for over 1 week. Its daily flux measured with the Fermi-Large Area Telescope at photon energies E>100 MeV reached F 100 = 22 ± 1 × 10–6 photon cm–2 s–1, representing the highest daily flux of any blazar ever recorded in high-energy γ-rays. It again became the brightest source in the sky in 2010 April, triggering a pointed-mode observation by Fermi. The correlated γ-ray temporal and spectral properties during these exceptional events are presented and discussed. The main results show flux variability over time scales less than 3 hr and very mild spectral variability with an indication of gradual hardening preceding major flares. The light curves during periods of enhanced activity in 2008 July-August and 2010 December show strong resemblance, with a flux plateau of a few days preceding the major flare. No consistent loop pattern emerged in the γ-ray spectral index versus the flux plane as would be expected in acceleration and cooling scenarios. The maximum energy of a photon from 3C 454.3 is ≈20 GeV and a minimum Doppler factor of ≈13 is derived. The γ-ray spectrum of 3C 454.3 shows a significant spectral break between ≈2 and 3 GeV that is very weakly dependent on the flux state, even when the flux changes by an order of magnitude.