Abdo, A. A. AA ; Ackermann, M. ; Ajello, M. ; Cavazzuti, E. ; Giommi, P. ;
Jul - 2009
ISSN : 0004-637X ;
journal : The Astrophysical Journal

Issue : 1
type: Article Journal

This is the first report of Fermi Gamma-Ray Space Telescope observations of the quasar 3C 454.3, which has been undergoing pronounced long-term outbursts since 2000. The data from the Large Area Telescope, covering 2008 July 7-October 6, indicate strong, highly variable ³-ray emission with an average flux of <3 × 106 photons cm2 s1, for energies >100 MeV. The ³-ray flux is variable, with strong, distinct, symmetrically shaped flares for which the flux increases by a factor of several on a timescale of about 3 days. This variability indicates a compact emission region, and the requirement that the source is optically thin to pair production implies relativistic beaming with Doppler factor ´>8, consistent with the values inferred from Very Long Baseline Interferometry observations of superluminal expansion (´ < 25). The observed ³-ray spectrum is not consistent with a simple power law, but instead steepens strongly above <2 GeV, and is well described by a broken power law with photon indices of <2.3 and <3.5 below and above the break, respectively. This is the first direct observation of a break in the spectrum of a high-luminosity blazar above 100 MeV, and it is likely direct evidence for an intrinsic break in the energy distribution of the radiating particles. Alternatively, the spectral softening above 2 GeV could be due to ³-ray absorption via photon-photon pair production on the soft X-ray photon field of the host active galactic nucleus, but such an interpretation would require the dissipation region to be located very close (r100 gravitational radii) to the black hole, which would be inconsistent with the X-ray spectrum of the source.

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