EARLY FERMI GAMMA-RAY SPACE TELESCOPE OBSERVATIONS OF THE QUASAR 3C 454.3

Abstract

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 γ-ray emission with an average flux of ∼3 × 10 −6 photons cm −2 s −1 , for energies >100 MeV. The γ-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 δ>8, consistent with the values inferred from Very Long Baseline Interferometry observations of superluminal expansion (δ ∼ 25). The observed γ-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 γ-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 (≲100 gravitational radii) to the black hole, which would be inconsistent with the X-ray spectrum of the source.