Fermi Observations of GRB 090902B: A Distinct Spectral Component in the Prompt and Delayed Emission.

摘要

We report on the observation of the bright, long gamma-ray burst, GRB 090902B, by the Gamma-ray Burst Monitor (GBM) and Large Area Telescope (LAT) instruments on-board the Fermi observatory. This was one of the brightest GRBs to have been observed by the LAT, which detected several hundred photons during the prompt phase. With a redshift of z = 1.822, this burst is among the most luminous detected by Fermi. Time-resolved spectral analysis reveals a significant power-law component in the LAT data that is distinct from the usual Band model emission that is seen in the sub-MeV energy range. This power-law component appears to extrapolate from the GeV range to the lowest energies and is more intense than the Band component both below 50 keV and above 100MeV. The Band component undergoes substantial spectral evolution over the entire course of the burst, while the photon index of the power-law com- ponent remains constant for most of the prompt phase, then hardens significantly towards the end.