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We report on the observation of the bright, long gamma-ray burst, GRB090902B, by the Gamma-ray Burst Monitor (GBM) and Large Area Telescope (LAT)instruments on-board the Fermi observatory. This was one of the brightest GRBsto have been observed by the LAT, which detected several hundred photons duringthe prompt phase. With a redshift of z = 1.822, this burst is among the mostluminous detected by Fermi. Time-resolved spectral analysis reveals asignificant power-law component in the LAT data that is distinct from the usualBand model emission that is seen in the sub-MeV energy range. This power-lawcomponent appears to extrapolate from the GeV range to the lowest energies andis more intense than the Band component both below $\sim$ 50 keV and above 100MeV. The Band component undergoes substantial spectral evolution over theentire course of the burst, while the photon index of the power-law componentremains constant for most of the prompt phase, then hardens significantlytowards the end. After the prompt phase, power-law emission persists in the LATdata as late as 1 ks post-trigger, with its flux declining as $t^{-1.5}$. TheLAT detected a photon with the highest energy so far measured from a GRB,$33.4_{-3.5}^{+2.7}$ GeV. This event arrived 82 seconds after the GBM triggerand $\sim$ 50 seconds after the prompt phase emission had ended in the GBMband. We discuss the implications of these results for models of GRB emissionand for constraints on models of the Extragalactic Background Light.

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