FERMI OBSERVATIONS OF GRB 090510: A SHORT-HARD GAMMA-RAY BURST WITH AN ADDITIONAL, HARD POWER-LAW COMPONENT FROM 10 keV TO GeV ENERGIES
Ackermann, M. ; Asano, K. ; Atwood, W. B. ;
Jun - 2010
ISSN : 0004-637X ;
journal : The Astrophysical Journal

Issue : 2
type: Article Journal

Abstract
We present detailed observations of the bright short-hard gamma-ray burst GRB 090510 made with the Gamma-ray Burst Monitor (GBM) and Large Area Telescope (LAT) on board the Fermi observatory. GRB 090510 is the first burst detected by the LAT that shows strong evidence for a deviation from a Band spectral fitting function during the prompt emission phase. The time-integrated spectrum is fit by the sum of a Band function with E peak = 3.9 ± 0.3 MeV, which is the highest yet measured, and a hard power-law component with photon index 1.62 ± 0.03 that dominates the emission below H20 keV and above H100 MeV. The onset of the high-energy spectral component appears to be delayed by <0.1 s with respect to the onset of a component well fit with a single Band function. A faint GBM pulse and a LAT photon are detected 0.5 s before the main pulse. During the prompt phase, the LAT detected a photon with energy 30.5+5.8 2.6 GeV, the highest ever measured from a short GRB. Observation of this photon sets a minimum bulk outflow Lorentz factor, “s 1200, using simple ³³ opacity arguments for this GRB at redshift z = 0.903 and a variability timescale on the order of tens of ms for the H100 keV-few MeV flux. Stricter high confidence estimates imply “ s 1000 and still require that the outflows powering short GRBs are at least as highly relativistic as those of long-duration GRBs. Implications of the temporal behavior and power-law shape of the additional component on synchrotron/synchrotron self-Compton, external-shock synchrotron, and hadronic models are considered.

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