paper

Fermi Observations of GRB 090510: A Short Hard Gamma-Ray Burst with an Additional, Hard Power-Law Component from 10 keV to GeV Energies

arXiv:1005.2141 · doi:10.1088/0004-637X/716/2/1178

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 $\Epeak = 3.9\pm 0.3$\,MeV, which is the highest yet measured, and a hard power-law component with photon index that dominates the emission below \,20\,keV and above \,100\,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 GeV, the highest ever measured from a short GRB. Observation of this photon sets a minimum bulk outflow Lorentz factor, $Γ\ga$\,1200, using simple opacity arguments for this GRB at redshift and a variability time scale on the order of tens of ms for the \,100\,keV--few MeV flux. Stricter high confidence estimates imply $Γ\ga 1000$ 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 (SSC), external-shock synchrotron, and hadronic models are considered.

33 pages, 7 figures, Accepted for publication in ApJ. Contact Authors: James Chiang ([email protected]), Jonathon Granot ([email protected]), Sylvain Guiriec ([email protected]), Masanori Ohno ([email protected])

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