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Ackermann M.,Kavli Institute for Particle Astrophysics and Cosmology | Asano K.,Tokyo Institute of Technology | Atwood W.B.,University of California at Santa Cruz | Axelsson M.,University of Stockholm | And 225 more authors.
Astrophysical Journal | Year: 2010

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 ± 0.3MeV, which is the highest yet measured, and a hard power-law component with photon index -1.62 ± 0.03 that dominates the emission below ≈ 20keV and above ≈ 100MeV. The onset of the high-energy spectral component appears to be delayed by ≈ 0.1s 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 0.5s before the main pulse. During the prompt phase, the LAT detected a photon with energy 30.5+5.8 -2.6GeV, the highest ever measured from a short GRB. Observation of this photon sets a minimum bulk outflow Lorentz factor, Γ≳1200, using simple γγ opacity arguments for this GRB at redshift z = 0.903 and a variability timescale on the order of tens of ms for the ≈ 100keV-few MeV flux. Stricter high confidence estimates imply Γ ≳ 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. © 2010. The American Astronomical Society. All rights reserved.


Ackermann M.,Kavli Institute for Particle Astrophysics and Cosmology | Ajello M.,Kavli Institute for Particle Astrophysics and Cosmology | Baldini L.,National Institute of Nuclear Physics, Italy | Ballet J.,University Paris Diderot | And 226 more authors.
Astrophysical Journal Letters | Year: 2010

The Fermi observatory is advancing our knowledge of gamma-ray bursts (GRBs) through pioneering observations at high energies, covering more than seven decades in energy with the two on-board detectors, the Large Area Telescope (LAT) and the Gamma-ray Burst Monitor (GBM). Here, we report on the observation of the long GRB 090217A which triggered the GBM and has been detected by the LAT with a significance greater than 9σ. We present the GBM and LAT observations and on-ground analyses, including the time-resolved spectra and the study of the temporal profile from 8 keV up to 1 GeV. All spectra are well reproduced by a Band model. We compare these observations to the first two LAT-detected, long bursts GRB 080825C and GRB 080916C. These bursts were found to have time-dependent spectra and exhibited a delayed onset of the high-energy emission, which are not observed in the case of GRB 090217A. We discuss some theoretical implications for the high-energy emission of GRBs. © 2010. The American Astronomical Society. All rights reserved..

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