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

Author(s): Ackermann M., Asano K., B. Atwood W., Axelsson M., Baldini L., Ballet J., Barbiellini G., Bellazzini R., Bruel P., M. Casandjian J., Charles E., Cohen-Tanugi J., Dumora D., Farnier C., J. Fegan S., A. Grenier I., Grondin M.-H., Guiriec Sylvain, Horan D., Knodlseder J., Lemoine-Goumard M., Lott B., Nuss E., Parent D., Pelassa V., Piron F., D. Scargle J., Sgro C., Tibaldo L., Vilchez N.

(Article) Published: The Astrophysical Journal / The Astrophysical Journal Letters, vol. 716 p.1178-1190 (2010)
Links openAccess full text : arxiv


Ref HAL: in2p3-00497160_v1
Ref Arxiv: 1005.2141
DOI: 10.1088/0004-637X/716/2/1178
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
246 citations
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 $-1.62\pm 0.03$ that dominates the emission below $\approx$\,20\,keV and above $\approx$\,100\,MeV. The onset of the high-energy spectral component appears to be delayed by $\sim$\,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, $\Gamma\ga$\,1200, using simple $\gamma\gamma$ opacity arguments for this GRB at redshift $z = 0.903$ and a variability time scale on the order of tens of ms for the $\approx$\,100\,keV--few MeV flux. Stricter high confidence estimates imply $\Gamma \ga 1000$ 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.



Comments: 33 pages, 7 figures, Accepted for publication in ApJ. Contact Authors: James Chiang (jchiang@slac.stanford.edu), Jonathon Granot (j.granot@herts.ac.uk), Sylvain Guiriec (sylvain.guiriec@lpta.in2p3.fr), Masanori Ohno (ohno@astro.isas.jaxa.jp)