Joint Center for Particle Nuclear Physics and Cosmology

Nanjing, China

Joint Center for Particle Nuclear Physics and Cosmology

Nanjing, China
SEARCH FILTERS
Time filter
Source Type

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..

Loading Joint Center for Particle Nuclear Physics and Cosmology collaborators
Loading Joint Center for Particle Nuclear Physics and Cosmology collaborators