FERMI constraints on the high energy, ~1 GeV, emission of long gamma ray bursts
Abstract
Aims: We investigate the constraints imposed on the luminosity function (LF) of long duration gamma ray bursts (LGRBs) by the flux distribution of bursts detected by the GBM at 1 MeV, and the implications of the non detection of the vast majority, 95%, of the LGRBs at higher energy, 1 GeV, by the LAT detector.
Methods: We find a LF that is consistent with those determined by BATSE and Swift. The non detections by LAT set upper limits on the ratio R of the prompt fluence at 1 GeV to that at 1 MeV. The upper limits are more stringent for brighter bursts, with upper limits on R<{0.1,0.3,1} for {5,30,60}% of the bursts. This implies that for most bursts the prompt 1 GeV emission may be comparable to the 1 MeV emission, but can not dominate it. The value of R is not universal: the measured values and upper limits imply that R ranges over (at least) an order of magnitude around 0.1. For several bright bursts with reliable determination of the photon spectral index at 1 MeV, the LAT non detection implies an upper limit to the 100 MeV flux which is <0.1 of the flux obtained by extrapolating the 1 MeV flux to high energy.
Results: For the widely accepted models, in which the 1 MeV power-law photon spectrum reflects the power-law energy distribution of fast cooling electrons, this suggests that either the electron energy distribution does not follow a power-law over a wide energy range, or that the high energy photons are absorbed. Requiring an order unity pair production optical depth at 100 MeV sets an upper limit for the Lorentz factor, Γ ⪉ 102.5.
- Publication:
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Astronomy and Astrophysics
- Pub Date:
- January 2011
- DOI:
- 10.1051/0004-6361/201014344
- arXiv:
- arXiv:1003.0566
- Bibcode:
- 2011A&A...525A..53G
- Keywords:
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- gamma rays: general;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 12 pages, 6 figures. Submitted to A&