Limits to the fraction of high-energy photon emitting gamma-ray bursts

TL;DR

This study analyzes gamma-ray bursts detected by Fermi and Swift, revealing that only a small fraction emit high-energy photons above 100 MeV, with an upper limit of about 9% of all GRBs.

Contribution

The paper provides the first constraints on the fraction of GRBs emitting high-energy photons above 100 MeV using combined Fermi and Swift data and image co-adding techniques.

Findings

High-energy photon emission in GRBs is limited to roughly 9% of events.

A small but significant high-energy photon signal was detected in previously uncorrelated GRBs.

The distribution of high-energy photons among GRBs follows a power-law.

Abstract

After almost 4 years of operation, the two instruments onboard the Fermi Gamma-ray Space Telescope have shown that the number of gamma-ray bursts with high energy photon emission above 100 MeV cannot exceed roughly 9% of the total number of all such events, at least at the present detection limits. In a recent paper (Zheng et al. 2012c), we found that GRBs with photons detected in the Large Area Telescope (LAT) have a surprisingly broad distribution with respect to the photon number above background. Extrapolation of our empirical fit to numbers of photons below our quoted detection limit suggests that the overall rate of such events could be determined by standard image co-adding techniques. In this case, we have taken advantage of the excellent angular resolution of the Swift mission to provide accurate reference points for 79 GRB events which have eluded any previous correlations with high energy photons. We find a small but significant signal. Guided by the power law fit obtained previously for the number distribution of GRBs, the data suggests that only a small fraction of GRBs are sources of high energy photons.