The average GeV-band Emission from Gamma-Ray Bursts

TL;DR

This study uses non-standard analysis to reveal that faint gamma-ray bursts emit significant GeV-band flux, lasting longer than keV emission, and contributes to understanding their role in cosmic ray energy flux.

Contribution

It introduces a method to estimate average GeV emission from faint GRBs by summing over many events, revealing their collective flux and spectral properties.

Findings

Faint GRBs emit significant GeV flux on average.

GeV emission lasts longer than keV emission.

The total energy flux from GRBs in GeV is a small fraction of cosmic ray flux.

Abstract

We analyze the emission in the 0.3-30 GeV energy range of Gamma-Ray Bursts detected with the Fermi Gamma-ray Space Telescope. We concentrate on bursts that were previously only detected with the Gamma-Ray Burst Monitor in the keV energy range. These bursts will then be compared to the bursts that were individually detected with the Large Area Telescope at higher energies. To estimate the emission of faint GRBs we use non-standard analysis methods and sum over many GRBs to find an average signal which is significantly above background level. We use a subsample of 99 GRBs listed in the Burst Catalog from the first two years of observation. Although mostly not individually detectable, the bursts not detected by the Large Area Telescope on average emit a significant flux in the energy range from 0.3 GeV to 30 GeV, but their cumulative energy fluence is only 8% of that of all GRBs. Likewise, the GeV-to-MeV flux ratio is less and the GeV-band spectra are softer. We confirm that the GeV-band emission lasts much longer than the emission found in the keV energy range. The average allsky energy flux from GRBs in the GeV band is 6.4*10^-4 erg cm^-2 yr^-1 or only 4% of the energy flux of cosmic rays above the ankle at 10^18.6 eV.