Constraining the High-Energy Emission from Gamma-ray Bursts with Fermi

TL;DR

This study analyzes 288 Fermi-detected gamma-ray bursts, setting upper limits on high-energy emission, revealing spectral breaks or steepening above the peak, and constraining the Lorentz factors of their relativistic jets.

Contribution

It provides the first comprehensive upper limits on GeV emission from a large sample of GRBs and investigates spectral properties and Lorentz factors using joint GBM and LAT data.

Findings

About half of GBM-detected bursts require spectral breaks or steepening above Epk.

Approximately 20% show evidence of high-energy spectral cut-offs, constraining Lorentz factors.

LAT-detected bursts tend to have higher Lorentz factors, indicating a wide distribution.

Abstract

We examine 288 GRBs detected by the Fermi Gamma-ray Space Telescope's Gamma-ray Burst Monitor (GBM) that fell within the field-of-view of Fermi's Large Area Telescope (LAT) during the first 2.5 years of observations, which showed no evidence for emission above 100 MeV. We report the photon flux upper limits in the 0.1-10 GeV range during the prompt emission phase as well as for fixed 30 s and 100 s integrations starting from the trigger time for each burst. We compare these limits with the fluxes that would be expected from extrapolations of spectral fits presented in the first GBM spectral catalog and infer that roughly half of the GBM-detected bursts either require spectral breaks between the GBM and LAT energy bands or have intrinsically steeper spectra above the peak of the {\nu}F{\nu} spectra (Epk). In order to distinguish between these two scenarios, we perform joint GBM and LAT spectral fits to the 30 brightest GBM-detected bursts and find that a majority of these bursts are indeed softer above Epk than would be inferred from fitting the GBM data alone. Approximately 20% of this spectroscopic subsample show statistically significant evidence for a cut-off in their high-energy spectra, which if assumed to be due to {\gamma}{\gamma} attenuation, places limits on the maximum Lorentz factor associated with the relativistic outflow producing this emission. All of these latter bursts have maximum Lorentz factor estimates that are well below the minimum Lorentz factors calculated for LAT- detected GRBs, revealing a wide distribution in the bulk Lorentz factor of GRB outflows and indicating that LAT-detected bursts may represent the high end of this distribution.