A Search for Short Duration Very High Energy Emission from Gamma-Ray Bursts

Abstract

Milagro is a water-Cherenkov detector capable of observing air showers produced by gamma rays with primary energies of approximately 100 GeV and higher. The wide field of view (~ 2 sr) and high duty cycle (>90%) of Milagro make it ideal for searching for transient emission from gamma-ray bursts (GRBs). The median energy of photons detected by Milagro is a few TeV, but the effective area is still relatively large at a few hundred GeV (~50 m^2 at 100 GeV). This results in a gamma-ray fluence sensitivity comparable to previous satellite detectors at keV energies. Measurements have been made of GRB spectra up to a few tens of GeV with no sign of a cutoff, however much is still unknown about the nature and existence of this Very High Energy (VHE) component. Additionally, gamma/gamma absorption from infrared background photons or from the optically thick region of the burst source complicate observations of this VHE component. However, many models predict VHE emission from GRBs through mechanisms such as synchrotron self-Compton processes. In the absence of a GRB localization provided by another instrument, the Milagro data is searched independently for VHE emission from GRBs. In 2.3 years of searching for bursts with durations ranging from 250 us to 40 s, no significant evidence was observed for VHE emission from GRBs. Models for different GRB parameters (such as redshift and isotropic energy distributions) are used to constrain the VHE spectrum of GRBs.