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

Milagro is a water‐Cherenkov detector capable of observing air showers produced by gamma rays with 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 GRBs.Measurements have been made of GRB spectra up to a few GeV with no sign of a cutoff, however much is still unknown about the nature and existence of this VHE component. While many models predict VHE emission from GRBs, gamma/gamma absorption from infrared background photons or the optically thick region of the burst source complicate observations.In three years of searching the Milagro data set for GRBs with durations ranging from 250μs to 40s, no significant evidence was found for VHE emission from GRBs. Models for GRB redshift and isotropic energy distributions are used to significantly constrain the VHE spectrum of GRBs.