Milagro Constraints on Very High Energy Emission from Short Duration Gamma-Ray Bursts

TL;DR

This study searched for very high energy gamma-ray emission from short-duration gamma-ray bursts using Milagro data but found no significant signals, setting limits on their VHE fluence and constraining GRB emission models.

Contribution

First search for >100 GeV emission from short GRBs with Milagro, providing fluence limits and implications for GRB mechanisms and EBL absorption effects.

Findings

No significant VHE emission detected from 17 short GRBs.

Fluence limits set for GRBs at various redshifts, especially z<0.5.

Constraints on GRB models based on non-detection at VHE energies.

Abstract

Recent rapid localizations of short, hard gamma-ray bursts (GRBs) by the Swift and HETE satellites have led to the observation of the first afterglows and the measurement of the first redshifts from this type of burst. Detection of >100 GeV counterparts would place powerful constraints on GRB mechanisms. Seventeen short duration (< 5 s) GRBs detected by satellites occurred within the field of view of the Milagro gamma-ray observatory between 2000 January and 2006 December. We have searched the Milagro data for >100 GeV counterparts to these GRBs and find no significant emission correlated with these bursts. Due to the absorption of high-energy gamma rays by the extragalactic background light (EBL), detections are only expected for redshifts less than ~0.5. While most long duration GRBs occur at redshifts higher than 0.5, the opposite is thought to be true of short GRBs. Lack of a detected VHE signal thus allows setting meaningful fluence limits. One GRB in the sample (050509b) has a likely association with a galaxy at a redshift of 0.225, while another (051103) has been tentatively linked to the nearby galaxy M81. Fluence limits are corrected for EBL absorption, either using the known measured redshift, or computing the corresponding absorption for a redshift of 0.1 and 0.5, as well as for the case of z=0.