Search for gamma-rays from the unusually bright GRB 130427A with the HAWC Gamma-ray Observatory

TL;DR

This study reports the first limits on very high-energy gamma-ray emission from GRB 130427A using HAWC, highlighting the potential for future detections with the full detector to study high-energy processes in gamma-ray bursts.

Contribution

The paper presents the first upper limits on >100 GeV emission from GRB 130427A with HAWC and discusses the detector's future capabilities for GRB observations.

Findings

No significant gamma-ray excess detected from GRB 130427A.

Current HAWC configuration could detect similar GRBs near zenith.

Full HAWC detector expected to detect 1-2 GRBs per year.

Abstract

The first limits on the prompt emission from the long gamma-ray burst (GRB) 130427A in the $>100\nobreakspace\rm{GeV}$ energy band are reported. GRB 130427A was the most powerful burst ever detected with a redshift $z\lesssim0.5$ and featured the longest lasting emission above $100\nobreakspace\rm{MeV}$. The energy spectrum extends at least up to $95\nobreakspace\rm{GeV}$, clearly in the range observable by the High Altitude Water Cherenkov (HAWC) Gamma-ray Observatory, a new extensive air shower detector currently under construction in central Mexico. The burst occurred under unfavourable observation conditions, low in the sky and when HAWC was running 10% of the final detector. Based on the observed light curve at MeV-GeV energies, eight different time periods have been searched for prompt and delayed emission from this GRB. In all cases, no statistically significant excess of counts has been found and upper limits have been placed. It is shown that a similar GRB close to zenith would be easily detected by the full HAWC detector, which will be completed soon. The detection rate of the full HAWC detector may be as high as one to two GRBs per year. A detection could provide important information regarding the high energy processes at work and the observation of a possible cut-off beyond the $\mathit{Fermi}$-LAT energy range could be the signature of gamma-ray absorption, either in the GRB or along the line of sight due to the extragalactic background light.