Prospects for GRB detection with HAWC scalers

Abstract

The High Altitude Water Cherenkov (HAWC) experiment is a high energy gamma ray observatory which is currently under construction in the Sierra Negra, Mexico, at an altitude of 4100 m. It will detect gamma-ray induced atmospheric particle showers on the ground. The water Cherenkov particle detection technique allows for full day observations of a large fraction of the sky. HAWC is the successor of Milagro. Because of the higher altitude, size and improved detector design, the sensitivity of HAWC will be ≈15× higher than the sensitivity of Milagro. Besides a triggered data acquisition system (DAQ) searching for individual air shower events, a scaler DAQ counting all signals in the HAWC photomultiplier tubes (PMTs) above a fixed threshold is available. The large sky coverage of about 1.5 sr and high duty cycle of HAWC increases its detection potential for transient sources like gamma ray bursts (GRBs). The sensitive energy range of the scaler DAQ overlaps at low energies with the energy range of the Large Area Telescope (LAT) aboard the Fermi satellite. We demonstrate that GRBs with a high energy spectral component ( E γ ≳ 30 GeV) seen by the LAT are also visible for the HAWC scaler DAQ. The properties of the GRB spectra at high energies provide information about e ±-pair attenuation inside of GRB jets, extra-galactic background light (EBL) absorption and the maximum energy to which particles can be accelerated in GRBs.