H.E.S.S. Observations of the Prompt and Afterglow Phases of GRB 060602B

TL;DR

This paper reports the first simultaneous VHE gamma-ray observations of a GRB using H.E.S.S., finding no signal and setting flux upper limits, with implications for understanding the burst's nature and emission mechanisms.

Contribution

It presents the first simultaneous VHE gamma-ray observation of a GRB with H.E.S.S., providing flux limits and discussing the burst's possible Galactic or cosmological origin.

Findings

No VHE gamma-ray signal detected during observations.

Flux upper limit set at 2.9x10^-9 erg cm^-2 s^-1 for the prompt phase.

The burst is likely a Galactic X-ray burster, but a cosmological origin cannot be excluded.

Abstract

We report on the first completely simultaneous observation of a gamma-ray burst (GRB) using an array of Imaging Atmospheric Cherenkov Telescopes which is sensitive to photons in the very-high-energy (VHE) gamma-ray range (>~100 GeV). On 2006 June 2, the Swift Burst Alert Telescope (BAT) registered an unusually soft gamma-ray burst (GRB 060602B). The burst position was under observation using the High Energy Stereoscopic System (H.E.S.S.) at the time the burst occurred. Data were taken before, during, and after the burst. A total of 5 hours of observations were obtained during the night of 2006 June 2-3, and 5 additional hours were obtained over the next 3 nights. No VHE gamma-ray signal was found during the period covered by the H.E.S.S. observations. The 99% confidence level flux upper limit (>1 TeV) for the prompt phase (9s) of GRB 060602B is 2.9x10^-9 erg cm^-2 s^-1. Due to the very soft BAT spectrum of the burst compared to other Swift GRBs and its proximity to the Galactic center, the burst is likely associated with a Galactic X-ray burster, although the possibility of it being a cosmological GRB cannot be ruled out. We discuss the implications of our flux limits in the context of these two bursting scenarios.