H.E.S.S. follow-up of BBH merger events

TL;DR

This paper reports on H.E.S.S. follow-up observations of four binary black hole merger events during LIGO/Virgo runs, setting upper limits on VHE gamma-ray emission and evaluating follow-up strategies.

Contribution

It provides the first detailed VHE gamma-ray upper limits for BBH mergers and assesses the effectiveness of follow-up strategies during gravitational wave observations.

Findings

No significant VHE emission detected from the four BBH events.

Derived upper limits on VHE luminosity for each event.

Evaluated and discussed the follow-up strategy for future GW observing runs.

Abstract

We present here, follow-up observations of four Binary black hole BBH events performed with the High Energy Stereoscopic System (H.E.S.S.) in the Very High Energy (VHE) gamma-ray domain during the second and third LIGO/Virgo observation runs. Detailed analyses of the obtained data did not show significant VHE emission. We derive integral upper limit maps considering a generic $E^{-2}$ source spectrum in the most sensitive H.E.S.S energy interval ranging from 1 to 10 TeV. We also consider Extragalactic Background Light absorption effects and derive integral upper limits over the full accessible energy range. We finally derive upper limits of the VHE luminosity for each event and compare them with the expected VHE emission from GRBs. These comparisons allow us to assess the H.E.S.S. gravitational wave follow-up strategies. For the fourth GW observing run O4, we do not expect to fundamentally alter our observing strategy, and will continue to prioritize sky coverage like for the previous runs