H.E.S.S. follow-up observations of Binary Black Hole Coalescence events during the second and third Gravitational Waves observing runs of Advanced LIGO and Advanced Virgo

TL;DR

H.E.S.S. conducted follow-up observations of four binary black hole mergers during LIGO/Virgo runs O2 and O3, setting upper limits on very high energy gamma-ray emission and preparing for increased detection during O4.

Contribution

This study reports the first systematic follow-up of BBH mergers by H.E.S.S., establishing upper limits on gamma-ray emission and demonstrating the potential for future observations in O4.

Findings

No significant gamma-ray signals detected from the four GW events.

H.E.S.S. covered 35-75% of the localization regions for each event.

Upper limits on gamma-ray luminosity were set, below 10^45 erg/s.

Abstract

We report on the observations of four well-localized binary black hole (BBH) mergers by the High Energy Stereoscopic System (H.E.S.S.) during the second and third observing runs of Advanced LIGO and Advanced Virgo, O2 and O3. H.E.S.S. can observe $\mathrm{20\,deg^2}$ of the sky at a time and follows up gravitational-wave (GW) events by ``tiling'' localization regions to maximize the covered localization probability. During O2 and O3, H.E.S.S. observed large portions of the localization regions, between 35\% and 75\%, for four BBH mergers (GW170814, GW190512\_180714, GW190728\_064510, and S200224ca). For these four GW events, we find no significant signal from a pointlike source in any of the observations, and set upper limits on the very high energy ($>$100 GeV) $\gamma$-ray emission. The 1-10 TeV isotropic luminosity of these GW events is below $10^{45}$ erg s$^{-1}$ at the times of the H.E.S.S. observations, around the level of the low-luminosity GRB 190829A. Assuming no changes are made to how follow-up observations are conducted, H.E.S.S. can expect to observe over 60 GW events per year in the fourth GW observing run, O4, of which eight would be observable with minimal latency.