The H.E.S.S. gravitational wave rapid follow-up program

Abstract

Gravitational Wave (GW) events are physical processes that significantly perturbate space-time, e.g. compact binary coalescenses, causing the production of GWs. The detection of GWs by a worldwide network of advanced interferometers offer unique opportunities for multi-messenger searches and electromagnetic counterpart associations. While carrying extremely useful information, searches for associated electromagnetic emission are challenging due to large sky localisation uncertainties provided by the current GW observatories LIGO and Virgo. Here we present the methods and procedures used within the High Energy Stereoscopic System (H.E.S.S.) in searches for very-high-energy (VHE) gamma-ray emission associated to the emission of GWs from extreme events. To do so we create several algorithms dedicated to schedule GW follow-up observations by creating optimized pointing paterns. We describe algorithms using 2-dimensional GW localisation information and algorithms correlating the galaxy distribution in the local universe, by using galaxy catalogs, with the 3-dimensional GW localisation information and evaluate their performances. The H.E.S.S. automatic GW follow-up chain, described in this paper, is optimized to initiate GW follow-up observations within less than 1 minute after the alert reception. These developements allowed H.E.S.S. observations of 6 GW events out of the 67 non-retracted GW events detected during the first three observation runs of LIGO and Virgo reaching VHE γ-ray coverages of up to 70% of the GW localisation.

The H.E.S.S. Gravitational Wave Rapid Follow-up Program during O2 and O3

“…ToO Alert System is part of the H.E.S.S. Transients Follow-up System described in [8]. It is responsible for handling ToO alerts by assessing their potential interest to H.E.S.S., their visibility and the possibility of follow-up observations.…”

“…Moreover, the FoV-targeted 3D search with galaxies as seeds has a very quick computation time for prompt triggers. More details on this decision can be found in [8]. Finally, the latency of H.E.S.S.…”

“…gathered data on 6 GW event: GW170814, GW170817, S190512at, S190728q, S200115j [20] and S200224ca [21]. These observations are presented in [8]. GW170817 was observed during a short-term and a long-term follow-up campaigns.…”

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

“…are developed for H.E.S.S. and are extensively described in [20] and [21] alongside the pointing pattern of the observations described here. The requirement for BBH merger observations is that at least 50% of the localisation map is covered by the observations.…”

Searching for very-high-energy electromagnetic counterparts to gravitational-wave events with the Cherenkov Telescope Array

“…Several observation scheduling algorithms have been developed to derive optimal pointing patterns which cover the largest total GW uncertainty region possible, an approach based on [30]. These algorithms are part of realistic observation scheduling simulations, which include the consideration of visibility conditions of both the North and South sites, i.e.…”