# Searches for counterparts of gravitational waves at very high energies with H.E.S.S

## TL;DR

This paper discusses the strategies and results of using H.E.S.S. telescopes to search for high-energy gamma-ray counterparts to gravitational wave events, highlighting successful campaigns and future prospects.

## Contribution

It introduces optimized follow-up strategies for gravitational wave events with H.E.S.S. and reports on observational campaigns and their constraints on high-energy emission.

## Key findings

- Successful observations during GW170814 and GW170817 campaigns.
- Constraints on very-high-energy emission from gravitational wave events.
- Plans for improving follow-up strategies with H.E.S.S.

## Abstract

The recent milestones in multi-messenger astronomy have opened new ways to study the Unverse. The unprecedented gravitational wave (GW) follow-up campaigns established the power that the combination of different messengers has to identify and study the nature and evolution of astrophysical phenomena. Here we focus on the search for high-energy gamma ray emission as electromagnetic counterpart of compact binary coalescences with the H.E.S.S. Imaging Air Cherenkov Telescopes (IACTs). In this contribution, the optimized strategies developed specifically for the prompt follow-up of gravitational wave events with H.E.S.S are presented. As illustration, the successful observation campaigns up to this time will be described, including the ones during Observation Run O2 on the binary black hole (BH-BH) merger GW170814 and the binary neutron star (NS-NS) merger GW170817, and an update on recent events occurring during O3. Results of these searches are presented and the constraints that prompt observations can put on very-high-energy, non-thermal emission, are briefly discussed. Finally, an outlook on further improvements for the gravitational waves follow-up program with H.E.S.S. will be provided.