[Paper Review] Searches for TeV gamma-ray counterparts to Gravitational Wave events with H.E.S.S

H. Ashka, Schüssler, Fabian+1 more|arXiv (Cornell University)|Jun 25, 2019

Gamma-ray bursts and supernovae4 citations

TL;DR

This study presents the first VHE gamma-ray follow-up observations of two gravitational wave events—GW170814 (binary black hole merger) and GW170817 (binary neutron star merger)—using the H.E.S.S. Cherenkov telescope array. Despite no significant gamma-ray emission detected, the analysis establishes the first integral upper limits on non-thermal TeV emission from these remnants, providing critical constraints for multi-messenger astrophysics models.

ABSTRACT

The search for electromagnetic counterparts for gravitational waves events is one of the main topics of multi-messenger Astrophysics. Among these searches is the one for high energy gamma-ray emission with the H.E.S.S. Imaging Atmospheric Cherenkov Telescopes in Namibia. During their second Observation Run O2, the Advanced Virgo detector in Italy and the two advanced LIGO detectors in Washington and Louisiana while conducting joint observations, detected for the first time, on August $14^{th}$, 2017 a transient GW signal due to the coalescence of two stellar masses black holes, an event labeled GW170814. The alert announcing the event was issued two hours later and H.E.S.S. observations could be scheduled for the nights of $16^{th}$, $17^{th}$ and $18^{th}$ August 2017. Three days after the binary BH merger, on August $17^{th}$, the coalescence of two neutron star was detected for the first time, followed by a GRB detection by Fermi's GBM starting a new era in multi-messenger Astronomy. Observations started 5.3 h after the merge and contained the counterpart SSS17a that was identified several hours later. It stands as the first data obtained by a ground-based pointing instrument on this object. In this contribution, we will present the results of the search of high-energy gamma ray emission as electromagnetic counterpart of these two GW events. No significant gamma ray emission was detected for either event. Nevertheless upper limit maps were derived constraining, for the first time, the non-thermal, high-energy emission on the remnant of a three detector binary black hole coalescence (GW170814), and a binary neutron star coalescence (GW170817).

Motivation & Objective

• To search for very-high-energy (VHE) gamma-ray counterparts to gravitational wave events detected by LIGO and Virgo during O2.
• To constrain non-thermal emission from the remnants of binary black hole and binary neutron star coalescences using ground-based VHE gamma-ray telescopes.
• To evaluate the performance and response efficiency of the H.E.S.S. array in reacting to GW alerts in real time.
• To provide the first VHE gamma-ray upper limits for a three-detector binary black hole merger (GW170814) and a binary neutron star merger (GW170817).

Proposed method

• H.E.S.S. conducted follow-up observations using its 28-meter and four 12-meter telescopes, sensitive to gamma rays from 50 GeV to 100 TeV.
• Observations were scheduled based on LALInference-based sky localization maps issued after GW triggers, with rapid response enabled by automated alert processing.
• Data were analyzed using standard H.E.S.S. analysis pipelines to produce significance maps and integral flux upper limits.
• Upper limits were computed for the energy range 250 GeV < E < 20 TeV assuming a generic E⁻² spectrum, accounting for angular dependence and telescope acceptance.
• The final 90% credible region for GW170814 was used to validate coverage, despite initial scheduling based on an earlier, less precise map.
• The H.E.S.S. reaction time was under 5 minutes after the LALInference map release for GW170817, enabling the first ground-based VHE observations of a neutron star merger.

Experimental results

Research questions

• RQ1 What is the level of very-high-energy gamma-ray emission from the remnant of a binary black hole merger detected by three-observatory LIGO-Virgo?
• RQ2 Can ground-based VHE gamma-ray telescopes detect electromagnetic counterparts to gravitational wave events within minutes of the trigger?
• RQ3 What are the first integral upper limits on non-thermal TeV emission from a binary neutron star merger?
• RQ4 How effectively can the H.E.S.S. array cover the localized regions of GW events with minimal latency?
• RQ5 To what extent do systematic shifts in GW localization maps affect the sensitivity of follow-up observations?

Key findings

• No significant VHE gamma-ray emission was detected during follow-up observations of GW170814 or GW170817.
• H.E.S.S. achieved a reaction time of less than 5 minutes after the LALInference sky map was released for GW170817, enabling the first ground-based VHE observations of a neutron star merger.
• For GW170814, the H.E.S.S. observations covered approximately 90% of the final 90% credible region despite initial scheduling based on a less precise localization map.
• Integral upper limits on non-thermal emission were derived for the energy range 250 GeV < E < 20 TeV, marking the first such constraints for a three-detector binary black hole merger.
• The upper limit map for GW170814 provides the first empirical constraint on TeV emission from a stellar-mass black hole binary remnant.
• The results from GW170817 set stringent upper limits in the 270 GeV to 8.55 TeV range, significantly constraining models of non-thermal emission from neutron star mergers.