[Paper Review] Gravitational wave alert follow-up strategy in the H.E.S.S. multi-messenger framework

M. Seglar-Arroyo, F. Schüßler|arXiv (Cornell University)|May 29, 2017

Pulsars and Gravitational Waves Research1 references5 citations

TL;DR

This paper presents an optimized gravitational wave (GW) alert follow-up strategy for the H.E.S.S. high-energy gamma-ray observatory, using galaxy catalogues, time-dependent sky visibility, and automated processing of GW uncertainty maps to prioritize pointing directions. It demonstrates that the 'galaxies-in-FoV' approach achieves high probability coverage with fewer pointings, while the 'one-galaxy' method enables rapid automatic response, significantly improving multi-messenger detection prospects for binary neutron star mergers.

ABSTRACT

The H.E.S.S. high-energy gamma-ray observatory is member of the Virgo/LIGO electromagnetic follow-up effort since early 2014. Its capability for transient follow-up studies benefits from its large field of view, rapid response time and high sensitivity. Drawing from the experience gained from other science cases like gamma-ray bursts and high-energy neutrino follow-ups we demonstrate the high perspectives for new types of analyses like the search for gravitational wave counterparts and the study of multi-messenger signals from binary neutron star mergers. This contribution aims to present the potential pointing strategy that the H.E.S.S. observatory would carry out following an alert from gravitational wave observatories. We will discuss several key points like the use of information from a galaxy catalogue, the time-dependent visibility of sky regions and the automatic handling of gravitational wave uncertainty maps, that will enable an optimized choice of the pointing directions. Finally, based on simulated binary neutron star mergers, the performance of the outlined gravitational wave-alert observations will be presented.

Motivation & Objective

• To develop an automated, efficient follow-up strategy for H.E.S.S. to respond to gravitational wave alerts from LIGO/Virgo.
• To improve the probability of detecting electromagnetic counterparts to GW events, especially from binary neutron star mergers.
• To integrate galaxy distribution data and time-dependent visibility constraints into the observation planning process.
• To evaluate and compare different pointing algorithms for optimal coverage under realistic observational constraints.
• To enable rapid, automated response during observing windows while supporting detailed offline scheduling for non-triggered events.

Proposed method

• Utilizes the GLADE galaxy catalogue to prioritize sky regions with high galaxy density, increasing the likelihood of detecting a counterpart.
• Processes gravitational wave uncertainty maps using time-dependent visibility functions to exclude non-observable sky regions at any given time.
• Implements two main algorithms: 'one-galaxy' for fast, automatic response (within minutes), and 'galaxies-in-FoV' for optimized probability coverage.
• Calculates total probability coverage based on both GW localization uncertainty and galaxy distribution to guide pointing decisions.
• Automates the entire chain at the H.E.S.S. site in Namibia to minimize response time and maximize detection efficiency.
• Validates performance using simulated binary neutron star merger events with realistic sky localization and galaxy distribution.

Experimental results

Research questions

• RQ1 How can H.E.S.S. maximize the probability of detecting a high-energy gamma-ray counterpart to a gravitational wave event?
• RQ2 What is the optimal trade-off between speed of response and probability coverage in GW follow-up observations?
• RQ3 How do visibility constraints and galaxy distribution affect the selection of pointing directions for H.E.S.S.?
• RQ4 Can automated algorithms achieve high probability coverage with minimal pointing time while respecting observatory constraints?
• RQ5 How do different strategies (one-galaxy vs. galaxies-in-FoV) compare in terms of coverage efficiency and computational cost?

Key findings

• The 'galaxies-in-FoV' strategy achieves significantly higher probability coverage per pointing compared to the 'one-galaxy' method, requiring fewer observations to reach high coverage.
• The one-galaxy approach enables fully automated, rapid response within minutes, suitable for real-time alert processing during observing nights.
• Sky visibility constraints limit coverage, especially at low elevations, and must be integrated into the pointing strategy to avoid observing unobservable regions.
• Simulations show that with current GW localization uncertainties (~100–1000 deg²), H.E.S.S. can achieve high probability coverage using optimized pointing strategies.
• Future improvements in GW localization (down to ~10–100 deg²) will further enhance detection prospects, making multi-messenger astronomy more effective.
• The combination of galaxy catalogue data and uncertainty map processing enables a robust, scalable framework for transient follow-up that can be extended to future observatories like CTA.