The H.E.S.S. Gravitational Wave and Gamma-Ray Burst Follow-Up Programs

TL;DR

The paper describes the H.E.S.S. Transient Follow-Up Program for rapid multi-messenger observations of GRBs and GWs, detailing an automated ToO pipeline that ingests public alerts from Fermi, Swift, and LVK sources, and promptly repoints the array when observable. It documents follow-up statistics for GRBs since 2020 and GW alerts during LVK Run O4, highlighting trigger origins, follow-up rates, and notable cases. Two concrete examples—GW S240422ed and GRB 240809A—illustrate the end-to-end workflow from alert reception to analysis and upper-limit derivation, including multi-wavelength context and EBL considerations. The paper also outlines future improvements, such as leveraging CT5 to lower energy thresholds and integrating new X-ray missions, to enhance the likelihood of detecting VHE counterparts and advancing multi-messenger astrophysics.

Abstract

Multi-wavelength and multi-messenger astrophysics have experienced rapid growth over the past decade, seeking a complete picture of different cosmic phenomena. Transient sources, in particular, benefit from the input of multi-messenger observations, offering complementary perspectives on the same event while maximizing the detection probability of a rapidly fading signal. In this context, Gravitational Wave (GW) detections serve as perfect triggers for potential counterpart detections. Notably, a GW alert could be associated with a Gamma-Ray Burst (GRB), jetted cataclysmic events produced either by the collision of a binary neutron star system or a core-collapse supernova. These sources also radiate across the electromagnetic spectrum, allowing detection by X- and gamma-ray instruments aboard various satellites and thus enabling multi-wavelength triggering opportunities. The strong interest in minimizing reaction time to capture the full-time evolution of the emission, together with the often challenging localization uncertainties of the alerts, underscores the need for rapid and well-coordinated follow-up programs such as the one developed by the H.E.S.S. Collaboration. This contribution will give an overview of the transient follow-up strategy carried out by the H.E.S.S. Collaboration, from the external alert trigger and the automatic reaction of the observatory to the various analysis steps of the obtained observations. To illustrate this comprehensive strategy, we will show two examples of follow-up observations of both GRBs and GWs, highlighting key results and challenges in the search for an associated high-energy gamma-ray emission.

Figures (5)

• Figure 1: Schematic view of the H.E.S.S. transient follow-up system, illustrating the steps taken following the reception of an alert from multi-wavelength and multi-messenger observatories. Inspired from hess_gw). More details on each of the system blocks are available in the dedicated paper hess_follow_up_system.
• Figure 2: Left: Pie chart describing all alerts that passed H.E.S.S. trigger criteria from 2020 to June 2025. From the 160 passed alerts, 113 were followed by the array whereas 47 were cancelled for various reasons (see text). Right: Scatter plot of H.E.S.S. exposure time versus GRB observation delay, with marginal histograms showing the distributions of both variables. Marker shapes indicate redshift information, and colors represent the alerting instrument. The first bin in the observation delay distribution corresponds to automatically triggered observations, consistent with the $\sim$15% duty cycle of IACTs.
• Figure 3: Left: Multi-wavelength situation with possible counterparts of S240422ed on April 27th. Some of the possible counterparts were already ruled out at the moment. H.E.S.S. pointings tried to cover the most promising counterparts, especially X-ray sources. Right: Evolution of the multi-wavelength situation on May 2nd after five full nights of observations. Full H.E.S.S. pointings are represented, as well as new candidates and rejections from the intial ones.
• Figure 4: Excess and significance maps derived from H.E.S.S. observations of the regions around GRB 240809A. The inner circles illustrate the size of the H.E.S.S. point spread function.
• Figure 5: Left: SED of GRB 240809A comprising Swift-XRT flux extracted using the time-sliced analysis tool from the Swift burst analyzer and H.E.S.S. upper limits computed on the same time range. Right: Light-curve with automatic Swift-XRT data extracted using swifttools and H.E.S.S. integral upper limits for an energy range of [0.17-0.52 TeV]. Multiple XRT observations around the H.E.S.S. observations $\pm$ 4h are then combined and refit (dark blue, 1$\sigma$ uncertainty).