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 offers 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.
Highlights
Here rely on this assumption and focus on Gravitational Wave (GW) follow-up strategies allowing for fast coverage of GW events directly after a GW detection
We present the methods and procedures used within H.E.S.S. for rapid searches for VHE γ-ray emission associated to GW events
The content of the distributed alert messages include a first classification into Compact Binary Coalescence (CBC) or Burst alert, depending on the detection pipeline, the detection time, the GW localisation map, and an event classification into categories of the initial system: binary black hole (BBH), binary neutron star (BNS), binaries comprising a neutron star and a black hole (NSBH) or signal due to terrestrial noise
Summary
Follow-up strategies are derived with the aim of covering the coordinates from which the GW signal was most probably emitted, and the associated multi-wavelength or multi-messenger counterpart emission, as fast as possible [20] and reaching deep observations with a low energy threshold. The observation schedule resulting from this technique can as well present important overlapping of covered regions, as it was the case for the Best-pixel algorithm Such galaxytargeted searches can be very performant for small FoV instruments, like optical and X-ray telescopes [22], the relatively large FoVs of IACTs motivate a further step in the selection of the observation coordinates. For 3D searches we use an enclosed region of 99% (instead of 90%) to make sure that we cover all galaxies at the edges of the GW skymaps
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