TITLE: GCN CIRCULAR NUMBER: 21673 SUBJECT: LIGO/Virgo G297595: H.E.S.S. very-high energy gamma-ray follow-up DATE: 17/08/24 10:25:01 GMT FROM: Fabian Schussler at HESS Observatory M. de Naurois on behalf of the H.E.S.S. collaboration The H.E.S.S. array of imaging atmospheric Cherenkov telescopes [1] was used to carry out follow-up observations of the GW trigger G297595 (GCN #21474) using the LALInference map (GCN #21493). Observations started August 17 at 00:10 UTC and a total of 11 pointings were obtained over three consecutive nights. Each of the observations has a sensitivity of about 20% of the flux from the Crab nebula at 5 sigma. The observations cover about 80% of the LALInference localisation probability. A preliminary analysis with an energy threshold of about 500GeV did not reveal significant gamma-ray emission in any of the observed fields (within a field-of-view of ~2.5deg radius around the coordinates given below). Further analyses of the data are on-going. The following regions-of-interests were observed. Given is the start time in UTC for each of the typically 28min long observations and the center of the ROI. Time (UTC) RA Dec 2017-08-17 00:10 2h38m19s -48d43m12s 2017-08-17 00:40 3h03m07s -45d35m24s 2017-08-18 00:10 2h36m58s -51d15m59s 2017-08-18 00:40 3h05m57s -42d45m49s 2017-08-18 01:10 3h13m53s -39d46m14s 2017-08-18 23:55 2h34m20s -46d11m42s 2017-08-19 00:25 2h59m24s -48d23m28s 2017-08-19 00:55 3h16m57s -37d07m49s 2017-08-19 01:25 2h49m10s -46d15m52s 2017-08-19 01:55 3h18m53s -34d30m06s 2017-08-19 02:25 3h19m28s -42d22m29s H.E.S.S. is an array of five imaging atmospheric Cherenkov telescopes for the detection of very-high-energy gamma-ray sources and is located in the Khomas Highlands in Namibia. It was constructed and is operated by researchers from Armenia, Australia, Austria, the Czech Republic, France, Germany, Ireland, Netherlands, Japan, Poland, South Africa, Sweden, UK, and the host country, Namibia. For more details see https://www.mpi-hd.mpg.de/hfm/HESS/