M. de Naurois on behalf of the H.E.S.S. collaboration
The H.E.S.S. array of imaging atmospheric Cherenkov telescopes was used to carry out follow-up observations of the GW trigger G298048 (GCN #21509) and monitoring of the possible counterpart SSS17a/DLT17ck (Coulter et al. GCN #21529; Yang et al., GCN #21531).
H.E.S.S. observations started August 17 at 18:00 UTC with three pointings derived from the HLV-Bayestar map (GCN #21513) without assumptions on the orientation of the system and cross correlated in three dimensions with the GLADE catalog . The first of these pointings covered the region of NGC 4993 and the potential EM counterpart SSS17a/DLT17ck. We continued to monitor the region around SSS17a/DLT17ck during the following nights until the location moved outside the H.E.S.S. visiblity window. The last pointing was taken August 22, 18:00 UTC.
Each of our observations has a sensitivity of about 20% of the flux from the Crab nebula at 5 sigma. A preliminary analysis with an energy threshold of about 500GeV did not reveal significant gamma-ray emission in any of the datasets. Further analyses of the data are on-going.
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/
 GLADE website: http://aquarius.elte.hu/glade/