GRB Observations with H.E.S.S. II

C. Hoischen, A. Blazer, E. Bissaldi, M. Füßling, T. Garrigoux, D. Gottschalk, M. Holler, A. Mitchell, P. O'Brien, R. Parsons, G. Pühlhofer, G. Rowell, F. Schüssler, P. H. T. Tam, S. Wagner, for the H. E. S. S. Collaboration

Published 2017-08-03Version 1

The High Energy Stereoscopic System (H.E.S.S.) has been searching for counterparts of Gamma Ray Bursts (GRBs) for many years. In 2012 the system was upgraded with a fifth $28$ m diameter telescope (CT5) which is equipped with faster motors for rapid repointing, marking the start of the second phase of H.E.S.S. operation (H.E.S.S. II). CT5s large light collection area of $600\,{\rm m}^{2}$ improves the sensitivity to low-energy gamma-rays and even extends the energy range below $100$ GeV. The search for counterparts continues now in the energy range of tens of GeV to tens of TeV. A detection in this energy range would open a new window to the part of the spectrum of these highly energetic explosions which Fermi-LAT has only successfully detected in a reduced subset of events, with rather limited statistics. In the past years, H.E.S.S. has performed followup observations based on GRB detections by Swift-BAT and Fermi-GBM/-LAT. This Target of Opportunity observation program was carried out with a generalised Target of Opportunity Alert system. This contribution will highlight key features of the Target of Opportunity Alert system, present follow-up statistics of GRBs as well as detailed results of promising follow-up observations.