H.E.S.S. observations of soft spectrum VHE gamma-ray emission from the recurrent nova RS Ophiuchi

ATel #14857; Stefan J. Wagner, for the H. E.S. S. collaboration
on 12 Aug 2021; 23:03 UT
Credential Certification: Stefan J. Wagner (swagner@lsw.uni-heidelberg.de)

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Following the detection of very-high-energy gamma-ray emission the recurrent nova RS Ophiuchi currently in outburst (ATel #14844), H.E.S.S. observations continued on August 10th and August 11th for about 5h each night. A continued VHE gamma-ray excess compatible with the direction of RS Ophiuchi is seen in these two nights, with comparable significance to the night of August 9 (ATel #14844). The Fermi/LAT collaboration (Cheung et al, ATel #14845) reports a brightening of the nova in >100 MeV gamma-rays with a hard spectrum (single power law spectral index 1.9 +/- 0.1) up to ~10 GeV. In the VHE energy band covered with the H.E.S.S. telescopes, for the three nights August 9-11, a preliminary onsite analysis shows a significantly softer power law spectral index (> 3) than that reported by Fermi-LAT. This implies that a spectral break occurs between the >100 MeV to ~13 GeV range of the Fermi-LAT data and the higher energy range of the H.E.S.S. data. Further H.E.S.S. observations are planned. We strongly encourage follow-up at all wavelengths (see also ATels #14834, #14838, #14840, #14845, #14846, #14848, #14849, #14850, #14851, #14852, #14855). 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, France, Germany, Ireland, Japan, the Netherlands, Poland, South Africa, Sweden, UK, and the host country, Namibia. For more details see https://www.mpi-hd.mpg.de/hfm/HESS/

H.E.S.S.