arXiv subjects

Halim Ashkar

Publications and source records attributed to Halim Ashkar.

21 records · Page 2Linked to original sources

arXiv2019

H.E.S.S. searches for TeV gamma rays associated to high-energy neutrinos

The detection of an astrophysical flux of high-energy neutrinos by IceCube is a major step forward in the search for the origin of cosmic rays, as this emission is expected to originate in hadronic interactions taking place in or near cosmic-ray accelerators. No neutrino point sources, or significant correlation with known astrophysical objects, have been identified in the IceCube data so far. The hadronic interactions responsible for the neutrino emission should also lead to the production of high-energy gamma rays. The search for neutrino sources can then be performed by studying the spatial and temporal correlations between neutrino events and very high energy (VHE, E > 100 GeV) gamma rays. We report here on the search for VHE gamma-ray emission with the H.E.S.S. imaging air Cherenkov telescopes (IACTs) at the reconstructed position of muon neutrino events detected by IceCube. We will provide an up-to-date summary of the extensive program to perform prompt IACT observations of realtime IceCube neutrino event positions. A recent highlight of this program are the H.E.S.S. observations during the broad multi-wavelength campaign that followed the detection of the neutrino event IceCube-170922A arriving from a direction consistent with the location of a flaring gamma-ray blazar TXS 0506+056 in September 2017. We'll present the H.E.S.S. observations obtained within ~4h hours of the neutrino detection as well as a complementary search for gamma-ray emission at longer timescales and put them into the multi-wavelength and multi-messenger context.

astro-ph.HE↗

arXiv2019

Searches for counterparts of gravitational waves at very high energies with H.E.S.S

The recent milestones in multi-messenger astronomy have opened new ways to study the Unverse. The unprecedented gravitational wave (GW) follow-up campaigns established the power that the combination of different messengers has to identify and study the nature and evolution of astrophysical phenomena. Here we focus on the search for high-energy gamma ray emission as electromagnetic counterpart of compact binary coalescences with the H.E.S.S. Imaging Air Cherenkov Telescopes (IACTs). In this contribution, the optimized strategies developed specifically for the prompt follow-up of gravitational wave events with H.E.S.S are presented. As illustration, the successful observation campaigns up to this time will be described, including the ones during Observation Run O2 on the binary black hole (BH-BH) merger GW170814 and the binary neutron star (NS-NS) merger GW170817, and an update on recent events occurring during O3. Results of these searches are presented and the constraints that prompt observations can put on very-high-energy, non-thermal emission, are briefly discussed. Finally, an outlook on further improvements for the gravitational waves follow-up program with H.E.S.S. will be provided.

astro-ph.HE↗

arXiv2019

Searches for TeV gamma-ray counterparts to Gravitational Wave events with H.E.S.S

The search for electromagnetic counterparts for gravitational waves events is one of the main topics of multi-messenger Astrophysics. Among these searches is the one for high energy gamma-ray emission with the H.E.S.S. Imaging Atmospheric Cherenkov Telescopes in Namibia. During their second Observation Run O2, the Advanced Virgo detector in Italy and the two advanced LIGO detectors in Washington and Louisiana while conducting joint observations, detected for the first time, on August $14^{th}$, 2017 a transient GW signal due to the coalescence of two stellar masses black holes, an event labeled GW170814. The alert announcing the event was issued two hours later and H.E.S.S. observations could be scheduled for the nights of $16^{th}$, $17^{th}$ and $18^{th}$ August 2017. Three days after the binary BH merger, on August $17^{th}$, the coalescence of two neutron star was detected for the first time, followed by a GRB detection by Fermi's GBM starting a new era in multi-messenger Astronomy. Observations started 5.3 h after the merge and contained the counterpart SSS17a that was identified several hours later. It stands as the first data obtained by a ground-based pointing instrument on this object. In this contribution, we will present the results of the search of high-energy gamma ray emission as electromagnetic counterpart of these two GW events. No significant gamma ray emission was detected for either event. Nevertheless upper limit maps were derived constraining, for the first time, the non-thermal, high-energy emission on the remnant of a three detector binary black hole coalescence (GW170814), and a binary neutron star coalescence (GW170817).

astro-ph.HE↗