A. Fiasson

Search for dark matter annihilations towards the inner Galactic halo from 10 years of observations with H.E.S.S

The inner region of the Milkyxa0Way halo harbors a large amount of dark matter (DM). Given its proximity, it is one of the most promising targets to look for DM. We report on a search for the annihilations of DM particles using γ-ray observations towards the inner 300xa0pc of the Milkyxa0Way, with the H.E.S.S. array of ground-based Cherenkov telescopes. The analysis is based on a 2D maximum likelihood method using Galactic Center (GC) data accumulated by H.E.S.S. over the last 10xa0years (2004-2014), and does not show any significant γ-ray signal above background. Assuming Einasto and Navarro-Frenk-White DM density profiles at the GC, we derive upper limits on the annihilation cross section ⟨σv⟩. These constraints are the strongest obtained so far in the TeV DM mass range and improve upon previous limits by a factor 5. For the Einasto profile, the constraints reach ⟨σv⟩ values of 6×10^{-26}u2009u2009cm^{3}u2009s^{-1} in the W^{+}W^{-} channel for a DM particle mass of 1.5xa0TeV, and 2×10^{-26}u2009u2009cm^{3}u2009s^{-1} in the τ^{+}τ^{-} channel for a 1xa0TeV mass. For the first time, ground-based γ-ray observations have reached sufficient sensitivity to probe ⟨σv⟩ values expected from the thermal relic density for TeV DM particles.

SUPERNOVA-ENHANCED COSMIC-RAY IONIZATION AND INDUCED CHEMISTRY IN A MOLECULAR CLOUD OF W51C

Cosmic rays (CRs) pervade the Galaxy and are thought to be accelerated in supernova shocks. The interaction of CRs with dense interstellar matter has two important effects: (1) high-energy (1 GeV) protons produce γ-rays by π0-meson decay and (2) low-energy (1 GeV) CRs (protons and electrons) ionize the gas. We present here new observations toward a molecular cloud close to the W51C supernova remnant and associated with a recently discovered TeV γ-ray source. Our observations show that the cloud ionization degree is highly enhanced, implying a CR ionization rate ~10–15 s–1, i.e., 100 times larger than the standard value in molecular clouds. This is consistent with the idea that the cloud is irradiated by an enhanced flux of freshly accelerated low-energy CRs. In addition, the observed high CR ionization rate leads to an instability in the chemistry of the cloud, which keeps the electron fraction high, ~10–5, in a large fraction (A v ≥ 6 mag) of the cloud and low, ~10–7, in the interior. The two states have been predicted in the literature as high- and low-ionization phases (HIP and LIP). This is the observational evidence of their simultaneous presence in a cloud.

H.E.S.S. observations of RX J1713.7-3946 with improved angular and spectral resolution; evidence for gamma-ray emission extending beyond the X-ray emitting shell

Supernova remnants exhibit shock fronts (shells) that can accelerate charged particles up to very high energies. In the past decade, measurements of a handful of shell-type supernova remnants in very-high-energy gamma rays have provided unique insights into the acceleration process. Among those objects, RX J1713.7-3946 (also known as G347.3-0.5) has the largest surface brightness, allowing us in the past to perform the most comprehensive study of morphology and spatially resolved spectra of any such very-high-energy gamma-ray source. Here we present extensive new H.E.S.S. measurements of RX J1713.7-3946, almost doubling the observation time compared to our previous publication. Combined with new improved analysis tools, the previous sensitivity is more than doubled. The H.E.S.S. angular resolution of 0.048∘ (0.036∘ above 2 TeV) is unprecedented in gamma-ray astronomy and probes physical scales of 0.8 (0.6) parsec at the remnants location. nThe new H.E.S.S. image of RX J1713.7-3946 allows us to reveal clear morphological differences between X-rays and gamma rays. In particular, for the outer edge of the brightest shell region, we find the first ever indication for particles in the process of leaving the acceleration shock region. By studying the broadband energy spectrum, we furthermore extract properties of the parent particle populations, providing new input to the discussion of the leptonic or hadronic nature of the gamma-ray emission mechanism.

H.E.S.S. limits on linelike dark matter signatures in the 100 GeV to 2 TeV energy range close to the galactic center

A search for dark matter linelike signals iss performed in the vicinity of the Galactic Center by the H.E.S.S. experiment on observational data taken in 2014. An unbinned likelihood analysis iss developed to improve the sensitivity to linelike signals. The upgraded analysis along with newer data extend the energy coverage of the previous measurement down to 100xa0GeV. The 18xa0h of data collected with the H.E.S.S. array allow one to rule out at 95%xa0C.L. the presence of a 130xa0GeV line (at l=-1.5°, b=0° and for a dark matter profile centered at this location) previously reported in Fermi-LAT data. This new analysis overlaps significantly in energy with previous Fermi-LAT and H.E.S.S.nnnRESULTSnNo significant excess associated with dark matter annihilations was found in the energy range of 100xa0GeV to 2xa0TeV and upper limits on the gamma-ray flux and the velocity weighted annihilation cross section are derived adopting an Einasto dark matter halo profile. Expected limits for present and future large statistics H.E.S.S. observations are also given.

The supernova remnant W49B as seen with H.E.S.S. and Fermi-LAT

The supernova remnant (SNR) W49B originated from a core-collapse supernova that occurred between one and four thousand years ago, and subsequently evolved into a mixed-morphology remnant, which is interacting with molecular clouds (MC). γ-ray observations of SNR/MC associations are a powerful tool to constrain the origin of Galactic cosmic-rays, as they can probe the acceleration of hadrons through their interaction with the surrounding medium and subsequent emission of non-thermal photons. The detection of a γ-ray source coincident with W49B at very high energies (VHE; E > 100 GeV) with the H.E.S.S. Cherenkov telescopes is reported together with a study of the source with 5 years of Fermi-LAT high energy γ-ray (0.06 - 300 GeV) data. The smoothly-connected combined source spectrum, measured from 60 MeV to multi-TeV energies, shows two significant spectral breaks at 304±20 MeV and 8.4+2.2−2.5 GeV, the latter being constrained by the joint fit from the two instruments. The detected spectral features are similar to those observed in several other SNR/MC associations and are found to be indicative of γ-ray emission produced through neutral-pion decay.

Deeper H.E.S.S. Observations of Vela Junior (RX J0852.0-4622): Morphology Studies and Resolved Spectroscopy

Aims. The gamma-ray emission from the shell-type supernova remnant (SNR) RX J0852.0-4622 is studied in order to better characterize its spectral properties and its distribution over the SNR. nMethods. The analysis of an extended H.E.S.S. data set at very-high energies (E > 100 GeV) permits detailed studies of the morphology and the spectrum of the whole RX J0852.0-4622 region, as well as spatially-resolved spectroscopy. The H.E.S.S. data are combined with archival data from other wavebands and interpreted in the framework of leptonic and hadronic models. The joint Fermi-LAT-H.E.S.S. spectrum allows the direct determination of the spectral characteristics of the parent particle population in leptonic and hadronic scenarios using only GeV-TeV data. nResults. An updated analysis of the H.E.S.S. data shows that the spectrum of the entire SNR connects smoothly to the high-energy spectrum measured by Fermi-LAT. The increased data set makes it possible to demonstrate that the H.E.S.S. spectrum deviates significantly from a power law and is well described by both a curved power law and a power law with an exponential cut-off at an energy of Ecut = (6.7 +/- 1.2_stat +/- 1.2_syst) TeV. The joint Fermi-LAT-H.E.S.S. spectrum allows the unambiguous identification of the spectral shape as a power law with an exponential cut-off. No significant evidence is found for a variation of the spectral parameters across the SNR, suggesting similar conditions of particle acceleration across the remnant. A simple modeling using one particle population to model the SNR emission demonstrates that both leptonic and hadronic emission scenarios remain plausible. It is also shown that at least a part of the shell emission is likely due to the presence of a pulsar wind nebula around PSR J0855-4644.

The camera of the fifth H.E.S.S. telescope. Part I: System description

In July 2012, as the four ground-based gamma-ray telescopes of the H.E.S.S. (High Energy Stereoscopic System) array reached their tenth year of operation in Khomas Highlands, Namibia, a fifth telescope took its first data as part of the system. This new Cherenkov detector, comprising a 614.5 m

Optimization of multivariate analysis for IACT stereoscopic systems

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A search for very high energy flares from the microquasars GRS 1915+105, Circinus X-1, and V4641 Sgr using contemporaneous H.E.S.S. and RXTE observations

reflector with a highly pixellized camera in its focal plane, improves the sensitivity of the current array by a factor two and extends its energy domain down to a few tens of GeV. The present part I of the paper gives a detailed description of the fifth H.E.S.S. telescopes camera, presenting the details of both the hardware and the software, emphasizing the main improvements as compared to previous H.E.S.S. camera technology.

First limits on the very-high energy gamma-ray afterglow emission of a fast radio burst: H.E.S.S. observations of FRB 150418

Multivariate methods have been recently introduced and successfully applied for the discrimination of signal from background in the selection of genuine very-high energy gamma-ray events with the H.E.S.S. Imaging Atmospheric Cerenkov Telescope. The complementary performance of three independent reconstruction methods developed for the H.E.S.S. data analysis, namely Hillas, model and 3D-model suggests the optimization of their combination through the application of a resulting efficient multivariate estimator. In this work the boosted decision tree method is proposed leading to a significant increase in the signal over background ratio compared to the standard approaches. The improved sensitivity is also demonstrated through a comparative analysis of a set of benchmark astrophysical sources.