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Astronomy and astrophysics 612, A2 (2018). doi:10.1051/0004-6361/201629377, The nine-year H.E.S.S. Galactic Plane Survey (HGPS) has yielded the most uniform observation scan of the inner Milky Way in the TeV gamma-ray band to date. The sky maps and source catalogue of the HGPS allow for a systematic study of the population of TeV pulsar wind nebulae found throughout the last decade. To investigate the nature and evolution of pulsar wind nebulae, for the first time we also present several upper limits for regions around pulsars without a detected TeV wind nebula. Our data exhibit a correlation of TeV surface brightness with pulsar spin-down power Ė. This seems to be caused both by an increase of extension with decreasing Ė, and hence with time, compatible with a power law R$_{PWN}$(Ė) ~Ė$^{−0.65±0.20}$, and by a mild decrease of TeV gamma-ray luminosity with decreasing Ė, compatible with L$_{1−10 TeV}$ ~Ė$^{0.59±0.21}$. We also find that the offsets of pulsars with respect to the wind nebula centre with ages around 10 kyr are frequently larger than can be plausibly explained by pulsar proper motion and could be due to an asymmetric environment. In the present data, it seems that a large pulsar offset is correlated with a high apparent TeV efficiency L$_{1−10 TeV}$∕Ė. In addition to 14 HGPS sources considered firmly identified pulsar wind nebulae and 5 additional pulsar wind nebulae taken from literature, we find 10 HGPS sources that are likely TeV pulsar wind nebula candidates. Using a model that subsumes the present common understanding of the very high-energy radiative evolution of pulsar wind nebulae, we find that the trends and variations of the TeV observables and limits can be reproduced to a good level, drawing a consistent picture of present-day TeV data and theory., Published by EDP Sciences, Les Ulis

Astronomy and astrophysics 606, A59 (2017). doi:10.1051/0004-6361/201731200, Very high-energy gamma-rays (VHE, E>100 GeV) propagating over cosmological distances can interact with the low-energy photons of the extragalactic background light (EBL) and produce electron-positron pairs. The transparency of the universe to VHE gamma-rays is then directly related to the spectral energy distribution (SED) of the EBL. The observation of features in the VHE energy spectra of extragalactic sources allows the EBL to be measured, which otherwise is very difficult to determine. An EBL-model independent measurement of the EBL SED with the H.E.S.S. array of Cherenkov telescopes is presented. It is obtained by extracting the EBL absorption signal from the reanalysis of high-quality spectra of blazars. From H.E.S.S. data alone the EBL signature is detected at a significance of 9.5 sigma, and the intensity of the EBL obtained in different spectral bands is presented together with the associated gamma-ray horizon., Published by EDP Sciences, Les Ulis

Astronomy and astrophysics 612, A10 (2016). doi:10.1051/0004-6361/201527773, Context. Microquasars are potential γ-$\gamma$ray emitters. Indications of transient episodes of $\gamma$-ray emission were recently reported in at least two systems: Cyg X-1 and Cyg X-3. The identification of additional $\gamma$-ray-emitting microquasars is required to better understand how $\gamma$-ray emission can be produced in these systems.Aim. Theoretical models have predicted very high-energy (VHE) $\gamma$-ray emission from microquasars during periods of transient outburst. Observations reported herein were undertaken with the objective of observing a broadband flaring event in the $\gamma$-ray and X-ray bands.Methods. Contemporaneous observations of three microquasars, GRS 1915+105, Circinus X-1, and V4641 Sgr, were obtained using the High Energy Spectroscopic System (H.E.S.S.) telescope array and the Rossi X-ray Timing Explorer (RXTE) satellite. X-ray analyses for each microquasar were performed and VHE $\gamma$-ray upper limits from contemporaneous H.E.S.S. observations were derived.Results. No significant $\gamma$-ray signal has been detected in any of the three systems. The integral $\gamma$-ray photon flux at the observational epochs is constrained to be $I$(>560 GeV) < 7.3 × $10^{−13} cm^{−2} s^{−1}$, $I$(>560 GeV ) < 1.2 × $10^{−12} cm^{−2} s^{−1}$, and $I$(>240 GeV) < 4.5 × $10^{−12} cm^{−2} s^{−1}$ for GRS 1915+105, Circinus X-1, and V4641 Sgr, respectively.Conclusions. The $\gamma$-ray upper limits obtained using H.E.S.S. are examined in the context of previous Cherenkov telescope observations of microquasars. The effect of intrinsic absorption is modelled for each target and found to have negligible impact on the flux of escaping $\gamma$-rays. When combined with the X-ray behaviour observed using RXTE, the derived results indicate that if detectable VHE $\gamma$-ray emission from microquasars is commonplace, then it is likely to be highly transient., Published by EDP Sciences, Les Ulis

Astronomy and astrophysics 612, A5 (2016). doi:10.1051/0004-6361/201527843, 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). Gamma-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. We report 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 together with a study of the source with five 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 8.4−2.5+2.2 GeV; the latter is 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., Published by EDP Sciences, Les Ulis

Astronomy and astrophysics 612, A4 (2016). doi:10.1051/0004-6361/201526545, Aim. We aim for an understanding of the morphological and spectral properties of the supernova remnant RCW 86 and for insights into the production mechanism leading to the RCW 86 very high-energy γ-ray emission.Methods. We analyzed High Energy Spectroscopic System (H.E.S.S.) data that had increased sensitivity compared to the observations presented in the RCW 86 H.E.S.S. discovery publication. Studies of the morphological correlation between the 0.5–1 keV X-ray band, the 2–5 keV X-ray band, radio, and γ-ray emissions have been performed as well as broadband modeling of the spectral energy distribution with two different emission models.Results. We present the first conclusive evidence that the TeV γ-ray emission region is shell-like based on our morphological studies. The comparison with 2–5 keV X-ray data reveals a correlation with the 0.4–50 TeV γ-ray emission. The spectrum of RCW 86 is best described by a power law with an exponential cutoff at Ecut = (3.5 ± 1.2stat) TeV and a spectral index of Γ ≈ 1.6 ± 0.2. A static leptonic one-zone model adequately describes the measured spectral energy distribution of RCW 86, with the resultant total kinetic energy of the electrons above 1 GeV being equivalent to ~0.1% of the initial kinetic energy of a Type Ia supernova explosion (1051 erg). When using a hadronic model, a magnetic field of B ≈ 100 μG is needed to represent the measured data. Although this is comparable to formerly published estimates, a standard E−2 spectrum for the proton distribution cannot describe the γ-ray data. Instead, a spectral index of Γp ≈ 1.7 would be required, which implies that ∼7 × 1049/ncm−3 has been transferred into high-energy protons with the effective density ncm−3 = n/1 cm−3. This is about 10% of the kinetic energy of a typical Type Ia supernova under the assumption of a density of 1 cm−3., Published by EDP Sciences, Les Ulis