ALBERT

Description: The flat spectrum radio quasar 3C 279 is known to exhibit pronounced variability in the high-energy (100 MeV 〈  E 〈  100 GeV) γ-ray band, which is continuously monitored with Fermi-LAT. During two periods of high activity in April 2014 and June 2015 target-of-opportunity observations were undertaken with the High Energy Stereoscopic System (H.E.S.S.) in the very-high-energy (VHE, E 〉  100 GeV) γ-ray domain. While the observation in 2014 provides an upper limit, the observation in 2015 results in a signal with 8.7σ significance above an energy threshold of 66 GeV. No VHE variability was detected during the 2015 observations. The VHE photon spectrum is soft and described by a power-law index of 4.2 ± 0.3. The H.E.S.S. data along with a detailed and contemporaneous multiwavelength data set provide constraints on the physical parameters of the emission region. The minimum distance of the emission region from the central black hole was estimated using two plausible geometries of the broad-line region and three potential intrinsic spectra. The emission region is confidently placed at r ≳ 1.7 × 1017 cm from the black hole, that is beyond the assumed distance of the broad-line region. Time-dependent leptonic and lepto-hadronic one-zone models were used to describe the evolution of the 2015 flare. Neither model can fully reproduce the observations, despite testing various parameter sets. Furthermore, the H.E.S.S. data were used to derive constraints on Lorentz invariance violation given the large redshift of 3C 279.

Description: Aims. Colliding wind binary systems have long been suspected to be high-energy (HE; 100 MeV 〈 E 〈 100 GeV) γ-ray emitters. η Car is the most prominent member of this object class and is confirmed to emit phase-locked HE γ rays from hundreds of MeV to ~100 GeV energies. This work aims to search for and characterise the very-high-energy (VHE; E 〉100 GeV) γ-ray emission from η Car around the last periastron passage in 2014 with the ground-based High Energy Stereoscopic System (H.E.S.S.). Methods. The region around η Car was observed with H.E.S.S. between orbital phase p = 0.78−1.10, with a closer sampling at p ≈ 0.95 and p ≈ 1.10 (assuming a period of 2023 days). Optimised hardware settings as well as adjustments to the data reduction, reconstruction, and signal selection were needed to suppress and take into account the strong, extended, and inhomogeneous night sky background (NSB) in the η Car field of view. Tailored run-wise Monte-Carlo simulations (RWS) were required to accurately treat the additional noise from NSB photons in the instrument response functions. Results. H.E.S.S. detected VHE γ-ray emission from the direction of η Car shortly before and after the minimum in the X-ray light-curve close to periastron. Using the point spread function provided by RWS, the reconstructed signal is point-like and the spectrum is best described by a power law. The overall flux and spectral index in VHE γ rays agree within statistical and systematic errors before and after periastron. The γ-ray spectrum extends up to at least ~400 GeV. This implies a maximum magnetic field in a leptonic scenario in the emission region of 0.5 Gauss. No indication for phase-locked flux variations is detected in the H.E.S.S. data.

Description: Context. Flat-spectrum radio-quasars (FSRQs) are rarely detected at very high energies (E ≥ 100 GeV) due to their low-frequency-peaked spectral energy distributions. At present, only six FSRQs are known to emit very high-energy (VHE) photons, representing only 7% of the VHE extragalactic catalog, which is largely dominated by high-frequency-peaked BL Lacertae objects. Aims. Following the detection of MeV–GeV γ-ray flaring activity from the FSRQ PKS 0736+017 (z = 0.189) with Fermi-LAT, the H.E.S.S. array of Cherenkov telescopes triggered target-of-opportunity (ToO) observations on February 18, 2015, with the goal of studying the γ-ray emission in the VHE band. Methods. H.E.S.S. ToO observations were carried out during the nights of February 18, 19, 21, and 24, 2015. Together with Fermi-LAT, the multi-wavelength coverage of the flare includes Swift observations in soft X-ray and optical-UV bands, and optical monitoring (photometry and spectro-polarimetry) by the Steward Observatory, and the ATOM, the KAIT, and the ASAS-SN telescopes. Results. VHE emission from PKS 0736+017 was detected with H.E.S.S. only during the night of February 19, 2015. Fermi-LAT data indicate the presence of a γ-ray flare, peaking at the time of the H.E.S.S. detection, with a flux doubling timescale of around six hours. The γ-ray flare was accompanied by at least a 1 mag brightening of the non-thermal optical continuum. No simultaneous observations at longer wavelengths are available for the night of the H.E.S.S. detection. The γ-ray observations with H.E.S.S. and Fermi-LAT are used to put constraints on the location of the γ-ray emitting region during the flare: it is constrained to be just outside the radius of the broad-line region rBLR with a bulk Lorentz factor Γ ≃ 20, or at the level of the radius of the dusty torus rtorus with Γ ≃ 60. Conclusions. PKS 0736+017 is the seventh FSRQ known to emit VHE photons, and at z = 0.189 is the nearest so far. The location of the γ-ray emitting region during the flare can be tightly constrained thanks to opacity, variability, and collimation arguments.

Description: Aims. We report on the measurement and investigation of pulsed high-energy γ-ray emission from the Vela pulsar, PSR B0833−45, based on observations with the largest telescope of H.E.S.S., CT5, in monoscopic mode, and on data obtained with the Fermi-LAT. Methods. Data from 40.3 h of observations carried out with the H.E.S.S. II array from 2013 to 2015 have been used. A dedicated very low-threshold event reconstruction and analysis pipeline was developed to achieve the lowest possible energy threshold. Eight years of Fermi-LAT data were analysed and also used as reference to validate the CT5 telescope response model and analysis methods. Results. A pulsed γ-ray signal at a significance level of more than 15σ is detected from the P2 peak of the Vela pulsar light curve. Of a total of 15 835 events, more than 6000 lie at an energy below 20 GeV, implying a significant overlap between H.E.S.S. II-CT5 and the Fermi-LAT. While the investigation of the pulsar light curve with the LAT confirms characteristics previously known up to 20 GeV in the tens of GeV energy range, CT5 data show a change in the pulse morphology of P2, i.e. an extreme sharpening of its trailing edge, together with the possible onset of a new component at 3.4σ significance level. Assuming a power-law model for the P2 spectrum, an excellent agreement is found for the photon indices (Γ ≃ 4.1) obtained with the two telescopes above 10 GeV and an upper bound of 8% is derived on the relative offset between their energy scales. Using data from both instruments, it is shown however that the spectrum of P2 in the 10–100 GeV has a pronounced curvature; this is a confirmation of the sub-exponential cut-off form found at lower energies with the LAT. This is further supported by weak evidence of an emission above 100 GeV obtained with CT5. In contrast, converging indications are found from both CT5 and LAT data for the emergence of a hard component above 50 GeV in the leading wing (LW2) of P2, which possibly extends beyond 100 GeV. Conclusions. The detection demonstrates the performance and understanding of CT5 from 100 GeV down to the sub-20 GeV domain, i.e. unprecedented low energy for ground-based γ-ray astronomy. The extreme sharpening of the trailing edge of the P2 peak found in the H.E.S.S. II light curve of the Vela pulsar and the possible extension beyond 100 GeV of at least one of its features, LW2, provide further constraints to models of γ-Ray emission from pulsars.

Description: Context. PSR B1259–63/LS 2883 is a gamma-ray binary system consisting of a pulsar in an eccentric orbit around a bright Oe stellar-type companion star that features a dense circumstellar disc. The bright broad-band emission observed at phases close to periastron offers a unique opportunity to study particle acceleration and radiation processes in binary systems. Observations at gamma-ray energies constrain these processes through variability and spectral characterisation studies. Aims. The high- and very-high-energy (HE, VHE) gamma-ray emission from PSR B1259–63/LS 2883 around the times of its periastron passage are characterised, in particular, at the time of the HE gamma-ray flares reported to have occurred in 2011, 2014, and 2017. Short-term and average emission characteristics of PSR B1259–63/LS 2883 are determined. Super-orbital variability is searched for in order to investigate possible cycle-to-cycle VHE flux changes due to different properties of the companion star’s circumstellar disc and/or the conditions under which the HE gamma-ray flares develop. Methods. Spectra and light curves were derived from observations conducted with the H.E.S.S-II array in 2014 and 2017. Phase-folded light curves are compared with the results obtained in 2004, 2007, and 2011. Fermi-LAT observations from 2010/11, 2014, and 2017 are analysed. Results. A local double-peak profile with asymmetric peaks in the VHE light curve is measured, with a flux minimum at the time of periastron tp and two peaks coinciding with the times at which the neutron star crosses the companion’s circumstellar disc (~tp ± 16 d). A high VHE gamma-ray flux is also observed at the times of the HE gamma-ray flares (~tp + 30 d) and at phases before the first disc crossing (~tp − 35 d). The spectral energy range now extends to below 200 GeV and up to ~45 TeV. Conclusions. PSR B1259–63/LS 2883 displays periodic flux variability at VHE gamma-rays without clear signatures of super-orbital modulation in the time span covered by the monitoring of the source with the H.E.S.S. telescopes. This flux variability is most probably caused by the changing environmental conditions, particularly at times close to periastron passage at which the neutron star is thought to cross the circumstellar disc of the companion star twice. In contrast, the photon index remains unchanged within uncertainties for about 200 d around periastron. At HE gamma-rays, PSR B1259–63/LS 2883 has now been detected also before and after periastron, close to the disc crossing times. Repetitive flares with distinct variability patterns are detected in this energy range. Such outbursts are not observed at VHEs, although a relatively high emission level is measured. The spectra obtained in both energy regimes displays a similar slope, although a common physical origin either in terms of a related particle population, emission mechanism, or emitter location is ruled out.

Description: Young core-collapse supernovae with dense-wind progenitors may be able to accelerate cosmic-ray hadrons beyond the knee of the cosmic-ray spectrum, and this may result in measurable gamma-ray emission. We searched for gamma-ray emission from ten supernovae observed with the High Energy Stereoscopic System (H.E.S.S.) within a year of the supernova event. Nine supernovae were observed serendipitously in the H.E.S.S. data collected between December 2003 and December 2014, with exposure times ranging from 1.4 to 53 h. In addition we observed SN 2016adj as a target of opportunity in February 2016 for 13 h. No significant gamma-ray emission has been detected for any of the objects, and upper limits on the 〉1 TeV gamma-ray flux of the order of ~10−13 cm−2s−1 are established, corresponding to upper limits on the luminosities in the range ~2 × 1039 to ~1 × 1042 erg s−1. These values are used to place model-dependent constraints on the mass-loss rates of the progenitor stars, implying upper limits between ~2 × 10−5 and ~2 × 10−3 M⊙ yr−1 under reasonable assumptions on the particle acceleration parameters.

Description: Context. Pulsar wind nebulae (PWNe) represent the most prominent population of Galactic very-high-energy gamma-ray sources and are thought to be an efficient source of leptonic cosmic rays. Vela X is a nearby middle-aged PWN, which shows bright X-ray and TeV gamma-ray emission towards an elongated structure called the cocoon. Aims. Since TeV emission is likely inverse-Compton emission of electrons, predominantly from interactions with the cosmic microwave background, while X-ray emission is synchrotron radiation of the same electrons, we aim to derive the properties of the relativistic particles and of magnetic fields with minimal modelling. Methods. We used data from the Suzaku XIS to derive the spectra from three compact regions in Vela X covering distances from 0.3 to 4 pc from the pulsar along the cocoon. We obtained gamma-ray spectra of the same regions from H.E.S.S. observations and fitted a radiative model to the multi-wavelength spectra. Results. The TeV electron spectra and magnetic field strengths are consistent within the uncertainties for the three regions, with energy densities of the order 10−12 erg cm−3. The data indicate the presence of a cutoff in the electron spectrum at energies of ~ 100 TeV and a magnetic field strength of ~6 μG. Constraints on the presence of turbulent magnetic fields are weak. Conclusions. The pressure of TeV electrons and magnetic fields in the cocoon is dynamically negligible, requiring the presence of another dominant pressure component to balance the pulsar wind at the termination shock. Sub-TeV electrons cannot completely account for the missing pressure, which may be provided either by relativistic ions or from mixing of the ejecta with the pulsar wind. The electron spectra are consistent with expectations from transport scenarios dominated either by advection via the reverse shock or by diffusion, but for the latter the role of radiative losses near the termination shock needs to be further investigated in the light of the measured cutoff energies. Constraints on turbulent magnetic fields and the shape of the electron cutoff can be improved by spectral measurements in the energy range ≳ 10 keV.

Description: Context. We present a detailed view of the pulsar wind nebula (PWN) HESS J1825–137. We aim to constrain the mechanisms dominating the particle transport within the nebula, accounting for its anomalously large size and spectral characteristics. Aims. The nebula was studied using a deep exposure from over 12 years of H.E.S.S. I operation, together with data from H.E.S.S. II that improve the low-energy sensitivity. Enhanced energy-dependent morphological and spatially resolved spectral analyses probe the very high energy (VHE, E 〉 0.1 TeV) γ-ray properties of the nebula. Methods. The nebula emission is revealed to extend out to 1.5° from the pulsar, ~1.5 times farther than previously seen, making HESS J1825–137, with an intrinsic diameter of ~100 pc, potentially the largest γ-ray PWN currently known. Characterising the strongly energy-dependent morphology of the nebula enables us to constrain the particle transport mechanisms. A dependence of the nebula extent with energy of R ∝ Eα with α = −0.29 ± 0.04stat ± 0.05sys disfavours a pure diffusion scenario for particle transport within the nebula. The total γ-ray flux of the nebula above 1 TeV is found to be (1.12 ± 0.03stat ± 0.25sys) × 10−11 cm−2 s−1, corresponding to ~64% of the flux of the Crab nebula. Results. HESS J1825–137 is a PWN with clearly energy-dependent morphology at VHE γ-ray energies. This source is used as a laboratory to investigate particle transport within intermediate-age PWNe. Based on deep observations of this highly spatially extended PWN, we produce a spectral map of the region that provides insights into the spectral variation within the nebula.

Description: The analysis and combination of data from different gamma-ray instruments involves the use of collaboration proprietary software and case-by-case methods. The effort of defining a common data format for high-level data, namely event lists and instrument response functions (IRFs), has recently started for very-high-energy gamma-ray instruments, driven by the upcoming Cherenkov Telescope Array (CTA). In this work we implemented this prototypical data format for a small set of MAGIC, VERITAS, FACT, and H.E.S.S. Crab nebula observations, and we analyzed them with the open-source gammapy software package. By combining data from Fermi-LAT, and from four of the currently operating imaging atmospheric Cherenkov telescopes, we produced a joint maximum likelihood fit of the Crab nebula spectrum. Aspects of the statistical errors and the evaluation of systematic uncertainty are also commented upon, along with the release format of spectral measurements. The results presented in this work are obtained using open-access on-line assets that allow for a long-term reproducibility of the results.