arXiv subjects

S. Klepser

Publications and source records attributed to S. Klepser.

At least 37 records · Page 2Linked to original sources

arXiv2019

A very-high-energy component deep in the Gamma-ray Burst afterglow

Gamma-ray bursts (GRBs) are brief flashes of gamma rays, considered to be the most energetic explosive phenomena in the Universe. The emission from GRBs comprises a short (typically tens of seconds) and bright prompt emission, followed by a much longer afterglow phase. During the afterglow phase, the shocked outflow -- produced by the interaction between the ejected matter and the circumburst medium -- slows down, and a gradual decrease in brightness is observed. GRBs typically emit most of their energy via gamma-rays with energies in the kiloelectronvolt-to-megaelectronvolt range, but a few photons with energies of tens of gigaelectronvolts have been detected by space-based instruments. However, the origins of such high-energy (above one gigaelectronvolt) photons and the presence of very-high-energy (more than 100 gigaelectronvolts) emission have remained elussive. Here we report observations of very-high-energy emission in the bright GRB 180720B deep in the GRB afterglow -ten hours after the end of the prompt emission phase, when the X-ray flux had already decayed by four orders of magnitude. Two possible explanations exist for the observed radiation: inverse Compton emission and synchrotron emission of ultrarelativistic electrons. Our observations show that the energy fluxes in the X-ray and gamma-ray range and their photon indices remain comparable to each other throughout the afterglow. This discovery places distinct constraints on the GRB environment for both emission mechanisms, with the inverse Compton explanation alleviating the particle energy requirements for the emission observed at late times. The late timing of this detection has consequences for the future observations of GRBs at the highest energies.

astro-ph.HE↗

arXiv2019

H.E.S.S. detection of very-high-energy gamma-ray emission from the quasar PKS 0736+017

Flat-spectrum radio-quasars (FSRQs) are rarely detected at very-high-energies (VHE; E>100 GeV) due to their low-frequency-peaked SEDs. At present, only 6 FSRQs are known to emit VHE photons, representing only 7% of the VHE extragalactic catalog. Following the detection of MeV-GeV gamma-ray flaring activity from the FSRQ PKS 0736+017 (z=0.189) with Fermi, the H.E.S.S. array of Cherenkov telescopes triggered ToO observations on February 18, 2015, with the goal of studying the gamma-ray emission in the VHE band. 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-rays and optical/UV, and optical monitoring (photometry and spectro-polarimetry) by the Steward Observatory, the ATOM, the KAIT and the ASAS-SN telescope. VHE emission from PKS 0736+017 was detected with H.E.S.S. during the night of February 19, 2015, only. Fermi data indicate the presence of a gamma-ray flare, peaking at the time of the H.E.S.S. detection, with a flux doubling time-scale of around six hours. The gamma-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 gamma-ray observations with H.E.S.S. and Fermi are used to put constraints on the location of the gamma-ray emitting region during the flare: it is constrained to be just outside the radius of the broad-line-region with a bulk Lorentz factor $\simeq 20$, or at the level of the radius of the dusty torus with Gamma > 60. 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 gamma-ray emitting region during the flare can be tightly constrained thanks to opacity, variability, and collimation arguments.

astro-ph.HE↗

arXiv2019

Resolving the Crab pulsar wind nebula at teraelectronvolt energies

The Crab nebula is one of the most studied cosmic particle accelerators, shining brightly across the entire electromagnetic spectrum up to very high-energy gamma rays. It is known from radio to gamma-ray observations that the nebula is powered by a pulsar, which converts most of its rotational energy losses into a highly relativistic outflow. This outflow powers a pulsar wind nebula (PWN), a region of up to 10~light-years across, filled with relativistic electrons and positrons. These particles emit synchrotron photons in the ambient magnetic field and produce very high-energy gamma rays by Compton up-scattering of ambient low-energy photons. While the synchrotron morphology of the nebula is well established, it was up to now not known in which region the very high-energy gamma rays are emitted. Here we report that the Crab nebula has an angular extension at gamma-ray energies of 52 arcseconds (assuming a Gaussian source width), significantly larger than at X-ray energies. This result closes a gap in the multi-wavelength coverage of the nebula, revealing the emission region of the highest energy gamma rays. These gamma rays are a new probe of a previously inaccessible electron and positron energy range. We find that simulations of the electromagnetic emission reproduce our new measurement, providing a non-trivial test of our understanding of particle acceleration in the Crab nebula.

astro-ph.HE↗

arXiv2019

Constraints on the emission region of 3C 279 during strong flares in 2014 and 2015 through VHE gamma-ray observations with H.E.S.S

The flat spectrum radio quasar 3C 279 is known to exhibit pronounced variability in the high-energy ($100\,$MeV$ 100\,$GeV) $γ$-ray domain. While the observation in 2014 provides an upper limit, the observation in 2015 results in a signal with $8.7\,σ$ significance above an energy threshold of $66\,$GeV. No VHE variability has been detected during the 2015 observations. The VHE photon spectrum is soft and described by a power-law index of $4.2\pm 0.3$. 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 is estimated using two plausible geometries of the broad-line region and three potential intrinsic spectra. The emission region is confidently placed at $r\gtrsim 1.7\times10^{17}\,$cm from the black hole, i.e., beyond the assumed distance of the broad-line region. Time-dependent leptonic and lepto-hadronic one-zone models are 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 are used to derive constraints on Lorentz invariance violation given the large redshift of 3C 279.

astro-ph.HE↗

arXiv2019

Upper Limits on Very-High-Energy Gamma-ray Emission from Core-Collapse Supernovae Observed with H.E.S.S

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 hours to 53 hours. In addition we observed SN 2016adj as a target of opportunity in February 2016 for 13 hours. 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 $\sim$10$^{-13}$ cm$^{-2}$s$^{-1}$ are established, corresponding to upper limits on the luminosities in the range $\sim$2 $\times$ 10$^{39}$ erg s$^{-1}$ to $\sim$1 $\times$ 10$^{42}$ 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 $\sim$2 $\times 10^{-5}$ and $\sim$2 $\times 10^{-3}$M$_{\odot}$yr$^{-1}$ under reasonable assumptions on the particle acceleration parameters.

astro-ph.HE↗

arXiv2019

H.E.S.S. observations of the flaring gravitationally lensed galaxy PKS 1830-211

PKS 1830-211 is a known macrolensed quasar located at a redshift of z=2.5. Its high-energy gamma-ray emission has been detected with the Fermi-LAT instrument and evidence for lensing was obtained by several authors from its high-energy data. Observations of PKS 1830-211 were taken with the H.E.S.S. array of Imaging Atmospheric Cherenkov Telescopes in August 2014, following a flare alert by the Fermi- LAT collaboration. The H.E.S.S observations were aimed at detecting a gamma-ray flare delayed by 20-27 days from the alert flare, as expected from observations at other wavelengths. More than twelve hours of good quality data were taken with an analysis threshold of $\sim67$ GeV. The significance of a potential signal is computed as a function of the date as well as the average significance over the whole period. Data are compared to simultaneous observations by Fermi-LAT. No photon excess or significant signal is detected. An upper limit on PKS 1830-211 flux above 67 GeV is computed and compared to the extrapolation of the Fermi-LAT flare spectrum.

astro-ph.HE↗

arXiv2019

The 2014 TeV Gamma-ray Flare of Mrk 501 Seen with H.E.S.S.: Temporal and Spectral Constraints on Lorentz Invariance Violation

The blazar Mrk 501 (z=0.034) was observed at very-high-energy (VHE, $E\gtrsim 100$~GeV) gamma-ray wavelengths during a bright flare on the night of 2014 June 23-24 (MJD 56832) with the H.E.S.S. phase-II array of Cherenkov telescopes. Data taken that night by H.E.S.S. at large zenith angle reveal an exceptional number of gamma-ray photons at multi-TeV energies, with rapid flux variability and an energy coverage extending significantly up to 20 TeV. This data set is used to constrain Lorentz invariance violation (LIV) using two independent channels: a temporal approach considers the possibility of an energy dependence in the arrival time of gamma rays, whereas a spectral approach considers the possibility of modifications to the interaction of VHE gamma rays with extragalactic background light (EBL) photons. The non-detection of energy-dependent time delays and the non-observation of deviations between the measured spectrum and that of a supposed power-law intrinsic spectrum with standard EBL attenuation are used independently to derive strong constraints on the energy scale of LIV ($E_{\rm{QG}}$) in the subluminal scenario for linear and quadratic perturbations in the dispersion relation of photons. For the case of linear perturbations, the 95% confidence level limits obtained are $E_{\rm{QG},1} > 3.6 \times 10^{17} \ \rm{GeV} $ using the temporal approach and $E_{\rm{QG},1} > 2.6 \times 10^{19} \ \rm{GeV}$ using the spectral approach. For the case of quadratic perturbations, the limits obtained are $E_{\rm{QG},2} > 8.5 \times 10^{10} \ \rm{GeV} $ using the temporal approach and $E_{\rm{QG},2} > 7.8 \times 10^{11} \rm{ GeV}$ using the spectral approach.

astro-ph.HE↗

arXiv2018

Particle Transport within the Pulsar Wind Nebula HESS J1825-137

Aims: 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. Methods: The nebula is 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 improving the low energy sensitivity. Enhanced energy-dependent morphological and spatially-resolved spectral analyses probe the Very High Energy (VHE, E > 0.1 TeV) gamma-ray properties of the nebula. Results: The nebula emission is revealed to extend out to 1.5 degrees from the pulsar, ~1.5 times further than previously seen, making HESS J1825--137, with an intrinsic diameter of ~100 pc, potentially the largest gamma-ray PWN currently known. Characterisation of the nebula's strongly energy-dependent morphology enables the particle transport mechanisms to be constrained. A dependence of the nebula extent with energy of R $\propto$ E^αwith α= -0.29 +/- 0.04 (stat) +/- 0.05 (sys) disfavours a pure diffusion scenario for particle transport within the nebula. The total gamma-ray flux of the nebula above 1~TeV is found to be (1.12 +/- 0.03 (stat) +/- 0.25 (sys)) $\times 10^{-11}$ cm$^{-2}$ s$^{-1}$, corresponding to ~64% of the flux of the Crab Nebula. Conclusions: HESS J1825-137 is a PWN with clear energy-dependent morphology at VHE gamma-ray energies. This source is used as a laboratory to investigate particle transport within middle-aged PWNe. Deep observations of this highly spatially-extended PWN enable a spectral map of the region to be produced, providing insights into the spectral variation within the nebula.

astro-ph.HE↗

arXiv2018

VHE $γ$-ray discovery and multi-wavelength study of the blazar 1ES 2322-409

A hotspot at a position compatible with the BL Lac object 1ES 2322-409 was serendipitously detected with H.E.S.S. during observations performed in 2004 and 2006 on the blazar PKS 2316-423. Additional data on 1ES 2322-409 were taken in 2011 and 2012, leading to a total live-time of 22.3h. Point-like very-high-energy (VHE; E>100GeV) $γ$-ray emission is detected from a source centred on the 1ES 2322-409 position, with an excess of 116.7 events at a significance of 6.0$σ$. The average VHE $γ$-ray spectrum is well described with a power law with a photon index $Γ=3.40\pm0.66_{\text{stat}}\pm0.20_{\text{sys}}$ and an integral flux $Φ(E>200GeV) = (3.11\pm0.71_{\rm stat}\pm0.62_{\rm sys})\times10^{-12} cm^{-2} s^{-1}$, which corresponds to 1.1$\%$ of the Crab nebula flux above 200 GeV. Multi-wavelength data obtained with Fermi LAT, Swift XRT and UVOT, RXTE PCA, ATOM, and additional data from WISE, GROND and Catalina, are also used to characterise the broad-band non-thermal emission of 1ES 2322-409. The multi-wavelength behaviour indicates day-scale variability. Swift UVOT and XRT data show strong variability at longer scales. A spectral energy distribution (SED) is built from contemporaneous observations obtained around a high state identified in Swift data. A modelling of the SED is performed with a stationary homogeneous one-zone synchrotron-self-Compton (SSC) leptonic model. The redshift of the source being unknown, two plausible values were tested for the modelling. A systematic scan of the model parameters space is performed, resulting in a well-constrained combination of values providing a good description of the broad-band behaviour of 1ES 2322-409.

astro-ph.HE↗

arXiv2018

Searches for gamma-ray lines and `pure WIMP' spectra from Dark Matter annihilations in dwarf galaxies with H.E.S.S

Dwarf spheroidal galaxies are among the most promising targets for detecting signals of Dark Matter (DM) annihilations. The H.E.S.S. experiment has observed five of these systems for a total of about 130 hours. The data are re-analyzed here, and, in the absence of any detected signals, are interpreted in terms of limits on the DM annihilation cross section. Two scenarios are considered: i) DM annihilation into mono-energetic gamma-rays and ii) DM in the form of pure WIMP multiplets that, annihilating into all electroweak bosons, produce a distinctive gamma-ray spectral shape with a high-energy peak at the DM mass and a lower-energy continuum. For case i), upper limits at 95\% confidence level of about $\langle σv \rangle \lesssim 3 \times 10^{-25}$ cm$^3$ s$^{-1}$ are obtained in the mass range of 400 GeV to 1 TeV. For case ii), the full spectral shape of the models is used and several excluded regions are identified, but the thermal masses of the candidates are not robustly ruled out.

astro-ph.HE↗

arXiv2018

First Ground-based Measurement of Sub-20 GeV to 100 GeV $γ$-rays from the Vela Pulsar with H.E.S.S. II

We report on the measurement and investigation of pulsed high-energy $γ$-ray emission from the Vela pulsar, PSR B0833$-$45, based on 40.3 hours of observations with the largest telescope of H.E.S.S., CT5, in monoscopic mode, and on 8 years of data obtained with the Fermi-LAT. A dedicated very-low-threshold event reconstruction and analysis pipeline was developed and, together with the CT5 telescope response model, was validated using the Fermi-LAT data as reference. 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 15835 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 3.4$σ$ significance level. Assuming a power-law model for the P2 spectrum, an excellent agreement is found for the photon indices ($Γ\simeq$ 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 both instruments data, it is however shown that the spectrum of P2 in the 10-100 GeV has a pronounced curvature, i.e. a confirmation of the sub-exponential cutoff form found at lower energies with the LAT. This is further supported by the weak evidence for 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.

astro-ph.HE↗

arXiv2018

The starburst galaxy NGC 253 revisited by H.E.S.S. and Fermi-LAT

(Abridged) Context. NGC 253 is one of only two starburst galaxies found to emit $γ$-rays from hundreds of MeV to multi-TeV energies. Accurate measurements of the very-high-energy (VHE) (E $>$ 100 GeV) and high-energy (HE) (E $>$ 60 MeV) spectra are crucial to study the underlying particle accelerators and cosmic-ray interaction and transport. Aims. The measurement of the VHE $γ$-ray emission of NGC 253 published in 2012 by H.E.S.S. was limited by large systematic uncertainties. Here, a measurement of the $γ$-ray spectrum of NGC 253 is investigated in both HE and VHE $γ$-rays. Methods. The data of H.E.S.S. observations are reanalysed using an updated calibration and analysis chain. The $Fermi$-LAT analysis employs more than 8 years of data processed using pass 8. The cosmic-ray particle population is evaluated from the combined HE--VHE $γ$-ray spectrum using NAIMA. Results. The VHE $γ$-ray energy spectrum is best fit by a power-law with a flux normalisation of $(1.34\,\pm\,0.14^{\mathrm{stat}}\,\pm\,0.27^{\mathrm{sys}}) \times 10^{-13} \mathrm{cm^{-2} s^{-1} TeV^{-1}}$ at 1 TeV -- about 40 \% above, but compatible with the value obtained in Abramowski et al. (2012). The spectral index $Γ= 2.39 \pm 0.14^{\mathrm{stat}} \pm 0.25^{\mathrm{sys}}$ is slightly softer than but consistent with the previous measurement. At energies above $\sim$3 GeV the HE spectrum is consistent with a power-law ranging into the VHE part of the spectrum measured by H.E.S.S. Conclusions. Two scenarios for the starburst nucleus are tested, in which the gas in the starburst nucleus acts as a target for hadronic cosmic rays. In these two models, the level to which NGC\,253 acts as a calorimeter is estimated to a range of $f_{\rm cal} = 0.1$ to $1$ while accounting for the measurement uncertainties.

astro-ph.HE↗

arXiv2017

Performance of the upgraded H.E.S.S. cameras

The 14 years old cameras of the H.E.S.S. 12-m telescopes have been upgraded in 2015/2016, with the goals of reducing the system failure rate, reducing the dead time and improving the overall performance of the array. This conference contribution describes the various tests that were carried out on the cameras and their sub-components both in the lab and on site. It also gives an overview of the commissioning and calibration procedures adopted during and after the installation, including e.g. flat-fielding and trigger threshold scans. Finally, it reports in detail about the overall performance of the four new H.E.S.S. I cameras, using very recent data.

astro-ph.IM↗

arXiv2017

First Results of Eta Car Observations with H.E.S.S.II

Eta Carinae (or Eta Car) is a colliding-wind binary that shows non-thermal emission from hard X-rays to high-energy $γ$-rays. The $γ$-ray spectrum exhibits two spectral components, where the high-energy component extends up to 300 GeV. Previous observations of Eta Carinae with the High Energy Stereoscopic System (H.E.S.S.) resulted in upper limits. With the addition of the large 28-m central telescope to the H.E.S.S. array in 2012 the lower bound on the energy range has significantly been reduced. This lowers the energy threshold of the analysis compared to the published results due to the improved instrument sensitivity at energies below 400 GeV. Eta Carinae has been regularly observed in the following years with H.E.S.S. II. Here we report on the first results of Eta Carinae observations, which also cover the periastron passage in 2014 - the time of maximum $γ$-ray emission seen in GeV $γ$-rays with Fermi-LAT.

astro-ph.HE↗

arXiv2017

Hardware and software architecture of the upgraded H.E.S.S. cameras

In 2015/16, the photomultiplier cameras of the H.E.S.S. Cherenkov telescopes CT1-4 have undergone a major upgrade. The entire electronics has been replaced, using NECTAr chips for the front-end readout. A new ventilation system has been installed and several auxiliary components have been replaced. Besides this, the internal control and readout software was rewritten from scratch in a modern and modular way. Ethernet technology was used wherever possible to ensure both flexibility, stability and high bandwidth. An overview of the installed components will be given.

astro-ph.IM↗

arXiv2017

The population point of view on the evolution of TeV pulsar wind nebulae

To investigate the nature and evolution of TeV pulsar wind nebulae, we examine the firmly identified PWNe in the H.E.S.S. Galactic Plane Survey, along with the few other known detections from the literature, as well as the upper limits extracted from the H.E.S.S survey. These data exhibit a correlation of TeV surface brightness with pulsar spin-down power. It appears to be caused by both an increase of TeV extension and a decrease of TeV luminosity with decreasing spin-down power. We also find that the offsets of pulsars with ages around 10 kyr with respect to the wind nebula centres are frequently larger than can be plausibly explained by pulsar proper motion and could be due to an asymmetric environment. These and other results will be presented and put to context with a basic modelling of TeV pulsar wind nebula evolution.

astro-ph.HE↗

arXiv2016

Observations of Bow Shocks of Runaway Stars with H.E.S.S

Runaway stars form bow shocks by sweeping up interstellar matter in their direction of motion. Theoretical models predict a spectrally wide non-thermal component reaching up to gamma-ray energies at a flux level detectable with current instruments. They were motivated by a detection of non-thermal radio emission from the bow shock of BD$+43^\circ3654$ and a possible detection of non-thermal X-rays from AE Aurigae. A search in the high-energy regime using data from \textit{Fermi}-LAT resulted in flux upper limits for 27 candidates listed in the first E-BOSS catalogue. We perform the first systematic search for TeV emission from bow shocks of runaway stars. Using all available archival H.E.S.S. I data we search for very-high-energy gamma-ray emission at the positions of bow shock candidates listed in the second E-BOSS catalogue. This catalogue comprises 73 bow shock candidates, 32 of which have been observed with the H.E.S.S. telescopes. None of the observed bow shock candidates shows significant emission in the H.E.S.S. energy range. The resulting upper limits are used to constrain current models for non-thermal emission from these objects.

astro-ph.HE↗

arXiv2015

A major electronics upgrade for the H.E.S.S. Cherenkov telescopes 1-4

The High Energy Stereoscopic System (H.E.S.S.) is an array of imaging atmospheric Cherenkov telescopes (IACTs) located in the Khomas Highland in Namibia. It consists of four 12-m telescopes (CT1-4), which started operations in 2003, and a 28-m diameter one (CT5), which was brought online in 2012. It is the only IACT system featuring telescopes of different sizes, which provides sensitivity for gamma rays across a very wide energy range, from ~30 GeV up to ~100 TeV. Since the camera electronics of CT1-4 are much older than the one of CT5, an upgrade is being carried out; first deployment was in 2015, full operation is planned for 2016. The goals of this upgrade are threefold: reducing the dead time of the cameras, improving the overall performance of the array and reducing the system failure rate related to aging. Upon completion, the upgrade will assure the continuous operation of H.E.S.S. at its full sensitivity until and possibly beyond the advent of CTA. In the design of the new components, several CTA concepts and technologies were used and are thus being evaluated in the field: The upgraded read-out electronics is based on the NECTAR readout chips; the new camera front- and back-end control subsystems are based on an FPGA and an embedded ARM computer; the communication between subsystems is based on standard Ethernet technologies. These hardware solutions offer good performance, robustness and flexibility. The design of the new cameras is reported here.

astro-ph.IM↗