arXiv subjects

F. Krayzel

Publications and source records attributed to F. Krayzel.

8 recordsLinked to original sources

arXiv2013

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^2 reflector with a highly pixelized 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. telescope's 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.

astro-ph.IM↗

arXiv2013

Improved sensitivity of H.E.S.S.-II through the fifth telescope focus system

The Imaging Atmospheric Cherenkov Telescope (IACT) works by imaging the very short flash of Cherenkov radiation generated by the cascade of relativistic charged particles produced when a TeV gamma ray strikes the atmosphere. This energetic air shower is initiated at an altitude of 10-30 km depending on the energy and the arrival direction of the primary gamma ray. Whether the best image of the shower is obtained by focusing the telescope at infinity and measuring the Cherenkov photon angles or focusing on the central region of the shower is a not obvious question. This is particularly true for large size IACT for which the depth of the field is much smaller. We address this issue in particular with the fifth telescope (CT5) of the High Energy Stereoscopic System (H.E.S.S.); a 28 m dish large size telescope recently entered in operation and sensitive to an energy threshold of tens of GeVs. CT5 is equipped with a focus system, its working principle and the expected effect of focusing depth on the telescope sensitivity at low energies (50-200 GeV) is discussed.

astro-ph.IM↗

arXiv2012

Measurement of the extragalactic background light imprint on the spectra of the brightest blazars observed with H.E.S.S

The extragalactic background light (EBL) is the diffuse radiation with the second highest energy density in the Universe after the cosmic microwave background. The aim of this study is the measurement of the imprint of the EBL opacity to gamma-rays on the spectra of the brightest extragalactic sources detected with the High Energy Stereoscopic System (H.E.S.S.). The originality of the method lies in the joint fit of the EBL optical depth and of the intrinsic spectra of the sources, assuming intrinsic smoothness. Analysis of a total of ~10^5 gamma-ray events enables the detection of an EBL signature at the 8.8 std dev level and constitutes the first measurement of the EBL optical depth using very-high energy (E>100 GeV) gamma-rays. The EBL flux density is constrained over almost two decades of wavelengths (0.30-17 microns) and the peak value at 1.4 micron is derived as 15 +/- 2 (stat) +/- 3 (sys) nW / m^2 sr.

astro-ph.HE↗

arXiv2012

Probing the extent of the non-thermal emission from the Vela X region at TeV energies with H.E.S.S

Vela X is a region of extended radio emission in the western part of the Vela constellation: one of the nearest pulsar wind nebulae (PWNe), and associated with the energetic Vela pulsar (PSR B0833-45). Extended very-high-energy (VHE) $\gamma$-ray emission (HESS $\mathrm{J0835\mhyphen 455}$) was discovered using the H.E.S.S. experiment in 2004. The VHE $\gamma$-ray emission was found to be coincident with a region of X-ray emission discovered with ${\it ROSAT}$ above 1.5 keV (the so-called \textit{Vela X cocoon}): a filamentary structure extending southwest from the pulsar to the centre of Vela X. A deeper observation of the entire Vela X nebula region, also including larger offsets from the cocoon, has been performed with H.E.S.S. This re-observation was carried out in order to probe the extent of the non-thermal emission from the Vela X region at TeV energies and to investigate its spectral properties. In order to increase the sensitivity to the faint $\gamma$-ray emission from the very extended Vela X region, a multivariate analysis method combining three complementary reconstruction techniques of Cherenkov-shower images is applied for the selection of $\gamma$-ray events. The analysis is performed with the On/Off background method, which estimates the background from separate observations pointing away from Vela X; towards regions free of $\gamma$-ray sources but with comparable observation conditions. The $\gamma$-ray surface brightness over the large Vela X region reveals that the detection of non-thermal VHE $\gamma$-ray emission from the PWN HESS $\mathrm{J0835\mhyphen 455}$ is statistically significant over a region of radius 1.2$^{\circ}$ around the position $\alpha$ = 08$^{\mathrm{h}}$ 35$^{\mathrm{m}}$ 00$^{\mathrm{s}}$, $\delta$ = -45$^{\circ}$ 36$^{\mathrm{\prime}}$ 00$^{\mathrm{\prime}\mathrm{\prime}}$ (J2000).

astro-ph.HE↗

arXiv2012

Constraints on the gamma-ray emission from the cluster-scale AGN outburst in the Hydra A galaxy cluster

In some galaxy clusters powerful AGN have blown bubbles with cluster scale extent into the ambient medium. The main pressure support of these bubbles is not known to date, but cosmic rays are a viable possibility. For such a scenario copious gamma-ray emission is expected as a tracer of cosmic rays from these systems. Hydra A, the closest galaxy cluster hosting a cluster scale AGN outburst, located at a redshift of 0.0538, is investigated for being a gamma-ray emitter with the High Energy Stereoscopic System (H.E.S.S.) array and the Fermi Large Area Telescope (Fermi-LAT). Data obtained in 20.2 hours of dedicated H.E.S.S. observations and 38 months of Fermi-LAT data, gathered by its usual all-sky scanning mode, have been analyzed to search for a gamma-ray signal. No signal has been found in either data set. Upper limits on the gamma-ray flux are derived and are compared to models. These are the first limits on gamma-ray emission ever presented for galaxy clusters hosting cluster scale AGN outbursts. The non-detection of Hydra A in gamma-rays has important implications on the particle populations and physical conditions inside the bubbles in this system. For the case of bubbles mainly supported by hadronic cosmic rays, the most favorable scenario, that involves full mixing between cosmic rays and embedding medium, can be excluded. However, hadronic cosmic rays still remain a viable pressure support agent to sustain the bubbles against the thermal pressure of the ambient medium. The largest population of highly-energetic electrons which are relevant for inverse-Compton gamma-ray production is found in the youngest inner lobes of Hydra A. The limit on the inverse-Compton gamma-ray flux excludes a magnetic field below half of the equipartition value of 16 muG in the inner lobes.

astro-ph.CO↗

arXiv2012

Spectral analysis and interpretation of the \gamma-ray emission from the Starburst galaxy NGC 253

Very-high-energy (VHE; E >100 GeV) and high-energy (HE; 100 MeV < E < 100 GeV) data from \gamma-ray observations performed with the H.E.S.S. telescope array and the Fermi-LAT instrument, respectively, are analysed in order to investigate the non-thermal processes in the starburst galaxy NGC 253. The VHE \gamma-ray data can be described by a power law in energy with differential photon index \Gamma=2.14 \pm 0.18_stat \pm 0.30_sys and differential flux normalisation at 1 TeV of F_0 = (9.6 \pm 1.5_stat (+5.7,-2.9)_sys) x 10^{-14} TeV^{-1} cm^{-2} s^{-1}. A power-law fit to the differential HE \gamma-ray spectrum reveals a photon index of \Gamma=2.24 \pm 0.14_stat \pm 0.03_sys and an integral flux between 200 MeV and 200 GeV of F(0.2-200 GeV) = (4.9 \pm 1.0_stat \pm 0.3_sys) x 10^{-9} cm^{-2} s^{-1}. No evidence for a spectral break or turnover is found over the dynamic range of both the LAT instrument and the H.E.S.S. experiment: a combined fit of a power law to the HE and VHE \gamma-ray data results in a differential photon index \Gamma=2.34 \pm 0.03 with a p-value of 30%. The \gamma-ray observations indicate that at least about 20% of the energy of the cosmic rays capable of producing hadronic interactions is channeled into pion production. The smooth alignment between the spectra in the HE and VHE \gamma-ray domain suggests that the same transport processes dominate in the entire energy range. Advection is most likely responsible for charged particle removal from the starburst nucleus from GeV to multiple TeV energies. In a hadronic scenario for the \gamma-ray production, the single overall power-law spectrum observed would therefore correspond to the mean energy spectrum produced by the ensemble of cosmic-ray sources in the starburst region.

astro-ph.HE↗

arXiv2012

H.E.S.S. observations of the Carina nebula and its enigmatic colliding wind binary Eta Carinae

The massive binary system Eta Carinae and the surrounding HII complex, the Carina Nebula, are potential particle acceleration sites from which very-high-energy (VHE; E > 100 GeV) \gamma-ray emission could be expected. This paper presents data collected during VHE \gamma-ray observations with the H.E.S.S. telescope array from 2004 to 2010, which cover a full orbit of Eta Carinae. In the 33.1-hour data set no hint of significant \gamma-ray emission from Eta Carinae has been found and an upper limit on the \gamma-ray flux of 7.7 x 10-13 ph cm-2 s-1 (99% confidence level) is derived above the energy threshold of 470 GeV. Together with the detection of high-energy (HE; 0.1 GeV > E > 100 GeV) \gamma-ray emission by the Fermi-LAT up to 100 GeV, and assuming a continuation of the average HE spectral index into the VHE domain, these results imply a cut-off in the \gamma-ray spectrum between the HE and VHE \gamma-ray range. This could be caused either by a cut-off in the accelerated particle distribution or by severe \gamma-\gamma\ absorption losses in the wind collision region. Furthermore, the search for extended \gamma-ray emission from the Carina Nebula resulted in an upper limit on the \gamma-ray flux of 4.2 x 10-12 ph cm-2 s-1 (99% confidence level). The derived upper limit of ~23 on the cosmic-ray enhancement factor is compared with results found for the old-age mixed-morphology supernova remnant W 28.

astro-ph.HE↗

arXiv2012

Discovery of VHE \gamma-ray emission and multi-wavelength observations of the BL Lac object 1RXS J101015.9-311909

1RXS J101015.9-311909 is a galaxy located at a redshift of z=0.14 hosting an active nucleus belonging to the class of bright BL Lac objects. Observations at high (HE, E > 100 MeV) and very high (VHE, E > 100 GeV) energies provide insights into the origin of very energetic particles present in such sources and the radiation processes at work. We report on results from VHE observations performed between 2006-10 with H.E.S.S. H.E.S.S. data have been analysed with enhanced analysis methods, making the detection of faint sources more significant. VHE emission at a position coincident with 1RXS J101015.9-311909 is detected with H.E.S.S. for the first time. In a total good-quality livetime of about 49 h, we measure 263 excess counts, corresponding to a significance of 7.1\sigma. The photon spectrum above 0.2 TeV can be described by a power-law with a photon index of \Gamma\ = 3.08\pm0.42_{stat}\pm0.20_{sys}. The integral flux above 0.2 TeV is about 0.8% of the flux of the Crab nebula and shows no significant variability over the time reported. In addition, public Fermi/LAT data are analysed to search for high energy emission from the source. The Fermi/LAT HE emission is significant at 8.3\sigma\ in the chosen 25-month dataset. UV and X-ray contemporaneous observations with the Swift satellite in May 2007 are also reported, together with optical observations performed with the ATOM telescope located at the H.E.S.S. site. Swift observations reveal an absorbed X-ray flux of F_{0.3-7 keV} = 1.04^{+0.04}_{-0.05} \times 10^{-11} erg.cm^{-2}.s^{-1} in the 0.3-7 keV range. Finally, all the available data are used to study the source's multi-wavelength properties. The SED can be reproduced using a simple one-zone SSC model with emission from a region with a Doppler factor of 30 and a magnetic field between 0.025 and 0.16 G. These parameters are similar to those obtained for other sources of this type.

astro-ph.HE↗