Ref HAL: in2p3-00367126_v1
Ref Arxiv: 0903.1582
DOI: 10.1088/0004-637X/695/1/L40
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
163 citations
Abstract:

We report the discovery of faint very high energy (VHE, E>100 GeV) gamma-ray emission from the radio galaxy Centaurus A in observations performed with the H.E.S.S. experiment, an imaging atmospheric Cherenkov telescope array consisting of four telescopes located in Namibia. Centaurus A has been observed for more than 120 h. A signal with a statistical significance of 5.0 sigma is detected from the region including the radio core and the inner kpc jets. The integral flux above an energy threshold of ~250 GeV is measured to be ~0.8 % of the flux of the Crab Nebula (apparent luminosity: L(>250 GeV)~2.6x10^39 erg s^-1, adopting a distance of 3.8 Mpc. The spectrum can be described by a power law with a photon index of 2.7 +/- 0.5_stat +/- 0.2_sys. No significant flux variability is detected in the data set. However, the low flux only allows detection of variability on the timescale of days to flux increments above a factor of ~15-20 (3 sigma and 4 sigma, respectively). The discovery of VHE gamma-ray emission from Centaurus A reveals particle acceleration in the source to >TeV energies and, together with M 87, establishes radio galaxies as a class of VHE emitters.

Comments: 5 pages, 4 figures, accepted for publication in ApJ Letters

Ref HAL: in2p3-00354648_v1
Ref Arxiv: 0901.2187
DOI: 10.1051/0004-6361:200811072
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
38 citations
Abstract:

Very-high-energy (VHE; >~100 GeV) gamma-rays are expected from gamma-ray bursts (GRBs) in some scenarios. Exploring this photon energy regime is necessary for understanding the energetics and properties of GRBs. GRBs have been one of the prime targets for the H.E.S.S. experiment, which makes use of four Imaging Atmospheric Cherenkov Telescopes (IACTs) to detect VHE gamma-rays. Dedicated observations of 32 GRB positions were made in the years 2003-2007 and a search for VHE gamma-ray counterparts of these GRBs was made. Depending on the visibility and observing conditions, the observations mostly start minutes to hours after the burst and typically last two hours. Results from observations of 22 GRB positions are presented and evidence of a VHE signal was found neither in observations of any individual GRBs, nor from stacking data from subsets of GRBs with higher expected VHE flux according to a model-independent ranking scheme. Upper limits for the VHE gamma-ray flux from the GRB positions were derived. For those GRBs with measured redshifts, differential upper limits at the energy threshold after correcting for absorption due to extra-galactic background light are also presented.

Comments: 9 pages, 4 tables, 3 figures

Ref HAL: hal-00430148_v1
Ref Arxiv: 0809.0584
DOI: 10.1088/0004-637X/690/2/L101
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
86 citations
Abstract:

The High Energy Stereoscopic System (HESS) survey of the Galactic plane hasestablished the existence of a substantial number (~40) of Galactic TeVgamma-ray sources, a large fraction of which remain unidentified. HESSJ0632+057 is one of a small fraction of these objects which is point-like innature(<2' rms), and is one of only two point-like sources that remainunidentified. Follow-up observations of this object with XMM-Newton haverevealed an X-ray source coincident with the TeV source and with the massivestar MWC 148, of the spectral type B0pe. This source exhibits a hard spectrum,consistent with an absorbed power law with Gamma = 1.26 +/- 0.04, and showssignificant variability on hour timescales. We discuss this spatial coincidenceand the implied spectral energy distribution of this object and argue that itis likely a new gamma-ray binary system with a close resemblance to the threeknown members of this class, and in particular to LS I +61 303. Further X-ray,radio and optical observations of this system are needed to firmly establishHESS J0632+057 as a new member of this rare class of Galactic objects.

Ref HAL: in2p3-00348809_v1
Ref Arxiv: 0812.3762
DOI: 10.1051/0004-6361:200810912
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
27 citations
Abstract:

The rapidly varying (~10 minute timescale) non-thermal X-ray emission observed from Sgr A* implies that particle acceleration is occuring close to the event horizon of the supermassive black hole. The TeV gamma-ray source HESS J1745-290 is coincident with Sgr A* and may be closely related to its X-ray emission. Simultaneous X-ray and TeV observations are required to elucidate the relationship between these objects. We report on joint H.E.S.S./Chandra observations performed in July 2005, during which an X-ray flare was detected. Despite a factor of 9 increase in the X-ray flux of Sgr A*, no evidence is found for an increase in the TeV gamma-ray flux from this region. We find that an increase in the gamma-ray flux of a factor of 2 or greater can be excluded at a confidence level of 99%. This finding disfavours scenarios in which the keV and TeV emission are associated with the same population of accelerated particles and in which the bulk of the gamma-ray emission is produced within ~10^{14} cm (~100 R_S) of the supermassive black hole.

Comments: 5 pages, 2 figures

Ref HAL: in2p3-00345966_v1
Ref Arxiv: 0812.1638
DOI: 10.1051/0004-6361:200811372
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
37 citations
Abstract:

Aims: The nearby galaxy clusters Abell 496 and Abell 85 are studied in the very high energy (VHE, E > 100 GeV) band to investigate VHE cosmic rays (CRs) in this class of objects which are the largest gravitationally bound systems in the Universe. Methods: H.E.S.S., an array of four Imaging Atmospheric Cherenkov Telescopes (IACT), is used to observe the targets in the range of VHE gamma rays. Results: No significant gamma-ray signal is found at the respective position of the two clusters with several different source size assumptions for each target. In particular, emission regions corresponding to the high density core, to the extension of the entire X-ray emission in these clusters, and to the very extended region where the accretion shock is expected, are investigated. Upper limits are derived for the gamma-ray flux at energies E>570 GeV for Abell 496 and E>460 GeV for Abell 85. Conclusions: From the non-detection in VHE gamma rays, upper limits on the total energy of hadronic CRs in the clusters are calculated. If the cosmic-ray energy density follows the large scale gas density profile, the limit on the fraction of energy in these non-thermal particles with respect to the total thermal energy of the intra-cluster medium (ICM) is 51% for Abell 496 and only 8% for Abell 85 due to its larger mass and higher gas density. These upper limits are compared with theoretical estimates. They predict about ~10% of the thermal energy of the ICM in non-thermal particles. The observations presented here can constrain these predictions especially for the case of the Abell 85 cluster.

Comments: 11 pages, 4 figures, accepted for publication in A&A

Ref HAL: in2p3-00341406_v1
Ref Arxiv: 0811.3894
DOI: 10.1103/PhysRevLett.101.261104
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
465 citations
Abstract:

The very large collection area of ground-based gamma-ray telescopes gives them a substantial advantage over balloon/satellite based instruments in the detection of very-high-energy (>600 GeV) cosmic-ray electrons. Here we present the electron spectrum derived from data taken with the H.E.S.S. system of imaging atmospheric Cherenkov telescopes. In this measurement, the first of this type, we are able to extend the measurement of the electron spectrum beyond the range accessible to direct measurements. We find evidence for a substantial steepening in the energy spectrum above 600 GeV compared to lower energies.

Comments: 5 pages, 3 figures. Accepted for publication by Phys. Rev. Letters; modified to include latest ATIC data points

Ref HAL: in2p3-00332607_v1
Ref Arxiv: 0810.3475
DOI: 10.1103/PhysRevLett.101.170402
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
93 citations
Abstract:

In the past few decades, several models have predicted an energy-dependence of the speed of light in the context of quantum gravity. For cosmological sources such as active galaxies, this minuscule effect can add up to measurable photon-energy dependent time lags. In this paper a search for such time lags during the H.E.S.S. observations of the exceptional very high energy flare of the active galaxy PKS 2155-304 on 28 July in 2006 is presented. Since no significant time lag is found, lower limits on the energy scale of speed of light modifications are derived.

Comments: Accepted for publication by Phys. Rev. Letters