Ref HAL: in2p3-00118136_v1
Ref Arxiv: astro-ph/0612016
DOI: 10.1126/science.1134408
Ref. & Cit.: NASA ADS
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252 citations
Abstract:

The detection of fast variations of the TeV (10^12 eV) gamma-ray flux, on time-scales of days, from the nearby radio galaxy M 87 is reported. These variations are ~10 times faster than that observed in any other waveband and imply a very compact emission region with a dimension similar to the Schwarzschild radius of the central black hole. We thus can exclude several other sites and processes of the gamma-ray production. The observations confirm that TeV gamma-rays are emitted by extragalactic sources other than blazars, where jets are not relativistically beamed towards the observer.

Comments: 16 pages, 5 figures

Ref HAL: in2p3-00137679_v1
Ref Arxiv: astro-ph/0610509
DOI: 10.1103/PhysRevLett.97.221102
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
185 citations
Abstract:

The detection of gamma rays from the source HESS J1745-290 in the Galactic Center (GC) region with the High Energy Spectroscopic System (HESS) array of Cherenkov telescopes in 2004 is presented. After subtraction of the diffuse gamma-ray emission from the GC ridge, the source is compatible with a point source with spatial extent less than 1.2[prime](stat) (95% C.L.). The measured energy spectrum above 160 GeV is compatible with a power law with photon index of 2.25±0.04(stat)±0.10(syst) and no significant flux variation is detected. It is finally found that the bulk of the very high energy emission must have non-dark-matter origin.

Comments: 5 pages, 2 figures, published in Physical Review Letters Correct some references The accepted version set on astro-ph was slightly too long for publication. This is a slightly shorter version

Ref HAL: in2p3-00121031_v1
DOI: 10.1051/0004-6361:20065546
Exporter : BibTex | endNote
138 citations
Abstract:

Aims. We present results from deep $\gamma$-ray observations of the Galactic pulsar wind nebula HESS J1825-137 performed with the HESS array. Methods. Detailed morphological and spatially resolved spectral studies reveal the very high-energy (VHE) $\gamma$-ray aspects of this object with unprecedented precision. Results. We confirm previous results obtained in a survey of the Galactic Plane in 2004. The $\gamma$-ray emission extends asymmetrically to the south and south-west of the energetic pulsar PSR J1826-1334 , that is thought to power the pulsar wind nebula. The differential $\gamma$-ray spectrum of the whole emission region is measured over more than two orders of magnitude, from 270 GeV to 35 TeV, and shows indications for a deviation from a pure power law. Spectra have also been determined for spatially separated regions of HESS J1825-137. The photon indices from a power-law fit in the different regions show a softening of the spectrum with increasing distance from the pulsar and therefore an energy dependent morphology. Conclusions. This is the first time that an energy dependent morphology has been detected in the VHE $\gamma$-ray regime. The VHE $\gamma$-ray emission of HESS J1825-137 is phenomenologically discussed in the scenario where the $\gamma$-rays are produced by VHE electrons via Inverse Compton scattering. The high $\gamma$-ray luminosity of the source cannot be explained on the basis of constant spin-down power of the pulsar and requires higher injection power in past.

Ref HAL: in2p3-00107167_v1
Ref Arxiv: astro-ph/0607333
DOI: 10.1051/0004-6361:20065351
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
524 citations
Abstract:

The Crab nebula was observed with the H.E.S.S. stereoscopic Cherenkov-telescope array between October 2003 and January 2005 for a total of 22.9 hours (after data quality selection). Observations were made with three operational telescopes in late 2003 and with the complete 4 telescope array in January - February 2004 and October 2004 - January 2005. The observations are discussed and used as an example to detail the flux and spectral analysis procedures of H.E.S.S., and to evaluate the systematic uncertainties in H.E.S.S. flux measurements. The flux and spectrum of gamma-rays from the source are calculated on run-by-run and monthly time-scales, and a correction is applied for long-term variations in the detector sensitivity. Comparisons of the measured flux and spectrum over the observation period, along with the results from a number of different analysis procedures are used to estimate systematic uncertainties in the measurements. The energy spectrum is found to follow a power law with an exponential cutoff, with photon index $\Gamma = 2.39 \pm 0.03\stat$ and cutoff energy $E_{c} = (14.3 \pm 2.1\stat) \textrm{TeV}$ between 440 GeV and 40 TeV. The observed integral flux above 1 TeV is $(2.26 \pm 0.08\stat) \times 10^{-11} cm^{-2} s^{-1}$. The estimated systematic error on the flux measurement is estimated to be 20%, while the estimated systematic error on the spectral slope is 0.1.

Comments: Accepted for publication in A&A