Ref HAL: in2p3-00023668_v1
Ref Arxiv: astro-ph/0501512
DOI: 10.1051/0004-6361:200500008
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14 citations
Résumé:

The region around PSR B1706-44 has been observed with the H.E.S.S. imaging atmospheric Cherenkov telescopes in 2003. No evidence for gamma-ray emission in the TeV range was found at the pulsar position or at the radio arc which corresponds to the supernova remnant G 343.1-2.3. The 99% confidence level flux upper limit at the pulsar position is Ful(E > 350GeV) = 1.4e-12 /cm^2/s assuming a power law with photon index of 2.5 and Ful(E > 500GeV) = 1.3e-12 /cm^2/s without an assumption on the spectral shape. The reported upper limits correspond to 8% of the flux from an earlier detection by the CANGAROO experiment.

Ref HAL: in2p3-00024875_v1
Ref Arxiv: astro-ph/0506280
DOI: 10.1051/0004-6361:20052983
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265 citations
Résumé:

We report the discovery of very-high-energy (VHE) $\gamma$-ray emission of the binary system PSR B1259-63/SS 2883 of a radio pulsar orbiting a massive, luminous Be star in a highly eccentric orbit. The observations around the 2004 periastron passage of the pulsar were performed with the four 13 m Cherenkov telescopes of the H.E.S.S. experiment, recently installed in Namibia and in full operation since December 2003. Between February and June 2004, a $\gamma$-ray signal from the binary system was detected with a total significance above 13 $\sigma$. The flux was found to vary significantly on timescales of days which makes PSR B1259-63 the first variable galactic source of VHE $\gamma$-rays observed so far. Strong emission signals were observed in pre- and post-periastron phases with a flux minimum around periastron, followed by a gradual flux decrease in the months after. The measured time-averaged energy spectrum above a mean threshold energy of 380 GeV can be fitted by a simple power law $F_0(E/1 TeV)^{-\Gamma}$ with a photon index $\Gamma = 2.7\pm0.2_{stat}\pm0.2_{sys}$ and flux normalisation $F_0 = (1.3\0.1_{stat}\pm0.3_{sys})\times10^{-12} TeV^{-1} cm^{-2} s^{-1}$. This detection of VHE $\gamma$-rays provides unambiguous evidence for particle acceleration to multi-TeV energies in the binary system. In combination with coeval observations of the X-ray synchrotron emission by the RXTE and INTEGRAL instruments, and assuming the VHE $\gamma¤-ray emission to be produced by the inverse Compton mechanism, the magnetic field strength can be directly estimated to be of the order of 1 G.

Commentaires: 10 pages, 8 figures, accepted in Astronomy and Astrophysics on 2 June 2005, replace: document unchanged, replaced author field in astro-ph entry - authors are all members of the H.E.S.S. collaboration and three additional authors (99+3, see document)

Ref HAL: in2p3-00024349_v1
Ref Arxiv: astro-ph/0506593
DOI: 10.1051/0004-6361:20053353
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89 citations
Résumé:

The High Energy Stereoscopic System (H.E.S.S.) has observed the high-frequency peaked BL Lac object PKS2155-304 in 2003 between October 19 and November 26 in Very High Energy (VHE) $\gamma$-rays (E $\geq$ 160 GeV for these observations). Observations were carried out simultaneously with the Proportional Counter Array (PCA) on board the Rossi X-ray Timing Explorer satellite (RXTE), the Robotic Optical Transient Search Experiment (ROTSE) and the Nancay decimetric radiotelescope (NRT). Intra-night variability is seen in the VHE band, the source being detected with a high significance on each night it was observed. Variability is also found in the X-ray and optical bands on kilosecond timescales, along with flux-dependent spectral changes in the X-rays. The average H.E.S.S. spectrum shows a very soft power law shape with a photon index of 3.37 $\pm$ 0.07${stat} \pm 0.10 {syst}. The energy outputs in the 2-10 keV and in the VHE gamma-ray range are found to be similar, with the X-rays and the optical fluxes at a level comparable to some of the lowest historical measurements, indicating that PKS2155-304 was in a low or quiescent state during the observations. Both a leptonic and a hadronic model are used to derive source parameters from these observations.

Commentaires: 16 pages, 13 figures, Accepted (June 22) for publication in A&A

Ref HAL: in2p3-00024534_v1
Ref Arxiv: astro-ph/0507207
DOI: 10.1051/0004-6361:20053478
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60 citations
Résumé:

A sample of selected active galactic nuclei (AGN) was observed in 2003 and 2004 with the High Energy Stereoscopic System (H.E.S.S.), an array of imaging atmospheric-Cherenkov telescopes in Namibia. The redshifts of these candidate very-high-energy (VHE, >100 GeV) gamma-ray emitters range from z=0.00183 to z=0.333. Significant detections were already reported for some of these objects, such as PKS 2155-304 and Markarian 421. Marginal evidence (3.1 sigma) for a signal is found from large-zenith-angle observations of Markarian 501, corresponding to an integral flux of I(>1.65 TeV) = (1.5 +/- 0.6 (stat) +/- 0.3 (syst)) x 10^{-12} cm^{-2} s^{-1} or ~15% of the Crab Nebula flux. Integral flux upper limits for 19 other AGN, based on exposures of ~1 to ~8 hrs live time, and with average energy thresholds between 160 GeV and 610 GeV, range from 0.4% to 5.1% of the Crab Nebula flux. All the upper limits are the most constraining ever reported for these objects.

Ref HAL: in2p3-00024438_v1
Ref Arxiv: astro-ph/0507370
DOI: 10.1051/0004-6361:20053521
Ref. & Cit.: NASA ADS
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17 citations
Résumé:

We present the result of 28 hours of observations of the nearby starburst galaxy NGC 253 with the H.E.S.S. detector in 2003. We find no evidence for very high energy gamma-ray emission from this object. Gamma-ray emission above 400 GeV from NGC 253 had been reported by the CANGAROO collaboration in 2002. From the H.E.S.S. data we derive upper limits on the flux above 300 GeV of 1.9 * 10^-12 photons cm^-2 s^-1 for a point-like source and 6.3 * 10^-12 photons cm^-2 s^-1 for a source of radius 0.5 degrees as reported by CANGAROO, both at a confidence level of 99%. These upper limits are inconsistent with the spectrum reported by CANGAROO. The expected very high energy gamma-ray emission from this object is discussed in the framework of a galactic wind propagation model.

Commentaires: 7 pages, 3 figures

Ref HAL: in2p3-00023654_v1
Ref Arxiv: astro-ph/0411582
DOI: 10.1051/0004-6361:20041853
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157 citations
Résumé:

The high-frequency peaked BL Lac PKS 2155-304 at redshift z=0.117 has been detected with high significance (~45 sigma) at energies greater than 160 GeV, using the H.E.S.S. stereoscopic array of imaging air-Cherenkov telescopes in Namibia. A strong signal is found in each of the data sets corresponding to the dark periods of July and October, 2002, and June-September, 2003. The observed flux of VHE gamma rays shows variability on time scales of months, days, and hours. The monthly-averaged integral flux above 300 GeV varies between 10% and 60% of the flux observed from the Crab Nebula. Energy spectra are measured for these individual periods of data taking and are characterized by a steep power law with a time-averaged photon index of 3.32 +/- 0.06. An improved chi-square per degree of freedom is found when either a power law with an exponential cutoff energy or a broken power law are fit to the time-averaged energy spectrum. However, the significance of the improvement is marginal (~2 sigma). The suggested presence of features in the energy spectrum may be intrinsic to the emission from the blazar, or an indication of absorption of TeV gamma rays by the extragalactic infrared background light.

Ref HAL: in2p3-00023664_v1
Ref Arxiv: astro-ph/0501265
DOI: 10.1051/0004-6361:200500022
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155 citations
Résumé:

Very high energy (> 100 GeV) gamma-ray emission has been detected for the first time from the composite supernova remnant G0.9+0.1 using the H.E.S.S. instrument. The source is detected with a significance of 13 sigma, and a photon flux above 200 GeV of (5.7+/-0.7 stat +/- 1.2 sys) * 10^-12 cm^-2 s^-1, making it one of the weakest sources ever detected at TeV energies. The photon spectrum is compatible with a power law (dN/dE \propto E^-Gamma) with photon index Gamma = 2.40 +/- 0.11 stat +/- 0.20 sys. The gamma-ray emission appears to originate in the plerionic core of the remnant, rather than the shell, and can be plausibly explained as inverse Compton scattering of relativistic electrons.