Ref HAL: in2p3-00023654_v1
Ref Arxiv: astro-ph/0411582
DOI: 10.1051/0004-6361:20041853
Ref. & Cit.: NASA ADS
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157 citations
Abstract:

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
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
155 citations
Abstract:

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.

Ref HAL: in2p3-00024064_v1
Ref Arxiv: astro-ph/0504380
DOI: 10.1126/science.1108643
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
275 citations
Abstract:

Very high energy gamma-rays probe the long-standing mystery of the origin of cosmic rays. Produced in the interactions of accelerated particles in astrophysical objects, they can be used to image cosmic particle accelerators. A first sensitive survey of the inner part of the Milky Way with the High Energy Stereoscopic System (H.E.S.S.) reveals a population of eight previously unknown firmly detected sources of very high energy gamma-rays. At least two have no known radio or X-ray counterpart and may be representative of a new class of `dark' nucleonic cosmic ray sources.

Ref HAL: in2p3-00024066_v1
Ref Arxiv: astro-ph/0504120
DOI: 10.1051/0004-6361:200500105
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122 citations
Abstract:

The Supernova Remnant MSH 15-52 has been observed in very high energy (VHE) gamma-rays using the H.E.S.S. 4-telescope array located in Namibia. A gamma-ray signal is detected at the 25 sigma level during an exposure of 22.1 hours live time. The image reveals an elliptically shaped emission region around the pulsar PSR B1509-58, with semi-major axis 6' in the NW-SE direction and semi-minor axis 2' approximately. This morphology coincides with the diffuse pulsar wind nebula as observed at X-ray energies by ROSAT. The overall energy spectrum from 280 GeV up to 40 TeV can be fitted by a power law with photon index Gamma = 2.27 +/- 0.03(stat.) +/- 0.20(syst.). The detected emission can be plausibly explained by inverse Compton scattering of accelerated relativistic electrons with soft photons.

Ref HAL: in2p3-00024315_v1
Ref Arxiv: astro-ph/0504520
DOI: 10.1051/0004-6361:200500113
Ref. & Cit.: NASA ADS
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57 citations
Abstract:

The high-frequency peaked BL Lac PKS 2005-489 (z=0.071) was observed in 2003 and 2004 with the H.E.S.S. stereoscopic array of imaging atmospheric-Cherenkov telescopes in Namibia. A signal was detected at the 6.7 sigma level in the 2004 observations (24.2 hrs live time), but not in the 2003 data set (27.3 h live time). PKS 2005-489 is the first blazar independently discovered by H.E.S.S. to be an emitter of VHE photons, and only the second such blazar in the Southern Hemisphere. The integral flux above 200 GeV observed in 2004 is (6.9$\pm$ 1.0$_{stat} \pm 1.4_{syst}$) x 10$^{-12}$ cm$^{-2}$ s$^{-1}$, corresponding to $\sim$ 2.5% of the flux observed from the Crab Nebula. The 99% upper limit on the flux in 2003, I(>200 GeV) < 5.2 x 10$^{-12}$ cm$^{-2}$ s$^{-1}$, is smaller than the flux measured in 2004, suggesting an increased level of activity in 2004. However, the data show no evidence for significant variability on any time scale less than a year. An energy spectrum is measured and is characterized by a very soft power law (photon index of $\Gamma$= 4.0 $\pm$ 0.4).

Ref HAL: in2p3-00024439_v1
Ref Arxiv: astro-ph/0506319
DOI: 10.1051/0004-6361:20053050
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69 citations
Abstract:

Mkn 421 was observed during a high flux state for nine nights in April and May 2004 with the fully operational High Energy Stereoscopic System (H.E.S.S.) in Namibia. The observations were carried out at zenith angles of 60$^\circ$--65$^\circ$, which result in an average energy threshold of 1.5 TeV and a collection area reaching 2 km$^2$ at 10 TeV. Roughly 7000 photons from Mkn 421 were accumulated with an average gamma-ray rate of 8 photons/min. The overall significance of the detection exceeds 100 standard deviations. The light-curve of integrated fluxes above 2 TeV shows changes of the diurnal flux up to a factor of 4.3. For nights of high flux, intra-night variability is detected with a decay time of less than 1 hour. The time averaged energy spectrum is curved and is well described by a power-law with a photon index $\egamm$ and an exponential cutoff at $\ecut$ TeV and an average integral flux above 2 TeV of 3 Crab flux units. Significant variations of the spectral shape are detected with a spectral hardening as the flux increases. Contemporaneous multi-wavelength observations at lower energies (X-rays and gamma-rays above $\approx 300$ GeV) indicate smaller relative variability amplitudes than seen above 2 TeV during high flux state observed in April 2004.

Comments: 5 pages, 4 figures

Ref HAL: in2p3-00024436_v1
Ref Arxiv: astro-ph/0502239
DOI: 10.1051/0004-6361:20042522
Ref. & Cit.: NASA ADS
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41 citations
Abstract:

Observations of the shell-type supernova remnant SN1006 have been carried out with the HESS system of Cherenkov telescopes during 2003 (18.2 h with two operating telescopes) and 2004 (6.3 h with all four telescopes). No evidence for TeV $\gamma$-ray emission from any compact or extended region associated with the remnant is seen and resulting upper limits at the 99.9% confidence level are up to a factor 10 lower than previously-published fluxes from CANGAROO. For SN1006 at its current epoch of evolution we define limits for a number of important global parameters. Upper limits on the $\gamma$-ray luminosity (for E = 0.26 to 10 TeV, distance d = 2 kpc) of $L_\gamma < 1.7$ $\times$ 1033 erg s-1, and the total energy in corresponding accelerated protons, $W_{\rm p}<1.6$ $\times$ 1050 erg are estimated (for proton energies $E_{\rm p} \sim 1.5$ to 60 TeV and assuming the lowest value n=0.05 cm-3 of the ambient target density discussed in literature). Extending this estimate to cover the range of proton energies observed in the cosmic ray spectrum up to the knee (we take here $E_{\rm p} \sim$ 1 GeV to 3 PeV, assuming a differential particle index -2) gives $W_{\rm p}<6.3$ $\times$ 1050 erg. A lower limit on the post-shock magnetic field of $B>25~\mu$G results when considering the synchrotron/inverse-Compton framework for the observed X-ray flux and $\gamma$-ray upper limits.