Ref HAL: in2p3-00402427_v1
Ref Arxiv: 0907.0727
DOI: 10.1051/0004-6361/200912086
Ref. & Cit.: NASA ADS
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59 citations
Résumé:

The H.E.S.S. (High Energy Stereoscopic System) telescopes observed Coma for ~8hr in a search for gamma-ray emission at energies >1TeV. The large 3.5deg FWHM field of view of H.E.S.S. is ideal for viewing a range of targets at various sizes including the Coma cluster core, the radio-relic (1253+275) and merger/infall (NGC 4839) regions to the southwest, and features greater than deg away. No evidence for point-like nor extended TeV gamma-ray emission was found and upper limits to the TeV flux F(E) for E>1, >5, and >10TeV were set for the Coma core and other regions. Converting these limits to an energy flux E^2F(E) the lowest or most constraining is the E>5TeV upper limit for the Coma core (0.2deg radius) at ~8Crab flux units or ~10^{-13}ph cm^{-2} s^{-1}. The upper limits for the Coma core were compared with a prediction for the gamma-ray emission from proton--proton interactions, the level of which ultimately scales with the mass of the Coma cluster. A direct constraint using our most stringent limit for E>5 TeV, on the total energy content in non-thermal protons with injection energy spectrum proportional to E^{-2.1} and spatial distribution following the thermal gas in the cluster, is found to be ~0.2 times the thermal energy, or ~10^{62}erg. The E>5 TeV gamma-ray threshold in this case corresponds to cosmic-ray proton energies >50TeV. Our upper limits rule out the most optimistic theoretical models for gamma ray emission from clusters and complement radio observations which constrain the cosmic ray content in clusters at significantly lower proton energies, subject to assumptions on the magnetic field strength.

Commentaires: 7 pages, 3 figures, accepted in Astronomy and Astrophysics

Ref HAL: in2p3-00410643_v1
Ref Arxiv: 0906.2002
DOI: 10.1051/0004-6361/200912128
Ref. & Cit.: NASA ADS
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81 citations
Résumé:

Simultaneous HESS/CHANDRA/optical observations were performed on the BL Lac object PKS 2155-304 in the night of July 29-30 2006, when the source underwent its second major gamma-ray outburst in Summer 2006. This event took place about 44 hours after the July 28 outburst, known for its ultrafast variability. An unprecedented 6 to 8 hours of uninterrupted coverage was achieved, with spectra and light curves measured down to 7 and 2-minute timescales, respectively. The gamma-ray flux reached a maximum of 11x the Crab flux (>400 GeV), with rise/decay timescales of ~1 hour, plus a few smaller-amplitude flares superimposed on the decaying phase. The emission in the X-ray and VHE bands is strongly correlated, both in flux and spectrum, with no evidence of lags. The VHE spectrum shows a curvature that is variable with time and stronger at higher fluxes. The huge VHE variations (22x) are only accompanied by small-amplitude X-ray and optical variations (factor 2 and 15% respectively). The source has shown for the first time in an HBL a large Compton dominance (L_C/L_S ~10) -- rapidly evolving -- and a cubic relation between VHE and X-ray flux variations, during a decaying phase. These results challenge the common scenarios for the TeV-blazar emission.

Commentaires: 22 pages, 16 figures, accepted for publication in A&A

Ref HAL: in2p3-00410625_v1
Ref Arxiv: 0906.1247
DOI: 10.1051/0004-6361/200811569
Ref. & Cit.: NASA ADS
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100 citations
Résumé:

A detailed study of the spectrum and variability of the source HESS J1745-290 in the Galactic Center (GC) region using new data from the H.E.S.S. array of Cherenkov telescopes is presented. Flaring activity and Quasi Periodic Oscillations (QPO) of HESS J1745-290 are investigated. The image analysis is performed with a combination of a semi-analytical shower model and the statistical moments based Hillas technique. The spectrum and lightcurves of HESS J1745-290 are derived with a likelihood method based on a spectral shape hypothesis. Rayleigh tests and Fourier analysis of the H.E.S.S. GC signal are used to study the periodicity of the source. With three-fold increase in statistics compared to previous work, a deviation from a simple power law spectrum is detected for the first time. The measured energy spectrum over the three years 2004, 2005 and 2006 of data taking is compatible with both a power law spectrum with an exponential cut-off and a broken power law spectrum. The curvature of the energy spectrum is likely to be intrinsic to the photon source as opposed to effects of interstellar absorption. No significant flux variation is found. Increases in the gamma-ray flux of HESS J1745-290 by at least a factor of two would be required for a 3 sigma detection of a flare of time scales of an hour. Investigation of possible QPO activity at periods claimed to be detected in X-rays does not show any periodicities in the H.E.S.S. signal.

Commentaires: 15 pages, 8 figures. Accepted in A&A. Referee format

Ref HAL: hal-00398778_v1
Ref Arxiv: 0906.3411
DOI: 10.1111/j.1365-2966.2009.15272.x
Ref. & Cit.: NASA ADS
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35 citations
Résumé:

The few known gamma-ray binary systems are all associated with variable radioand X-ray emission. The TeV source HESS J0632+057, apparently associated withthe Be star MWC148, is plausibly a new member of this class. Following theidentification of a variable X-ray counterpart to the TeV source we conductedGMRT and VLA observations in June-September 2008 to search for the radiocounterpart of this object. A point-like radio source at the position of thestar is detected in both 1280 MHz GMRT and 5 GHz VLA observations, with anaverage spectral index, alpha, of ~0.6. In the VLA data there is significantflux variability on ~month timescales around the mean flux density of ~0.3 mJy.These radio properties (and the overall spectral energy distribution) areconsistent with an interpretation of HESS J0632+057 as a lower power analogueof the established gamma-ray binary systems.

Ref HAL: in2p3-00392048_v1
Ref Arxiv: 0904.0361
DOI: 10.1051/0004-6361/200811564
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
22 citations
Résumé:

Observations of the globular cluster 47 Tucanae (NGC 104), which contains at least 23 millisecond pulsars, were performed with the H.E.S.S. telescope system. The observations lead to an upper limit of F(E>800 GeV) < 6.7e-13 / cm^2 s on the integral gamma-ray photon flux from 47 Tucanae. Considering millisecond pulsars as the unique potential source of gamma-rays in the globular cluster, constraints based on emission models are derived: on the magnetic field in the average pulsar nebula and on the conversion efficiency of spin-down power to gamma-ray photons or to relativistic leptons.

Commentaires: 5 pages, 2 figures, accepted for publication in A&A

Ref HAL: in2p3-00379058_v1
DOI: 10.1063/1.3125791
Exporter : BibTex | endNote
Résumé:

Very high energy gamma-ray emission has been recently detected by H.E.S.S. from the direction of associations between supernova remnants and molecular clouds. In such associations dense molecular clouds may reveal accelerated cosmic rays in the vicinity of supernova remnant forward shocks. Hadronic interactions could explain part or all of the observed gamma-ray fluxes. The discovery of a new VHE gamma-ray source, HESS J1923+141, coincident with the supernova remnant W51C, is reported. Amongst possible associations for this source is a shocked molecular cloud.

Ref HAL: in2p3-00378403_v1
Ref Arxiv: 0903.2924
DOI: 10.1088/0004-637X/696/2/L150
Ref. & Cit.: NASA ADS
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109 citations
Résumé:

We report on the first simultaneous observations that cover the optical, X-ray, and high energy gamma-ray bands of the BL Lac object PKS 2155-304. The gamma-ray bands were observed for 11 days, between 25 August and 6 September 2008, jointly with the Fermi Gamma-ray Space Telescope and the H.E.S.S. atmospheric Cherenkov array, providing the first simultaneous MeV-TeV spectral energy distribution with the new generation of gamma-ray telescopes. The ATOM telescope and the RXTE and Swift observatories provided optical and X-ray coverage of the low-energy component over the same time period. The object was close to the lowest archival X-ray and Very High Energy state, whereas the optical flux was much higher. The light curves show relatively little (~30%$) variability overall when compared to past flaring episodes, but we find a clear optical/VHE correlation and evidence for a correlation of the X-rays with the high energy spectral index. Contrary to previous observations in the flaring state, we do not find any correlation between the X-ray and VHE components. Although synchrotron self-Compton models are often invoked to explain the SEDs of BL Lac objects, the most common versions of these models are at odds with the correlated variability we find in the various bands for PKS 2155-304.

Commentaires: Accepted for publication in the Astrophysical Journal