Ref HAL: in2p3-00445870_v1
DOI: 10.1051/0004-6361/200913389
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10 citations
Résumé:

Context. High energy particles reside in the relativistic jets of microquasars, making them possible sources of very high energy radiation (VHE, >100 GeV). Detecting this emission would provide a new handle on jet physics. Aims. Observations of the microquasar GRS 1915+105 with the HESS telescope array were undertaken in 2004–2008 to search for VHE emission. Methods. Stereoscopic imaging of Cherenkov radiation from extensive air showers is used to reconstruct the energy and direction of the incident gamma rays. Results. There is no evidence for a VHE gamma-ray signal either from the direction of the microquasar or its vicinity. An upper limit of $6.1\times 10^$ ph cm-2 s-1 (99.9% confidence level) is set on the photon flux above 410 GeV, equivalent to a VHE luminosity of ${\sim} 10^$ erg s-1 at 11 kpc. Conclusions. The VHE to X-ray luminosity ratio in GRS 1915+105 is at least four orders of magnitude lower than the ratio observed in gamma-ray binaries. The VHE radiative efficiency of the compact jet is less than 0.01% based on its estimated total power of 1038 erg s-1. Particle acceleration in GRS 1915+105 is not efficient at high energies and/or the magnetic field is too strong. It is also possible that VHE gamma-rays are produced by GRS 1915+105, but the emission is highly time-dependent.

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: in2p3-00438793_v1
DOI: 10.1051/0004-6361/200912339
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60 citations
Résumé:

Aims. This article presents very-high-energy (VHE; E > 100 GeV) data from the $\gamma$-ray binary PSR B1259-63 as taken during the years 2005, 2006 and before as well as shortly after the 2007 periastron passage. These data extend the knowledge of the lightcurve of this object to all phases of the 3.4 year binary orbit. The lightcurve constrains physical mechanisms present in this TeV source. Methods. Observations of VHE $\gamma$-rays with the HESS telescope array using the Imaging Atmospheric Cherenkov Technique were performed. The HESS instrument features an angular resolution of < 0.1° and an energy resolution of < 20%. Gamma-ray events in an energy range of 0.5-70 TeV were recorded. From these data, energy spectra and lightcurve with a monthly time sampling were extracted. Results. VHE $\gamma$-ray emission from PSR B1259-63 was detected with an overall significance of 9.5 standard deviations using 55h of exposure, obtained from April to August 2007. The monthly flux of $\gamma$-rays during the observation period was measured, yielding VHE lightcurve data for the early pre-periastron phase of the system for the first time. No spectral variability was found on timescales of months. The spectrum is described by a power law with a photon index of $\Gamma$ = 2.8 $\pm$ 0.2stat $\pm$ 0.2sys and flux normalisation $\Phi_{0}$ = (1.1 $\pm$ 0.1stat $\pm$ 0.2sys) $\times$ 10-12 TeV-1 cm-2 s-1. PSR B1259-63 was also monitored in 2005 and 2006, far from periastron passage, comprising 8.9 h and 7.5 h of exposure, respectively. No significant excess of $\gamma$-rays is seen in those observations. Conclusions. PSR B1259-63 has been re-confirmed as a variable TeV $\gamma$-ray emitter. The firm detection of VHE photons emitted at a true anomaly $\theta$$\approx$-0.35 of the pulsar orbit, i.e. already ~50 days prior to the periastron passage, disfavors the stellar disc target scenario as a primary emission mechanism, based on current knowledge about the companion star's disc inclination, extension, and density profile. Results. VHE $\gamma$-ray emission from PSR B1259-63 was detected with an overall significance of 9.5 standard deviations using 55h of exposure, obtained from April to August 2007. The monthly flux of $\gamma$-rays during the observation period was measured, yielding VHE lightcurve data for the early pre-periastron phase of the system for the first time. No spectral variability was found on timescales of months. The spectrum is described by a power law with a photon index of $\Gamma$ = 2.8 $\pm$ 0.2stat $\pm$ 0.2sys and flux normalisation $\Phi_{0}$ = (1.1 $\pm$ 0.1stat $\pm$ 0.2sys) $\times$ 10-12 TeV-1 cm-2 s-1. PSR B1259-63 was also monitored in 2005 and 2006, far from periastron passage, comprising 8.9 h and 7.5 h of exposure, respectively. No significant excess of $\gamma$-rays is seen in those observations. Conclusions. PSR B1259-63 has been re-confirmed as a variable TeV $\gamma$-ray emitter. The firm detection of VHE photons emitted at a true anomaly $\theta$$\approx$-0.35 of the pulsar orbit, i.e. already ~50 days prior to the periastron passage, disfavors the stellar disc target scenario as a primary emission mechanism, based on current knowledge about the companion star's disc inclination, extension, and density profile.

Ref HAL: hal-00430143_v1
Ref Arxiv: 0902.1561
Ref. & Cit.: NASA ADS
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The High Energy Stereoscopic System (H.E.S.S.) consists of four ImagingAtmospheric Cherenkov Telescopes (IACTs) in Namibia for the detection of cosmicvery-high-energy (VHE) gamma-rays. Gamma-ray bursts (GRBs) are among thepotential VHE gamma-ray sources. VHE gamma-emission from GRBs is predicted bymany GRB models. Because of its generally fast-fading nature in many wavebands,the time evolution of any VHE gamma-radiation is still unknown. In order toprobe the largely unexplored VHE gamma-ray spectra of GRBs, a GRB observingprogram has been set up by the H.E.S.S. collaboration. With the highsensitivity of the H.E.S.S. array, VHE gamma-ray flux levels predicted by GRBmodels are well within reach. We report the H.E.S.S. observations of andresults from some of the reported GRB positions during March 2003 - May 2006.

Commentaires: Proceedings of the meeting ``070228 - The Next Decade of GRB Afterglows'' held March 19-23, 2007 in Amsterdam

Ref HAL: hal-00430142_v1
Ref Arxiv: 0905.3593
Ref. & Cit.: NASA ADS
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We present an upper limit of galactic diffuse gamma-ray flux, measured withthe GAMMA experiment at energy about 175 TeV. The results were obtained usingselection of muon poor extensive air showers at mountain level (700 g/cm^2, Mt.Aragats, Armenia) for 5 GeV muon energy threshold. An upper limit for thedifferential flux at energy E=175+/-25 TeV is equal to (5.8-7.0)x10^-12 [ergm^2 s sr]^-1 for 95% confidence level.

Commentaires: Submitted to the 31-th ICRC, Lodz (Pologne) 2009, July 7-15

Ref HAL: hal-00430140_v1
Ref Arxiv: 0907.0642
Ref. & Cit.: NASA ADS
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In the survey of the Galactic plane conducted with H.E.S.S., many VHEgamma-ray sources were discovered for which no clear counterpart at otherwavelengths could be identified. HESS J1731-347 initially belonged to thissource class. Recently however, the new shell-type supernova remnant (SNR)G353.6-0.7 was discovered in radio data, positionally coinciding with the VHEsource. We will present new X-ray observations that cover a fraction of the VHEsource, revealing nonthermal emission that most likely can be interpreted assynchrotron emission from high-energy electrons. This, along with a largerH.E.S.S. data set which comprises more than twice the observation time used inthe discovery paper, allows us to test whether the VHE source may indeed beattributed to shell-type emission from that new SNR. If true, this would makeHESS J1731-347 a new object in the small but growing class of non-thermalshell-type supernova remnants with VHE emission.

Commentaires: 4 pages, 5 figures, to appear in proceedings of the 31st ICRC, Lodz, Poland

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