Ref HAL: hal-00337735_v1
Ref Arxiv: 0811.1506
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Résumé:

After a successful development of theoretical and numerical works on Fermi acceleration at relativistic shocks, some difficulties recently raised with the scattering issue, a crucial aspect of the process. Most pioneering works were developed assuming the scattering off magnetic fluctuations as given. Even in that case, when a mean field is considered, its orientation is mostly perpendicular to the shock normal in the front frame, and this tends to quench the scattering process. Solving this difficulty leads to address the issue of the generation of very intense magnetic fluctuations at short wave lengths. The relativistic motion of the shock front let the cosmic rays to visit upstream during a very short time only, making this generation of magnetic fluctuations very challenging. Anyway there is some hope to solve the problem. Thanks to a recent work by Spitkovsky (2008) \cite{AS}, we know that the process works without any mean field and now we have to investigate up to which intensity the mean field can be amplified for allowing Fermi process with appropriate fast instabilities. In this presentation, the collisionless shock structure in relativistic regime is sketched, the scattering issue is presented, and the instabilities that can provide the expected magnetic field amplification are presented as well. Although there exists observational evidence that particles are accelerated in relativistic flows and are distributed according to a power law suggesting a Fermi process, the drastic conditions for Fermi process to work are not always clearly fulfilled.

Ref HAL: in2p3-00332607_v1
Ref Arxiv: 0810.3475
DOI: 10.1103/PhysRevLett.101.170402
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93 citations
Résumé:

In the past few decades, several models have predicted an energy-dependence of the speed of light in the context of quantum gravity. For cosmological sources such as active galaxies, this minuscule effect can add up to measurable photon-energy dependent time lags. In this paper a search for such time lags during the H.E.S.S. observations of the exceptional very high energy flare of the active galaxy PKS 2155-304 on 28 July in 2006 is presented. Since no significant time lag is found, lower limits on the energy scale of speed of light modifications are derived.

Commentaires: Accepted for publication by Phys. Rev. Letters

Ref HAL: in2p3-00331427_v1
Ref Arxiv: 0810.2689
DOI: 10.1088/0004-637X/692/2/1500
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85 citations
Résumé:

The shell-type supernova remnant (SNR) RCW 86, possibly associated with the historical supernova SN 185, with its relatively large size (about 40' in diameter) and the presence of non-thermal X-rays is a promising target for gamma-ray observations. The high sensitivity, good angular resolution of a few arc minutes and the large field of view of the High Energy Stereoscopic System (H.E.S.S.) make it ideally suited for the study of the gamma-ray morphology of such extended sources. H.E.S.S. observations have indeed led to the discovery of the SNR RCW 86 in very high energy (VHE; E > 100 GeV) gamma-rays. With 31 hours of observation time, the source is detected with a statistical significance of 8.5 sigma and is significantly more extended than the H.E.S.S. point spread function. Morphological studies have been performed and show that the gamma-ray flux does not correlate perfectly with the X-ray emission. The flux from the remnant is ~10% of the flux from the Crab nebula, with a similar photon index of about 2.5. Possible origins of the very high energy gamma-ray emission, via either Inverse Compton scattering by electrons or the decay of neutral pions produced by proton interactions, are discussed on the basis of spectral features obtained both in the X-ray and gamma-ray regimes.

Commentaires: 6 pages, 5 figures; Accepted for publication in ApJ on October 14, 2008

Ref HAL: in2p3-00324687_v1
Ref Arxiv: 0809.3894
DOI: 10.1088/0004-637X/691/1/175
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37 citations
Résumé:

A search for a dark matter (DM) annihilation signal into $\gamma$-rays toward the direction of the Canis Major (CMa) overdensity is presented. The nature of CMa is still controversial and one scenario represents it as a dwarf galaxy, making it an interesting candidate for DM annihilation searches. A total of 9.6 hours of high quality data were collected with the H.E.S.S. array of Imaging Atmospheric Cherenkov Telescopes (IACTs) and no evidence for a very high energy $\gamma$-ray signal is found. Upper limits on the CMa dwarf galaxy mass of the order of 10$^{9}$ M$_{\odot}$ are derived at the 95% C.L. assuming neutralino masses in the range 500 GeV - 10 TeV and relatively large annihilation cross-sections. Constraints on the velocity-weighted annihilation cross section $<\sigma v>$, are calculated for specific WIMP scenarios, using a NFW model for the DM halo profile and taking advantage of numerical simulations of hierarchical structure formation. 95% C.L. exclusion limits of the order of 5 $\times$ 10$^{-24}$ cm$^{3}$ s$^{-1}$ are reached in the 500 GeV - 10 TeV DM particle mass interval, assuming a total halo mass of 3 $\times$ 10$^{8}$ M$_{\odot}$.

Commentaires: Accepted for publication in ApJ

Ref HAL: in2p3-00321879_v1
Ref Arxiv: 0809.2334
DOI: 10.1088/0004-637X/690/2/1068
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21 citations
Résumé:

We report on the first completely simultaneous observation of a gamma-ray burst (GRB) using an array of Imaging Atmospheric Cherenkov Telescopes which is sensitive to photons in the very-high-energy (VHE) gamma-ray range (>~100 GeV). On 2006 June 2, the Swift Burst Alert Telescope (BAT) registered an unusually soft gamma-ray burst (GRB 060602B). The burst position was under observation using the High Energy Stereoscopic System (H.E.S.S.) at the time the burst occurred. Data were taken before, during, and after the burst. A total of 5 hours of observations were obtained during the night of 2006 June 2-3, and 5 additional hours were obtained over the next 3 nights. No VHE gamma-ray signal was found during the period covered by the H.E.S.S. observations. The 99% confidence level flux upper limit (>1 TeV) for the prompt phase (9s) of GRB 060602B is 2.9x10^-9 erg cm^-2 s^-1. Due to the very soft BAT spectrum of the burst compared to other Swift GRBs and its proximity to the Galactic center, the burst is likely associated with a Galactic X-ray burster, although the possibility of it being a cosmological GRB cannot be ruled out. We discuss the implications of our flux limits in the context of these two bursting scenarios.

Commentaires: 8 pages, 4 figures, 1 table; ApJ in press

Ref HAL: in2p3-00289653_v1
Ref Arxiv: 0806.3347
DOI: 10.1051/0004-6361:200809401
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22 citations
Résumé:

Observations of Kepler's supernova remnant (G4.5+6.8) with the H.E.S.S. telescope array in 2004 and 2005 with a total live time of 13 h are presented. Stereoscopic imaging of Cherenkov radiation from extensive air showers is used to reconstruct the energy and direction of the incident gamma rays. No evidence for a very high energy (VHE: >100 GeV) gamma-ray signal from the direction of the remnant is found. An upper limit (99% confidence level) on the energy flux in the range 230 GeV - 12.8 TeV of 8.6 x 10^{-13} erg cm^{-2} s^{-1} is obtained. In the context of an existing theoretical model for the remnant, the lack of a detectable gamma-ray flux implies a distance of at least 6.4 kpc. A corresponding upper limit for the density of the ambient matter of 0.7 cm^{-3} is derived. With this distance limit, and assuming a spectral index Gamma = 2, the total energy in accelerated protons is limited to E_p < 8.6 x 10^{49} erg. In the synchrotron/inverse Compton framework, extrapolating the power law measured by RXTE between 10 and 20 keV down in energy, the predicted gamma-ray flux from inverse Compton scattering is below the measured upper limit for magnetic field values greater than 52 muG.

Commentaires: 5 pages, 3 figures; Accepted for publication in A&A on June 12, 2008

Ref HAL: in2p3-00288992_v1
Ref Arxiv: 0806.2981
DOI: 10.1103/PhysRevD.78.072008
Ref. & Cit.: NASA ADS
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22 citations
Résumé:

The H.E.S.S. array of Cherenkov telescopes has performed, from 2004 to 2007, a survey of the inner Galactic plane at photon energies above 100 GeV. About 400 hours of data have been accumulated in the region between -30 and +60 degrees in Galactic longitude, and between -3 and +3 degrees in Galactic latitude. Assuming that dark matter is composed of Weakly Interacting Massive Particles, we calculate here the H.E.S.S. sensitivity map for dark matter annihilations, and derive the first experimental constraints on the ''mini-spikes'' scenario, in which a gamma-ray signal arises from dark matter annihilation around Intermediate Mass Black Holes. The data exclude the proposed scenario at a 90% confidence level for dark matter particles with velocity-weighted annihilation cross section sigma v above 10^28 cm3s^-1 and mass between 800 GeV and 10 TeV.

Commentaires: 12 pages, 7 figures, accepted for publication in Phys. Rev. D on June 16, 2008