Ref HAL: in2p3-00221478_v1
Ref Arxiv: 0801.3555
DOI: 10.1051/0004-6361:20077765
Ref. & Cit.: NASA ADS
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203 citations
Abstract:

We observed the W28 field (for ~40 h) at Very High Energy (VHE) gamma-ray energies (E>0.1 TeV) with the H.E.S.S. Cherenkov telescopes. A reanalysis of EGRET E>100 MeV data was also undertaken. Results from the NANTEN 4m telescope Galactic plane survey and other CO observations have been used to study molecular clouds. We have discovered VHE gamma-ray emission (HESSJ1801-233) coincident with the northeastern boundary of W28, and a complex of sources (HESSJ1800-240A, B and C) ~0.5 deg south of W28, in the Galactic disc. The VHE differential photon spectra are well fit by pure power laws with indices Gamma~2.3 to 2.7. The NANTEN ^{12}CO(J=1-0) data reveal molecular clouds positionally associating with the VHE emission, spanning a ~15 km s^{-1} range in local standard of rest velocity. The VHE/molecular cloud association could indicate a hadronic origin for HESSJ1801-233 and HESSJ1800-240, and several cloud components in projection may contribute to the VHE emission. The clouds have components covering a broad velocity range encompassing the distance estimates for W28 (~2 kpc), and extending up to ~4 kpc. Assuming a hadronic origin, and distances of 2 and 4 kpc for cloud components, the required cosmic ray density enhancement factors (with respect to the solar value) are in the range ~10 to ~30. If situated at 2 kpc distance, such cosmic ray densities may be supplied by a SNR like W28. Additionally and/or alternatively, particle acceleration may come from several catalogued SNRs and SNR candidates, the energetic ultra compact HII region W28A2, and the HII regions M8 and M20 along with their associated open clusters. Further sub-mm observations would be recommended to probe in detail the dynamics of the molecular clouds at velocites >10 km s^{-1}, and their possible connection to W28.

Comments: 10 pages, 4 figures. Accepted for publication in Astronomy & Astrophysics. (Abstract shortened)

Ref HAL: hal-00491483_v1
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Abstract:

Recent advances in very-high-energy (VHE) gamma-ray astronomy have opened a new observational window on the physics of pulsars. The high sensitivity of current imaging atmospheric Cherenkov telescopes, and in particular of the H.E.S.S. array, has already led to the discovery of about a dozen VHE-emitting pulsar wind nebulae (PWN) and PWN candidates. These include the plerions in the composite supernova remnants MSH 15-52, G 21.5-0.9, Kes 75, and Vela, two sources in the Kookaburra, and the nebula of PSR B 1823-13. This VHE emission is generally interpreted as inverse Compton emission from the relativistic electrons and positrons accelerated by the pulsar and its wind; as such, it can yield a more direct spatial and spectral view of the accelerated particles than can be inferred from observations of their synchrotron emission. The VHE-emitting PWNe detected by the H.E.S.S. telescopes are reviewed and the implications for pulsar physics discussed.

Ref HAL: hal-00430366_v1
Ref Arxiv: astro-ph/0702158
Ref. & Cit.: NASA ADS
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Abstract:

This work investigates some aspects of the transport of low-energy positronsin the interstellar medium (ISM). We consider resonance interactions withmagnetohydrodynamic waves above the resonance threshold. Below the threshold,collisions take over and deflect positrons in their motion parallel tomagnetic-field lines. Using Monte-Carlo simulations, we model the propagationand energy losses of positrons in the different phases of the ISM until theyannihilate. We suggest that positrons produced in the disk by an old populationof stars, with initial kinetic energies below 1 MeV, and propagating in thespiral magnetic field of the disk, can probably not penetrate the Galacticbulge.

Comments: 4 pages, 3 figures, accepted for publication in the proceeding of the 6th INTEGRAL workshop

Ref HAL: hal-00430264_v1
Ref Arxiv: 0709.4380
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Abstract:

Motivated by recent detections of pulsar wind nebulae in very-high-energy(VHE) gamma rays, a systematic search for VHE gamma-ray sources associated withenergetic pulsars was performed, using data obtained with the H.E.S.S. (HighEnergy Stereoscopic System) instrument. The search for VHE gamma-ray sourcesnear the pulsar PSR J1718-3825 revealed the new VHE gamma-ray source HESSJ1718-385. We report on the results from the HESS data analysis of this sourceand on possible associations with the pulsar and at other wavelengths. Weinvestigate the energy spectrum of HESS J1718-385 that shows a clear peak. Thisis only the second time a VHE gamma-ray spectral maximum from a cosmic sourcewas observed, the first being the Vela X pulsar wind nebula.

Comments: Edited by Rogelio Caballero, Juan Carlos D'Olivo, Gustavo Medina-Tanco, Lukas Nellen, Federico A. Sánchez, José F. Valdés-Galicia. Universidad Nacional Autónoma de México, Mexico City, Mexico,

Ref HAL: hal-00430155_v1
Ref Arxiv: 0710.2247
Ref. & Cit.: NASA ADS
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Abstract:

The deeper and more extended survey of the central parts of the GalacticPlane by H.E.S.S. during 2005-2007 has revealed a number of new point-like, aswell as, extended sources. Two point-like sources can be associated to tworemarkable objects around "Crab-like" young and energetic pulsars in ourGalaxy: G21.5-0.9 and Kes 75. The characteristics of each of the sources arepresented and possible interpretations are briefly discussed.

Comments: Edited by Rogelio Caballero, Juan Carlos D'Olivo, Gustavo Medina-Tanco, Lukas Nellen, Federico A. Sánchez, José F. Valdés-Galicia. Universidad Nacional Autónoma de México, Mexico City, Mexico.

Ref HAL: hal-00712961_v1
Ref Arxiv: 0710.0367
DOI: 10.1051/0004-6361:20078775
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16 citations

Ref HAL: in2p3-00169489_v1
Ref Arxiv: 0706.3150
DOI: 10.1088/1126-6708/2007/09/102
Ref. & Cit.: NASA ADS
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4 citations
Abstract:

Inclusive beauty-quark production in two-photon collisions has been measured at LEP using an integrated luminosity of 698 pb-1 collected by the ALEPH detector with sqrt(s) between 130 and 209 GeV. The b quarks were identified using lifetime information. The cross section is found to be sigma(e+ e- --> e+ e- b \bar{b} X) = (5.4 +/- 0.8 (stat) +/- 0.8 (syst)) pb which is consistent with Next-to-Leading Order QCD.

Comments: 15 pages, 8 figures. Submitted to JHEP