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-00430264_v1
Ref Arxiv: 0709.4380
Ref. & Cit.: NASA ADS
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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
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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: hal-00285058_v1
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Abstract:

The current generation of very-high-energy (VHE) gamma-ray telescopes has revealed emission from a large number of sources which can be clearly identified with known supernova remnants (SNRs) or pulsar wind nebulae (PWNe). Most of these objects are also non-thermal X-ray emitters, and in many cases there is a remarkable morphological correspondence between the X-ray and TeV gamma-ray images. The current status of observational knowledge in the VHE gamma-ray domain for both categories of objects will be reviewed. Particular attention will be paid to the evidence for accelerated high-energy nuclei in SNRs, and to the complementarity of VHE observations and synchrotron X-ray observations in PWNe.

Ref HAL: in2p3-00196769_v1
Ref Arxiv: 0712.1173
DOI: 10.1051/0004-6361:20078516
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160 citations
Abstract:

Context.The detection of gamma rays in the very-high-energy (VHE) energy range (100 GeV-100 TeV) provides a direct view of the parent population of ultra-relativistic particles found in astrophysical sources. For this reason, VHE gamma rays are useful for understanding the underlying astrophysical processes in non-thermal sources. Aims.We investigate unidentified VHE gamma-ray sources that have been discovered with HESS in the most sensitive blind survey of the Galactic plane at VHE energies conducted so far. Methods.The HESS array of imaging atmospheric Cherenkov telescopes (IACTs) has a high sensitivity compared with previous instruments (~ in 25 h observation time for a point-source detection), and with its large field of view, is well suited for scan-based observations. The on-going HESS survey of the inner Galaxy has revealed a large number of new VHE sources, and for each we attempt to associate the VHE emission with multi-wavelength data in the radio through X-ray wavebands. Results. For each of the eight unidentified VHE sources considered here, we present the energy spectra and sky maps of the sources and their environment. The VHE morphology is compared with available multi-wavelength data (mainly radio and X-rays). No plausible counterparts are found.