Ref HAL: in2p3-00438793_v1
DOI: 10.1051/0004-6361/200912339
Exporter : BibTex | endNote
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-00435196_v2
Ref Arxiv: 0911.4457
DOI: 10.1051/0004-6361/200913520
Ref. & Cit.: NASA ADS
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49 citations
Résumé:

Supernova remnants are believed to be a major source of cosmic-rays in the Galaxy. As their progenitors are commonly found clustered in OB associations, one has to consider the possibility of collective effects in the acceleration process. In this work we investigate the shape of the spectrum of protons accelerated inside the superbubbles blown around clusters of massive stars. To do so we embed semi-analytical models of particle acceleration and transport inside Monte-Carlo simulations of OB associations timelines. We consider regular acceleration at the shock front of supernova remnants, as well as stochastic re-acceleration and escape occurring between the shocks. We observe that particle spectra, although strongly intermittent, get a distinctive shape resulting from a competition between acceleration and escape: they are harder at the lowest energies and softer at the highest energies. The momentum at which this spectral break occurs depends on a single dimensionless parameter, which we evaluate for a selection of objects. The behaviour of a superbubble regarding acceleration depends on the magnetic turbulence: if B is low the superbubble is simply the host of a collection of individual supernovae shocks, but if B is high enough it acts as a global accelerator, producing distinctive spectra – which has important implications on the high-energy emission from these objects.

Commentaires: 11 p.

Ref HAL: hal-00430321_v1
Ref Arxiv: 0909.4022
DOI: 10.1051/0004-6361/200809830
Ref. & Cit.: NASA ADS
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38 citations
Résumé:

We seek to understand the propagation mechanisms of positrons in theinterstellar medium (ISM). This understanding is a key to determine whether thespatial distribution of the annihilation emission observed in our Galaxyreflects the spatial distribution of positron sources and, therefore, makes itpossible to place constraints on the origin of positrons. We review thedifferent processes that are likely to affect the transport of positrons in theISM. These processes fall into three broad categories: scattering offmagnetohydrodynamic waves, collisions with particles of the interstellar gasand advection with large-scale fluid motions. We assess the efficiency of eachprocess and describe its impact on the propagation of positrons. We alsodevelop a model of positron propagation, based on Monte-Carlo simulations,which enable us to estimate the distances traveled by positrons in thedifferent phases of the ISM. We find that low-energy (< 10 MeV) positronsgenerally have negligible interactions with magnetohydrodynamic waves, insofaras these waves are heavily damped. Positron propagation is mainly controlled bycollisions with gas particles. Under these circumstances, positrons can travelhuge distances (up to 30 kpc/n_H for 1 MeV positrons) along magnetic fieldlines before annihilating.

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-00398787_v1
Ref Arxiv: 0906.2637
Ref. & Cit.: NASA ADS
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Résumé:

Stellar clusters are potential acceleration sites of very-high-energy (VHE, E> 100GeV) particles since they host supernova remnants (SNRs) and pulsar windnebulae (PWNe). Additionally, in stellar clusters, particles can also beaccelerated e.g. at the boundaries of wind-blown bubbles, in colliding windzones in massive binary systems or in the framework of collective wind orwind/supernova(SN) ejecta scenarios. Motivated by the detection of VHEgamma-ray emission towards Westerlund 2 and assuming similar particleacceleration mechanisms at work, Westerlund 1 is an even more promising targetfor VHE gamma-ray observations given that massive star content and distance aremore favorable for detectable VHE gamma-ray emission compared to Westerlund 2.Here, H.E.S.S. observations of massive stellar clusters in general with specialemphasis on the most massive stellar cluster in the galaxy, Westerlund 1 aresummarized.

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.