Ref HAL: hal-00430336_v1
Ref Arxiv: 0808.1258
DOI: 10.1051/0004-6361:200809982
Ref. & Cit.: NASA ADS
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43 citations
Résumé:

Modelling the emission properties of compact high energy sources such asX-ray binaries, AGN or gamma-ray bursts represents a complex problem.Contributions of numerous processes participate non linearly to produce theobserved spectra: particle-particle, particle-photon and particle-waveinteractions. In the past decades, numerical simulations have been widely usedto address the key properties of the high energy plasmas present in thesesources. This article presents a code that has been designed to investigatethese questions. It includes most of the relevant processes needed to simulatethe emission of high energy sources. This code solves the time-dependentkinetic equations for homogeneous, isotropic distributions of photons,electrons and positrons. No assumption is made on the shape of thesedistributions. Have been included so far: syn- chrotron self-absorbedradiation, Compton scattering, pair production/annihilation, e-e and e-pCoulomb collisions and some prescriptions for additional particle heating andacceleration. We also present comparisons with earlier works and some examplesto illustrate the code computational capacities. Previous results arereproduced qualitatively but some differences are often found in the details ofthe particle As a first application of the code, we investigate acceleration bysecond order Fermi-like processes and we find that the energy threshold foracceleration has a crucial influence on the particle distribution and theemitted spectrum.

Ref HAL: hal-00430333_v1
Ref Arxiv: 0810.5639
Ref. & Cit.: NASA ADS
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Résumé:

The high energy emission of microquasars is thought to originate from highenergy particles. Depending on the spectral state, the distribution of theseparticles can be thermal with a high temperature (typically 100 keV) ornon-thermal and extending to even higher energy. The properties of high energyplasmas are governed by a rich microphysics involving particle-particlecollisions and particles-photons interactions. We present a new code developed to address the evolution of relativisticplasmas. This one-zone code focuses on the microphysics and solves the coupledkinetic equations for particles and photons, including Compton scattering,synchrotron emission and absorption, pair production and annihilation,bremsstrahlung emission and absorption, Coulomb interactions, and prescriptionsfor additional particle acceleration and heating. It can in particular describemechanisms such a thermalisation by synchrotron self-absorption and Coulombcollisions. Using the code, we investigate whether various acceleration processes, namelythermal heating, non-thermal acceleration and stochastic acceleration, canreproduce the different spectral states of microquasars. Premilinary resultsare presented.

Commentaires: 9 pages, 6 figures, proceedings of the VII Microquasar Workshop: Microquasars and Beyond, September 1-5 2008, Foca, Izmir, Turkey; accepted for publication in PoS

Ref HAL: hal-00430326_v1
Ref Arxiv: 0810.5640
Ref. & Cit.: NASA ADS
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Résumé:

Accreting sources such as AGN, X-ray binaries or gamma-ray bursts are knownto be strong, high energy emitters. The hard emission is though to originatefrom plasmas of thermal and/or non-thermal high energy particles. Not only doesthis emission allow to probe the unique properties of the matter in an extremeenvironment, but it also has a crucial backreaction on the energetics and thedynamics of the emitting medium itself. Understanding interactions betweenradiation and matter has become a key issue in the modelling of high energysources. Although most cross sections are well known, they are quite complexand the way all processes couple non-linearly is still an open issue. We present a new code that solves the local, kinetic evolution equations fordistributions of electrons, positrons and photons, interacting by radiationprocesses such as self-absorbed synchrotron and brems-strahlung radiation,Compton scattering, pair production/annihilation, and by Coulomb collisions.The code is very general and aimed to modelled various high energy sources. Asan application, we study the spectral states of X-ray binaries, includingthermalization by Coulomb collisions and synchrotron self-absorption. It isfound that the low-hard and high-soft states can be modelled with differentillumination but the same non-thermal acceleration mechanism.

Commentaires: SF2A-2008: Proceedings of the Annual meeting of the French Society of Astronomy and Astrophysics Eds.: C. Charbonnel, F. Combes and R. Samadi. Available online at http://proc.sf2a.asso.fr, p.171

Ref HAL: hal-00430148_v1
Ref Arxiv: 0809.0584
DOI: 10.1088/0004-637X/690/2/L101
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86 citations
Résumé:

The High Energy Stereoscopic System (HESS) survey of the Galactic plane hasestablished the existence of a substantial number (~40) of Galactic TeVgamma-ray sources, a large fraction of which remain unidentified. HESSJ0632+057 is one of a small fraction of these objects which is point-like innature(<2' rms), and is one of only two point-like sources that remainunidentified. Follow-up observations of this object with XMM-Newton haverevealed an X-ray source coincident with the TeV source and with the massivestar MWC 148, of the spectral type B0pe. This source exhibits a hard spectrum,consistent with an absorbed power law with Gamma = 1.26 +/- 0.04, and showssignificant variability on hour timescales. We discuss this spatial coincidenceand the implied spectral energy distribution of this object and argue that itis likely a new gamma-ray binary system with a close resemblance to the threeknown members of this class, and in particular to LS I +61 303. Further X-ray,radio and optical observations of this system are needed to firmly establishHESS J0632+057 as a new member of this rare class of Galactic objects.

Ref HAL: in2p3-00348809_v1
Ref Arxiv: 0812.3762
DOI: 10.1051/0004-6361:200810912
Ref. & Cit.: NASA ADS
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27 citations
Résumé:

The rapidly varying (~10 minute timescale) non-thermal X-ray emission observed from Sgr A* implies that particle acceleration is occuring close to the event horizon of the supermassive black hole. The TeV gamma-ray source HESS J1745-290 is coincident with Sgr A* and may be closely related to its X-ray emission. Simultaneous X-ray and TeV observations are required to elucidate the relationship between these objects. We report on joint H.E.S.S./Chandra observations performed in July 2005, during which an X-ray flare was detected. Despite a factor of 9 increase in the X-ray flux of Sgr A*, no evidence is found for an increase in the TeV gamma-ray flux from this region. We find that an increase in the gamma-ray flux of a factor of 2 or greater can be excluded at a confidence level of 99%. This finding disfavours scenarios in which the keV and TeV emission are associated with the same population of accelerated particles and in which the bulk of the gamma-ray emission is produced within ~10^{14} cm (~100 R_S) of the supermassive black hole.

Commentaires: 5 pages, 2 figures

Ref HAL: in2p3-00345966_v1
Ref Arxiv: 0812.1638
DOI: 10.1051/0004-6361:200811372
Ref. & Cit.: NASA ADS
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37 citations
Résumé:

Aims: The nearby galaxy clusters Abell 496 and Abell 85 are studied in the very high energy (VHE, E > 100 GeV) band to investigate VHE cosmic rays (CRs) in this class of objects which are the largest gravitationally bound systems in the Universe. Methods: H.E.S.S., an array of four Imaging Atmospheric Cherenkov Telescopes (IACT), is used to observe the targets in the range of VHE gamma rays. Results: No significant gamma-ray signal is found at the respective position of the two clusters with several different source size assumptions for each target. In particular, emission regions corresponding to the high density core, to the extension of the entire X-ray emission in these clusters, and to the very extended region where the accretion shock is expected, are investigated. Upper limits are derived for the gamma-ray flux at energies E>570 GeV for Abell 496 and E>460 GeV for Abell 85. Conclusions: From the non-detection in VHE gamma rays, upper limits on the total energy of hadronic CRs in the clusters are calculated. If the cosmic-ray energy density follows the large scale gas density profile, the limit on the fraction of energy in these non-thermal particles with respect to the total thermal energy of the intra-cluster medium (ICM) is 51% for Abell 496 and only 8% for Abell 85 due to its larger mass and higher gas density. These upper limits are compared with theoretical estimates. They predict about ~10% of the thermal energy of the ICM in non-thermal particles. The observations presented here can constrain these predictions especially for the case of the Abell 85 cluster.

Commentaires: 11 pages, 4 figures, accepted for publication in A&A

Ref HAL: in2p3-00341406_v1
Ref Arxiv: 0811.3894
DOI: 10.1103/PhysRevLett.101.261104
Ref. & Cit.: NASA ADS
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465 citations
Résumé:

The very large collection area of ground-based gamma-ray telescopes gives them a substantial advantage over balloon/satellite based instruments in the detection of very-high-energy (>600 GeV) cosmic-ray electrons. Here we present the electron spectrum derived from data taken with the H.E.S.S. system of imaging atmospheric Cherenkov telescopes. In this measurement, the first of this type, we are able to extend the measurement of the electron spectrum beyond the range accessible to direct measurements. We find evidence for a substantial steepening in the energy spectrum above 600 GeV compared to lower energies.

Commentaires: 5 pages, 3 figures. Accepted for publication by Phys. Rev. Letters; modified to include latest ATIC data points