TeV blazars as seen by the CAT telescope

To date, only two extragalactic objects have been firmly established as very high-energy gamma-ray sources in the Northern sky: these are the two blazars Markarian 501 and Markarian 421. This paper reviews the most striking results obtained from these sources by the CAT atmospheric Cherenkov imaging telescope, with a particular emphasis on the 1999-2000 and 2000-2001 observation campaigns of Markarian 421.Comment: 4 pages, 5 figures, to appear in Proc. of the SF2A conference "Semaine de l'Astrophysique Francaise", May 28 - June 1st, 2001 (Lyon, France

Gamma-Ray Bursts at high and very high energies

Gamma-Ray Bursts (GRBs) are extra-galactic and extremely energetic transient emissions of gamma rays, which are thought to be associated with the death of massive stars or the merger of compact objects in binary systems. Their huge luminosities involve the presence a newborn stellar-mass black hole emitting a relativistic collimated outflow, which accelerates particles and produces non-thermal emissions from the radio domain to the highest energies. In this article, I review recent progresses in the understanding of GRB jet physics above 100 MeV, based on Fermi observations of bright GRBs. I discuss the physical implications of these observations and their impact on GRB modeling, and I present some prospects for GRB observation at very high energies in the near future.Comment: 18 pages, 13 figures -- To be published in a topical review of Comptes Rendus Physique (French Academy of Sciences) on "Gamma-ray Astronomy", Eds. B. Degrange & G. Fontain

Observations of the gamma-Ray Emission Above 250 Gev from the Blazars Markarian 501 and Markarian 421 by the Cat Cherenkov Atmospheric Imaging Telescope

The Very High Energy (VHE) gamma-ray emission of the closest BL Lacertae objects Markarian 501 and Markarian 421 has been observed by the CAT telescope in 1997 and 1998. In 1997 Mrk 501 exhibited a remarkable series of flares, with a VHE emission peaking above 250 GeV. The source showed correlated emissions in the X-ray and VHE gamma-ray bands, together with intensity-spectral hardness correlation in the latter energy range. During small flares in 1998, Mrk 421 became the second extragalactic source detected by CAT. Its spectral properties are compared to those of Mrk 501. Theoretical implications for jet astrophysics are briefly discussed.Comment: 4 pages, 3 figures. To appear in the proceedings of the XIth rencontres de Blois, June 27 - July 3, 199

Very high-energy gamma-ray sources as seen by the CAT imaging telescope

To date, only three objects have been firmly established as very high-energy gamma-ray sources in the Northern sky: the Crab nebula, which is a plerion, and the two blazars Markarian 501 and Markarian 421. This paper reviews the most striking results obtained for these sources by the CAT atmospheric Cherenkov imaging telescope.Comment: 4 pages, 7 figures, to appear in Proc. of the 36th Rencontres de Moriond on "Very High-Energy Phenomena in the Universe'' (Les Arcs, France

Synchrotron Emissions in GRB Prompt Phase Using a Semi Leptonic and Hadronic Model

In this communication devoted to the prompt emission of GRBs, we claim that some important parameters associated to the magnetic field, such as its index profile, the index of its turbulence spectrum and its level of irregularities, will be measurable with GLAST. In particular the law relating the peak energy Epeak with the total energy E (like Amati's law) constrains the turbulence spectrum index and, among all existing theories of MHD turbulence, is compatible with the Kolmogorov scaling only. Thus, these data will allow a much better determination of the performances of GRBs as particle accelerators. This opens the possibility to characterize both electron and proton acceleration more seriously. We discuss the possible generation of UHECRs and of its signature through GeV-TeV synchrotron emission.Comment: 30th International Cosmic Ray Conference (ICRC2007) - Proceeding #107

Constraints on Lorentz Invariance Violation from Fermi-Large Area Telescope Observations of Gamma-Ray Bursts

We analyze the MeV/GeV emission from four bright Gamma-Ray Bursts (GRBs) observed by the Fermi-Large Area Telescope to produce robust, stringent constraints on a dependence of the speed of light in vacuo on the photon energy (vacuum dispersion), a form of Lorentz invariance violation (LIV) allowed by some Quantum Gravity (QG) theories. First, we use three different and complementary techniques to constrain the total degree of dispersion observed in the data. Additionally, using a maximally conservative set of assumptions on possible source-intrinsic spectral-evolution effects, we constrain any vacuum dispersion solely attributed to LIV. We then derive limits on the "QG energy scale" (the energy scale that LIV-inducing QG effects become important, E_QG) and the coefficients of the Standard Model Extension. For the subluminal case (where high energy photons propagate more slowly than lower energy photons) and without taking into account any source-intrinsic dispersion, our most stringent limits (at 95% CL) are obtained from GRB090510 and are E_{QG,1}>7.6 times the Planck energy (E_Pl) and E_{QG,2}>1.3 x 10^11 GeV for linear and quadratic leading order LIV-induced vacuum dispersion, respectively. These limits improve the latest constraints by Fermi and H.E.S.S. by a factor of ~2. Our results disfavor any class of models requiring E_{QG,1} \lesssim E_Pl.Comment: Accepted for publication by Physical Review

Study of time lags in HETE-2 Gamma-Ray Bursts with redshift: search for astrophysical effects and Quantum Gravity signature

The study of time lags between spikes in Gamma-Ray Bursts light curves in different energy bands as a function of redshift may lead to the detection of effects due to Quantum Gravity. We present an analysis of 15 Gamma-Ray Bursts with measured redshift, detected by the HETE-2 mission between 2001 and 2006 in order to measure time lags related to astrophysical effects and search for Quantum Gravity signature in the framework of an extra-dimension string model. The use of photon-tagged data allows us to consider various energy ranges. Systematic effects due to selection and cuts are evaluated. No significant Quantum Gravity effect is detected from the study of the maxima of the light curves and a lower limit at 95% Confidence Level on the Quantum Gravity scale parameter of 3.2x10**15 GeV is set.Comment: 4 pages, 5 figures. v3: Error corrected in Eq. 1. Results updated. Proceedings of the 30th ICRC, Merida, Mexico (2007

Dossier Mobilités spatiales et ressources métropolitaines : l'accessibilité en questions

Cet article, avant tout méthodologique, explore la dispersion spatiale des lieux d'activité (étude et travail) fréquentés au quotidien par des individus au regard de leurs caractéristiques sociodémographiques et de leur lieu de résidence, dans l'agglomération de São Paulo. L'étude repose sur l'exploitation de données d'enquêtes sur les systèmes de mobilités réalisées auprès d'un échantillon de ménages. L'accent est mis sur la démarche qui fait intervenir une série d'analyses centrographiques que l'on combine dans un deuxième temps à une analyse typologique. Nous montrons l'intérêt de recourir à ces outils pour appréhender et visualiser les inégalités d'accès aux ressources urbaines en lien avec le cycle de vie des individus, la hiérarchie sociale et le lieu de résidence