1,510 research outputs found
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 mĂ©thodologique, explore la dispersion spatiale des lieux d'activitĂ© (Ă©tude et travail) frĂ©quentĂ©s au quotidien par des individus au regard de leurs caractĂ©ristiques sociodĂ©mographiques et de leur lieu de rĂ©sidence, dans l'agglomĂ©ration de SĂŁo Paulo. L'Ă©tude repose sur l'exploitation de donnĂ©es d'enquĂȘtes sur les systĂšmes de mobilitĂ©s rĂ©alisĂ©es auprĂšs d'un Ă©chantillon de mĂ©nages. L'accent est mis sur la dĂ©marche qui fait intervenir une sĂ©rie d'analyses centrographiques que l'on combine dans un deuxiĂšme temps Ă une analyse typologique. Nous montrons l'intĂ©rĂȘt de recourir Ă ces outils pour apprĂ©hender et visualiser les inĂ©galitĂ©s d'accĂšs aux ressources urbaines en lien avec le cycle de vie des individus, la hiĂ©rarchie sociale et le lieu de rĂ©sidence
- âŠ