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: in2p3-00436584_v1
DOI: 10.1038/nature08574
Exporter : BibTex | endNote
382 citations
Résumé:

A cornerstone of Einstein's special relativity is Lorentz invariance-the postulate that all observers measure exactly the same speed of light in vacuum, independent of photon-energy. While special relativity assumes that there is no fundamental length-scale associated with such invariance, there is a fundamental scale (the Planck scale, l(Planck) approximate to 1.62 x 10(-33) cm or E-Planck = M(Planck)c(2) approximate to 1.22 x 10(19) GeV), at which quantum effects are expected to strongly affect the nature of space-time. There is great interest in the (not yet validated) idea that Lorentz invariance might break near the Planck scale. A key test of such violation of Lorentz invariance is a possible variation of photon speed with energy(1-7). Even a tiny variation in photon speed, when accumulated over cosmological light-travel times, may be revealed by observing sharp features in gamma-ray burst (GRB) light-curves(2). Here we report the detection of emission up to similar to 31GeV from the distant and short GRB090510. We find no evidence for the violation of Lorentz invariance, and place a lower limit of 1.2E(Planck) on the scale of a linear energy dependence (or an inverse wavelength dependence), subject to reasonable assumptions about the emission (equivalently we have an upper limit of l(Planck)/1.2 on the length scale of the effect). Our results disfavour quantum-gravity theories(3,6,7) in which the quantum nature of space-time on a very small scale linearly alters the speed of light.

Ref HAL: in2p3-00436385_v1
Ref Arxiv: 0909.2470
DOI: 10.1088/0004-637X/706/1/L138
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
330 citations
Résumé:

We report on the observation of the bright, long gamma-ray burst, GRB 090902B, by the Gamma-ray Burst Monitor (GBM) and Large Area Telescope (LAT) instruments on-board the Fermi observatory. This was one of the brightest GRBs to have been observed by the LAT, which detected several hundred photons during the prompt phase. With a redshift of z = 1.822, this burst is among the most luminous detected by Fermi. Time-resolved spectral analysis reveals a significant power-law component in the LAT data that is distinct from the usual Band model emission that is seen in the sub-MeV energy range. This power-law component appears to extrapolate from the GeV range to the lowest energies and is more intense than the Band component both below $\sim$ 50 keV and above 100 MeV. The Band component undergoes substantial spectral evolution over the entire course of the burst, while the photon index of the power-law component remains constant for most of the prompt phase, then hardens significantly towards the end. After the prompt phase, power-law emission persists in the LAT data as late as 1 ks post-trigger, with its flux declining as $t^{-1.5}$. The LAT detected a photon with the highest energy so far measured from a GRB, $33.4_{-3.5}^{+2.7}$ GeV. This event arrived 82 seconds after the GBM trigger and $\sim$ 50 seconds after the prompt phase emission had ended in the GBM band. We discuss the implications of these results for models of GRB emission and for constraints on models of the Extragalactic Background Light.

Commentaires: Accepted for publication in ApJ Letters. Contact Authors: Elisabetta Bissaldi (ebs@mpe.mpg.de), James Chiang (jchiang@slac.stanford.edu), Francesco de Palma (francesco.depalma@ba.infn.it), Sheila McBreen (smcbreen@mep.mpg.de)

Ref HAL: in2p3-00436279_v1
Ref Arxiv: 0910.5520
DOI: 10.1088/0004-637X/706/1/L56
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
102 citations
Résumé:

The first results from observations of the high mass X-ray binary LS 5039 using the Fermi Gamma-ray Space Telescope data between 2008 August and 2009 June are presented. Our results indicate variability that is consistent with the binary period, with the emission being modulated with a period of 3.903 +/- 0.005 days; the first detection of this modulation at GeV energies. The light curve is characterized by a broad peak around superior conjunction in agreement with inverse Compton scattering models. The spectrum is represented by a power law with an exponential cutoff, yielding an overall flux (100 MeV - 300 GeV) of 4.9 +/- 0.5(stat) +/- 1.8(syst) x 10^-7 photon cm^-2 s^-1, with a cutoff at 2.1 +/- 0.3(stat) +/- 1.1(syst) GeV and photon index Gamma = 1.9 +/- 0.1(stat) +/- 0.3(syst). The spectrum is observed to vary with orbital phase, specifically between inferior and superior conjunction. We suggest that the presence of a cutoff in the spectrum may be indicative of magnetospheric emission similar to the emission seen in many pulsars by Fermi.

Commentaires: 6 pages, 5 figures, accepted for publication in ApJ Letters. Contact authors: Adam B. Hill (adam.hill@obs.ujf-grenoble.fr), Richard Dubois (richard@slac.stanford.edu), Takaaki Tanaka (ttanaka@slac.stanford.edu), Robin Corbet (Robin.Corbet@nasa.gov)

Ref HAL: in2p3-00436134_v1
Ref Arxiv: 0910.0908
DOI: 10.1088/0004-637X/706/1/L1
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
194 citations
Résumé:

The discovery of bright gamma-ray emission coincident with supernova remnant (SNR) W51C is reported using the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. W51C is a middle-aged remnant (~10^4 yr) with intense radio synchrotron emission in its shell and known to be interacting with a molecular cloud. The gamma-ray emission is spatially extended, broadly consistent with the radio and X-ray extent of SNR W51C. The energy spectrum in the 0.2-50 GeV band exhibits steepening toward high energies. The luminosity is greater than 1x10^{36} erg/s given the distance constraint of D>5.5 kpc, which makes this object one of the most luminous gamma-ray sources in our Galaxy. The observed gamma-rays can be explained reasonably by a combination of efficient acceleration of nuclear cosmic rays at supernova shocks and shock-cloud interactions. The decay of neutral pi-mesons produced in hadronic collisions provides a plausible explanation for the gamma-ray emission. The product of the average gas density and the total energy content of the accelerated protons amounts to 5x10^{51}(D/6kpc)^2 erg/cm^3. Electron density constraints from the radio and X-ray bands render it difficult to explain the LAT signal as due to inverse Compton scattering. The Fermi LAT source coincident with SNR W51C sheds new light on the origin of Galactic cosmic rays.

Commentaires: 17 pages, 4 figures, 1 table. Accepted for ApJ Letters. Contact authors: Y. Uchiyama, S. Funk., H. Tajima, T. Tanaka

Ref HAL: in2p3-00429043_v1
Ref Arxiv: 0908.1171
DOI: 10.1088/0004-637X/703/2/1249
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
94 citations
Résumé:

Observations by the Large Area Telescope (LAT) on the \textit{Fermi} mission of diffuse $\gamma$-rays in a mid-latitude region in the third quadrant (Galactic longitude $l$ from $200\arcdeg$ to $260\arcdeg$ and latitude $| b |$ from $22\arcdeg$ to $60\arcdeg$) are reported. The region contains no known large molecular cloud and most of the atomic hydrogen is within 1 kpc of the solar system. The contributions of $\gamma$-ray point sources and inverse Compton scattering are estimated and subtracted. The residual $\gamma$-ray intensity exhibits a linear correlation with the atomic gas column density in energy from 100 MeV to 10 GeV. The measured integrated $\gamma$-ray emissivity is $(1.63 \pm 0.05) \times 10^{-26} {\rm photons s^{-1} sr^{-1} H\mathchar`-atom^{-1}}$ and $(0.66 \pm 0.02) \times 10^{-26} {\rm photons s^{-1} sr^{-1} H\mathchar`-atom^{-1}}$ above 100 MeV and above 300 MeV, respectively, with additional systematic error of $\sim 10%$. The differential emissivity in 100 MeV--10 GeV agrees with calculations based on cosmic ray spectra consistent with those directly measured, at the 10% level. The results obtained indicate that cosmic ray nuclei spectra within 1 kpc from the solar system in regions studied are close to the local interstellar spectra inferred from direct measurements at the Earth within $\sim 10%$.

Ref HAL: in2p3-00414457_v1
DOI: 10.1126/SCIENCE.1177023
Exporter : BibTex | endNote
69 citations
Résumé:

We report the detection of gamma-ray emissions above 200 megaelectron volts at a significance level of 17σ from the globular cluster 47 Tucanae, using data obtained with the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope. Globular clusters are expected to emit gamma rays because of the large populations of millisecond pulsars that they contain. The spectral shape of 47 Tucanae is consistent with gamma-ray emission from a population of millisecond pulsars. The observed gamma-ray luminosity implies an upper limit of 60 millisecond pulsars present in 47 Tucanae.