Ref HAL: in2p3-00453792_v1
DOI: 10.1088/0004-637X/708/2/1310
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39 citations
Résumé:

We report the observations of PG 1553+113 during the first ∼200 days of Fermi Gamma-ray Space Telescope science operations, from 2008 August 4 to 2009 February 22 (MJD 54682.7–54884.2). This is the first detailed study of PG 1553+113 in the GeV gamma-ray regime and it allows us to fill a gap of three decades in energy in its spectral energy distribution (SED). We find PG 1553+113 to be a steady source with a hard spectrum that is best fit by a simple power law in the Fermi energy band. We combine the Fermi data with archival radio, optical, X-ray, and very high energy (VHE) gamma-ray data to model its broadband SED and find that a simple, one-zone synchrotron self-Compton model provides a reasonable fit. PG 1553+113 has the softest VHE spectrum of all sources detected in that regime and, out of those with significant detections across the Fermi energy bandpass so far, the hardest spectrum in that energy regime. Thus, it has the largest spectral break of any gamma-ray source studied to date, which could be due to the absorption of the intrinsic gamma-ray spectrum by the extragalactic background light (EBL). Assuming this to be the case, we selected a model with a low level of EBL and used it to absorb the power-law spectrum from PG 1553+113 measured with Fermi (200 MeV–157 GeV) to find the redshift, which gave the best fit to the measured VHE data (90 GeV–1.1 TeV) for this parameterization of the EBL. We show that this redshift can be considered an upper limit on the distance to PG 1553+113.

Ref HAL: in2p3-00453756_v1
DOI: 10.1088/0004-637X/708/2/1254
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181 citations
Résumé:

We report on γ -ray observations of the Crab Pulsar and Nebula using 8 months of survey data with the Fermi Large Area Telescope (LAT). The high quality light curve obtained using the ephemeris provided by the Nan¸cay and Jodrell Bank radio telescopes shows two main peaks stable in phase with energy. The first γ -ray peak leads the radio main pulse by (281 ± 12 ± 21) μs, giving new constraints on the production site of non-thermal emission in pulsar magnetospheres. The first uncertainty is due to γ -ray statistics, and the second arises from the rotation parameters. The improved sensitivity and the unprecedented statistics afforded by the LAT enable precise measurement of the Crab Pulsar spectral parameters: cut-off energy at Ec = (5.8 ± 0.5 ± 1.2) GeV, spectral index of Γ = (1.97 ± 0.02 ± 0.06) and integral photon flux above 100 MeV of (2.09 ± 0.03 ± 0.18) × 10−6 cm−2 s−1. The first errors represent the statistical error on the fit parameters, while the second ones are the systematic uncertainties. Pulsed γ -ray photons are observed up to ∼20 GeV which precludes emission near the stellar surface, below altitudes of around 4–5 stellar radii in phase intervals encompassing the two main peaks. A detailed phase-resolved spectral analysis is also performed: the hardest emission from the Crab Pulsar comes from the bridge region between the two γ -ray peaks while the softest comes from the falling edge of the second peak. The spectrum of the nebula in the energy range 100 MeV–300 GeV is well described by the sum of two power laws of indices Γsync = (3.99 ± 0.12 ± 0.08) and ΓIC = (1.64 ± 0.05 ± 0.07), corresponding to the falling edge of the synchrotron and the rising edge of the inverse Compton (IC) components, respectively. This latter, which links up naturally with the spectral data points of Cherenkov experiments, is well reproduced via IC scattering from standard magnetohydrodynamic nebula models, and does not require any additional radiation mechanism.

Ref HAL: in2p3-00445182_v1
Ref Arxiv: 0911.2709
DOI: 10.1051/0004-6361/200913073
Ref. & Cit.: NASA ADS
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19 citations
Résumé:

VHE observations of PKS 2005-489 were made with HESS from 2004 through 2007, together with three simultaneous multi-wavelength campaigns performed with XMM-Newton and RXTE in 2004 and 2005. A strong VHE signal, ~17sigma total, is detected during the four years of HESS observations (90.3 hrs live time). The integral flux above the average analysis threshold of 400 GeV is ~3% of the Crab and varies weakly on time-scales from days to years. At X-ray energies the flux is observed to vary by more than an order of magnitude between 2004 and 2005. Strong changes in the X-ray spectrum (DeltaGamma~0.7) are also observed, which appear to be mirrored in the VHE band. The SED of PKS 2005-489, constructed for the first time with contemporaneous data on both humps, shows significant evolution. The large flux variations in the X-ray band, coupled with weak or no variations in the VHE band and a similar spectral behavior, suggest the emergence of a new, separate, harder emission component in September 2005.

Commentaires: 13 pages, 11 figures. Astronomy and Astrophysics, in press. Minor typographical/text changes

Ref HAL: in2p3-00460529_v1
Ref Arxiv: 0912.1868
DOI: 10.1103/PhysRevD.80.122004
Ref. & Cit.: NASA ADS
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55 citations
Résumé:

We report on measurements of the cosmic-ray induced gamma-ray emission of Earth's atmosphere by the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope. The LAT has observed the Earth during its commissioning phase and with a dedicated Earth-limb following observation in September 2008. These measurements yielded 6.4 x 10^6 photons with energies >100MeV and ~250hours total livetime for the highest quality data selection. This allows the study of the spatial and spectral distributions of these photons with unprecedented detail. The spectrum of the emission - often referred to as Earth albedo gamma-ray emission - has a power-law shape up to 500 GeV with spectral index Gamma = 2.79+-0.06.

Ref HAL: in2p3-00446920_v1
Ref Arxiv: 0912.0973
DOI: 10.1103/PhysRevLett.103.251101
Ref. & Cit.: NASA ADS
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156 citations
Résumé:

The diffuse Galactic gamma-ray emission is produced by cosmic rays (CRs) interacting with the interstellar gas and radiation field. Measurements by the Energetic Gamma-Ray Experiment Telescope (EGRET) instrument on the Compton Gamma-Ray Observatory indicated excess gamma-ray emission > 1 GeV relative to diffuse Galactic gamma-ray emission models consistent with directly measured CR spectra (the so-called ``EGRET GeV excess''). The excess emission was observed in all directions on the sky, and a variety of explanations have been proposed, including beyond-the-Standard-Model scenarios like annihilating or decaying dark matter. The Large Area Telescope (LAT) instrument on the Fermi Gamma-ray Space Telescope has measured the diffuse gamma-ray emission with improved sensitivity and resolution compared to EGRET. We report on LAT measurements of the diffuse gamma-ray emission for energies 100 MeV to 10 GeV and Galactic latitudes 10 deg. <= |b| <= 20 deg. The LAT spectrum for this region of the sky is well reproduced by a diffuse Galactic gamma-ray emission model that is consistent with local CR spectra and inconsistent with the EGRET GeV excess.

Commentaires: 2 figures, 1 table, accepted by Physical Review Letters, available online Dec. 18th, 2009

Ref HAL: in2p3-00445870_v1
DOI: 10.1051/0004-6361/200913389
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10 citations
Résumé:

Context. High energy particles reside in the relativistic jets of microquasars, making them possible sources of very high energy radiation (VHE, >100 GeV). Detecting this emission would provide a new handle on jet physics. Aims. Observations of the microquasar GRS 1915+105 with the HESS telescope array were undertaken in 2004–2008 to search for VHE emission. Methods. Stereoscopic imaging of Cherenkov radiation from extensive air showers is used to reconstruct the energy and direction of the incident gamma rays. Results. There is no evidence for a VHE gamma-ray signal either from the direction of the microquasar or its vicinity. An upper limit of $6.1\times 10^$ ph cm-2 s-1 (99.9% confidence level) is set on the photon flux above 410 GeV, equivalent to a VHE luminosity of ${\sim} 10^$ erg s-1 at 11 kpc. Conclusions. The VHE to X-ray luminosity ratio in GRS 1915+105 is at least four orders of magnitude lower than the ratio observed in gamma-ray binaries. The VHE radiative efficiency of the compact jet is less than 0.01% based on its estimated total power of 1038 erg s-1. Particle acceleration in GRS 1915+105 is not efficient at high energies and/or the magnetic field is too strong. It is also possible that VHE gamma-rays are produced by GRS 1915+105, but the emission is highly time-dependent.

Ref HAL: in2p3-00402427_v1
Ref Arxiv: 0907.0727
DOI: 10.1051/0004-6361/200912086
Ref. & Cit.: NASA ADS
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59 citations
Résumé:

The H.E.S.S. (High Energy Stereoscopic System) telescopes observed Coma for ~8hr in a search for gamma-ray emission at energies >1TeV. The large 3.5deg FWHM field of view of H.E.S.S. is ideal for viewing a range of targets at various sizes including the Coma cluster core, the radio-relic (1253+275) and merger/infall (NGC 4839) regions to the southwest, and features greater than deg away. No evidence for point-like nor extended TeV gamma-ray emission was found and upper limits to the TeV flux F(E) for E>1, >5, and >10TeV were set for the Coma core and other regions. Converting these limits to an energy flux E^2F(E) the lowest or most constraining is the E>5TeV upper limit for the Coma core (0.2deg radius) at ~8Crab flux units or ~10^{-13}ph cm^{-2} s^{-1}. The upper limits for the Coma core were compared with a prediction for the gamma-ray emission from proton--proton interactions, the level of which ultimately scales with the mass of the Coma cluster. A direct constraint using our most stringent limit for E>5 TeV, on the total energy content in non-thermal protons with injection energy spectrum proportional to E^{-2.1} and spatial distribution following the thermal gas in the cluster, is found to be ~0.2 times the thermal energy, or ~10^{62}erg. The E>5 TeV gamma-ray threshold in this case corresponds to cosmic-ray proton energies >50TeV. Our upper limits rule out the most optimistic theoretical models for gamma ray emission from clusters and complement radio observations which constrain the cosmic ray content in clusters at significantly lower proton energies, subject to assumptions on the magnetic field strength.

Commentaires: 7 pages, 3 figures, accepted in Astronomy and Astrophysics