108 research outputs found
Discovery of VHE gamma-rays from the vicinity of the shell-type SNR G318.2+0.1 with H.E.S.S
The on-going H.E.S.S. Galactic Plane Survey continues to reveal new sources
of VHE gamma-rays. In particular, recent re-observations of the region around
the shell-type supernova remnant (SNR) G318.2+0.1 have resulted in the
discovery of statistically-significant very-high-energy (VHE) gamma-ray
emission from an extended region. Although the source remains unidentified,
archival observations of CO12 in the region provide an opportunity to
investigate a potential SNR/molecular cloud interaction. The morphological
properties of this newly-discovered VHE gamma-ray source HESSJ1457-593 are
presented and discussed in light of the multi-wavelength data available.Comment: 7 pages, 2 figures. Proc. of the 25th TEXAS Symposium on Relativistic
Astrophysics. To appear in Proceedings of Scienc
The camera of the fifth H.E.S.S. telescope. Part I: System description
In July 2012, as the four ground-based gamma-ray telescopes of the H.E.S.S.
(High Energy Stereoscopic System) array reached their tenth year of operation
in Khomas Highlands, Namibia, a fifth telescope took its first data as part of
the system. This new Cherenkov detector, comprising a 614.5 m^2 reflector with
a highly pixelized camera in its focal plane, improves the sensitivity of the
current array by a factor two and extends its energy domain down to a few tens
of GeV.
The present part I of the paper gives a detailed description of the fifth
H.E.S.S. telescope's camera, presenting the details of both the hardware and
the software, emphasizing the main improvements as compared to previous
H.E.S.S. camera technology.Comment: 16 pages, 13 figures, accepted for publication in NIM
Gamma-ray flaring activity from the gravitationally lensed blazar PKS 1830-211 observed by Fermi LAT
The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope
routinely detects the highly dust-absorbed, reddened, and MeV-peaked flat
spectrum radio quasar PKS 1830-211 (z=2.507). Its apparent isotropic gamma-ray
luminosity (E>100 MeV) averaged over 3 years of observations and peaking
on 2010 October 14/15 at 2.9 X 10^{50} erg s^{-1}, makes it among the brightest
high-redshift Fermi blazars. No published model with a single lens can account
for all of the observed characteristics of this complex system. Based on radio
observations, one expects time delayed variability to follow about 25 days
after a primary flare, with flux about a factor 1.5 less. Two large gamma-ray
flares of PKS 1830-211 have been detected by the LAT in the considered period
and no substantial evidence for such a delayed activity was found. This allows
us to place a lower limit of about 6 on the gamma rays flux ratio between the
two lensed images. Swift XRT observations from a dedicated Target of
Opportunity program indicate a hard spectrum and with no significant
correlation of X-ray flux with the gamma-ray variability. The spectral energy
distribution can be modeled with inverse Compton scattering of thermal photons
from the dusty torus. The implications of the LAT data in terms of variability,
the lack of evident delayed flare events, and different radio and gamma-ray
flux ratios are discussed. Microlensing effects, absorption, size and location
of the emitting regions, the complex mass distribution of the system, an
energy-dependent inner structure of the source, and flux suppression by the
lens galaxy for one image path may be considered as hypotheses for
understanding our results.Comment: 14 pages, 6 figures, 2 tables. Accepted by the The Astrophysical
Journal. Corresponding authors: S. Ciprini (ASI ASDC & INAF OAR, Rome,
Italy), S. Buson (INAF Padova & Univ. of Padova, Padova, Italy), J. Finke
(NRL, Washington, DC, USA), F. D'Ammando (INAF IRA, Bologna, Italy
Fermi observations of high-energy gamma-ray emission from GRB 090217A
The Fermi observatory is advancing our knowledge of Gamma-Ray Bursts (GRBs)
through pioneering observations at high energies, covering more than 7 decades
in energy with the two on-board detectors, the Large Area Telescope (LAT) and
the Gamma-ray Burst Monitor (GBM). Here we report on the observation of the
long GRB 090217A which triggered the GBM and has been detected by the LAT with
a significance greater than 9 sigma. We present the GBM and LAT observations
and on-ground analyses, including the time-resolved spectra and the study of
the temporal profile from 8 keV up to 1 GeV. All spectra are well reproduced by
a Band model. We compare these observations to the first two LAT-detected, long
bursts GRB 080825C and GRB 080916C. These bursts were found to have
time-dependent spectra and exhibited a delayed onset of the high-energy
emission, which are not observed in the case of GRB 090217A. We discuss some
theoretical implications for the high-energy emission of GRBs.Comment: 17 pages, 4 figures. Contact Authors: Fred, Piron; Sara, Cutini;
Andreas, von Kienli
Constraints on the Cosmic-Ray Density Gradient beyond the Solar Circle from Fermi gamma-ray Observations of the Third Galactic Quadrant
We report an analysis of the interstellar -ray emission in the third
Galactic quadrant measured by the {Fermi} Large Area Telescope. The window
encompassing the Galactic plane from longitude 210\arcdeg to 250\arcdeg has
kinematically well-defined segments of the Local and the Perseus arms, suitable
to study the cosmic-ray densities across the outer Galaxy. We measure no large
gradient with Galactocentric distance of the -ray emissivities per
interstellar H atom over the regions sampled in this study. The gradient
depends, however, on the optical depth correction applied to derive the \HI\
column densities. No significant variations are found in the interstellar
spectra in the outer Galaxy, indicating similar shapes of the cosmic-ray
spectrum up to the Perseus arm for particles with GeV to tens of GeV energies.
The emissivity as a function of Galactocentric radius does not show a large
enhancement in the spiral arms with respect to the interarm region. The
measured emissivity gradient is flatter than expectations based on a cosmic-ray
propagation model using the radial distribution of supernova remnants and
uniform diffusion properties. In this context, observations require a larger
halo size and/or a flatter CR source distribution than usually assumed. The
molecular mass calibrating ratio, , is
found to be
in the Local-arm clouds and is not significantly sensitive to the choice of
\HI\ spin temperature. No significant variations are found for clouds in the
interarm region.Comment: Corresponding authors: I. A. Grenier ([email protected]); T.
Mizuno ([email protected]); L. Tibaldo
([email protected]) accepted for publication in Ap
Fermi Gamma-ray Space Telescope Observations of Recent Gamma-ray Outbursts from 3C 454.3
The flat spectrum radio quasar 3C~454.3 underwent an extraordinary outburst
in December 2009 when it became the brightest gamma-ray source in the sky for
over one week. Its daily flux measured with the Fermi Large Area Telescope at
photon energies E>100 MeV reached F = 22+/-1 x 10^-6 ph cm^-2 s^-1,
representing the highest daily flux of any blazar ever recorded in high-energy
gamma-rays. It again became the brightest source in the sky in 2010 April,
triggering a pointed-mode observation by Fermi. The correlated gamma-ray
temporal and spectral properties during these exceptional events are presented
and discussed. The main results show flux variability over time scales less
than 3 h and very mild spectral variability with an indication of gradual
hardening preceding major flares. No consistent loop pattern emerged in the
gamma-ray spectral index vs flux plane. A minimum Doppler factor of ~ 15 is
derived, and the maximum energy of a photon from 3C 454.3 is ~ 20 GeV. The
spectral break at a few GeV is inconsistent with Klein-Nishina softening from
power-law electrons scattering Ly_alpha line radiation, and a break in the
underlying electron spectrum in blazar leptonic models is implied.Comment: submitted to the Astrophysical Journa
Searches for Cosmic-Ray Electron Anisotropies with the Fermi Large Area Telescope
The Large Area Telescope on board the \textit{Fermi} satellite
(\textit{Fermi}-LAT) detected more than 1.6 million cosmic-ray
electrons/positrons with energies above 60 GeV during its first year of
operation. The arrival directions of these events were searched for
anisotropies of angular scale extending from 10 up to
90, and of minimum energy extending from 60 GeV up to 480 GeV. Two
independent techniques were used to search for anisotropies, both resulting in
null results. Upper limits on the degree of the anisotropy were set that
depended on the analyzed energy range and on the anisotropy's angular scale.
The upper limits for a dipole anisotropy ranged from to .Comment: 16 pages, 10 figures, accepted for publication in Physical Review D -
contact authors: M.N. Mazziotta and V. Vasileio
The radio/gamma-ray connection in Active Galactic Nuclei in the era of the Fermi Large Area Telescope
We present a detailed statistical analysis of the correlation between radio
and gamma-ray emission of the Active Galactic Nuclei (AGN) detected by Fermi
during its first year of operation, with the largest datasets ever used for
this purpose. We use both archival interferometric 8.4 GHz data (from the VLA
and ATCA, for the full sample of 599 sources) and concurrent single-dish 15 GHz
measurements from the Owens Valley Radio Observatory (OVRO, for a sub sample of
199 objects). Our unprecedentedly large sample permits us to assess with high
accuracy the statistical significance of the correlation, using a
surrogate-data method designed to simultaneously account for common-distance
bias and the effect of a limited dynamical range in the observed quantities. We
find that the statistical significance of a positive correlation between the cm
radio and the broad band (E>100 MeV) gamma-ray energy flux is very high for the
whole AGN sample, with a probability <1e-7 for the correlation appearing by
chance. Using the OVRO data, we find that concurrent data improve the
significance of the correlation from 1.6e-6 to 9.0e-8. Our large sample size
allows us to study the dependence of correlation strength and significance on
specific source types and gamma-ray energy band. We find that the correlation
is very significant (chance probability <1e-7) for both FSRQs and BL Lacs
separately; a dependence of the correlation strength on the considered
gamma-ray energy band is also present, but additional data will be necessary to
constrain its significance.Comment: Accepted for publications by ApJ. Contact authors: M. Giroletti, V.
Pavlidou, A. Reime
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