718 research outputs found
3D-reconstruction of gamma-ray showers with a stereoscopic system
We report on a new 3D-reconstruction of -ray showers which takes full advantage of the assets of a stereoscopic system of atmospheric Cherenkov telescopes and of the fine-grain imaging. The rich information collected by the cameras allows us to discriminate -ray showers and hadronic showers on the basis of two simple properties of electromagnetic showers : their rotational symmetry with respect to the axis and their relatively small lateral spread. The performance of the method is presented in terms of -ray efficiency, angular resolution and spectral resolution
Constraints on cosmic-ray efficiency in the supernova remnant RCW 86 using multi-wavelength observations
Several young supernova remnants (SNRs) have recently been detected in the
high-energy and very-high-energy gamma-ray domains. As exemplified by RX
J1713.7-3946, the nature of this emission has been hotly debated, and direct
evidence for the efficient acceleration of cosmic-ray protons at the SNR shocks
still remains elusive. We analyzed more than 40 months of data acquired by the
Large Area Telescope (LAT) on-board the Fermi Gamma-Ray Space Telescope in the
HE domain, and gathered all of the relevant multi-wavelength (from radio to VHE
gamma-rays) information about the broadband nonthermal emission from RCW 86.
For this purpose, we re-analyzed the archival X-ray data from the ASCA/Gas
Imaging Spectrometer (GIS), the XMM-Newton/EPIC-MOS, and the RXTE/Proportional
Counter Array (PCA). Beyond the expected Galactic diffuse background, no
significant gamma-ray emission in the direction of RCW 86 is detected in any of
the 0.1-1, 1-10 and 10-100 GeV Fermi-LAT maps. In the hadronic scenario, the
derived HE upper limits together with the HESS measurements in the VHE domain
can only be accommodated by a spectral index Gamma <= 1.8, i.e. a value
in-between the standard (test-particle) index and the asymptotic limit of
theoretical particle spectra in the case of strongly modified shocks. The
interpretation of the gamma-ray emission by inverse Compton scattering of high
energy electrons reproduces the multi-wavelength data using a reasonable value
for the average magnetic field of 15-25 muG. For these two scenarios, we
assessed the level of acceleration efficiency. We discuss these results in the
light of existing estimates of the magnetic field strength, the effective
density and the acceleration efficiency in RCW 86.Comment: Accepted for publication in A&A; 10 pages and 4 figure
Fermi Detection of the Pulsar Wind Nebula HESS J1640-465
We present observations of HESS J1640-465 with the Fermi-LAT. The source is
detected with high confidence as an emitter of high-energy gamma-rays. The
spectrum lacks any evidence for the characteristic cutoff associated with
emission from pulsars, indicating that the emission arises primarily from the
pulsar wind nebula. Broadband modeling implies an evolved nebula with a low
magnetic field resulting in a high gamma-ray to X-ray flux ratio. The Fermi
emission exceeds predictions of the broadband model, and has a steeper
spectrum, possibly resulting from a distinct excess of low energy electrons
similar to what is inferred for both the Vela X and Crab pulsar wind nebulae.Comment: 6 pages, 5 figures, accepted for publication in Ap
Detecting stable massive neutral particles through particle lensing
Stable massive neutral particles emitted by astrophysical sources undergo
deflection under the gravitational potential of our own galaxy. The deflection
angle depends on the particle velocity and therefore non-relativistic particles
will be deflected more than relativistic ones. If these particles can be
detected through neutrino telescopes, cosmic ray detectors or directional dark
matter detectors, their arrival directions would appear aligned on the sky
along the source-lens direction. On top of this deflection, the arrival
direction of non-relativistic particles is displaced with respect to the
relativistic counterpart also due to the relative motion of the source with
respect to the observer; this induces an alignment of detections along the sky
projection of the source trajectory. The final alignment will be given by a
combination of the directions induced by lensing and source proper motion. We
derive the deflection-velocity relation for the Milky Way halo and suggest that
searching for alignments on detection maps of particle telescopes could be a
way to find new particles or new astrophysical phenomena.Comment: 17 pages, 7 figures. Accepted by PR
Selection and 3D-Reconstruction of Gamma-Ray-induced Air Showers with a Stereoscopic System of Atmospheric Cherenkov Telescopes
A simple 3D-reconstruction method for gamma-ray induced air showers is
presented, which takes full advantage of the assets of a system of Atmospheric
Cherenkov Telescopes combining stereoscopy and fine-grain imaging like the High
Energy Stereoscopic System (H.E.S.S.). The rich information collected by the
cameras allows to select electromagnetic showers on the basis of their
rotational symmetry with respect to the incident direction, as well as of their
relatively small lateral spread. In the framework of a 3D-model of the shower,
its main parameters -- incident direction, shower core position on the ground,
slant depth of shower maximum, average lateral spread of Cherenkov photon
origins (or ``photosphere 3D-width'') and primary energy -- are fitted to the
pixel contents of the different images. For gamma-ray showers, the photosphere
3D-width is found to scale with the slant depth of shower maximum, an effect
related to the variation of the Cherenkov threshold with the altitude; this
property allows to define a dimensionless quantity omega (the ``reduced
3D-width''), which turns out to be an efficient and robust variable to
discriminate gamma-rays from primary hadrons. In addition, the omega
distribution varies only slowly with the gamma-ray energy and is practically
independent of the zenith angle. The performance of the method as applied to
H.E.S.S. is presented. Depending on the requirements imposed to reconstructed
showers, the angular resolution at zenith varies from 0.04 to 0.1 degrees and
the spectral resolution in the same conditions from 15% to 20%.Comment: 32 pages including 38 figures. Accepted by AstroParticle Physic
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