558 research outputs found
Optimization of multivariate analysis for IACT stereoscopic systems
Multivariate methods have been recently introduced and successfully applied
for the discrimination of signal from background in the selection of genuine
very-high energy gamma-ray events with the H.E.S.S. Imaging Atmospheric
Cerenkov Telescope. The complementary performance of three independent
reconstruction methods developed for the H.E.S.S. data analysis, namely Hillas,
model and 3D-model suggests the optimization of their combination through the
application of a resulting efficient multivariate estimator. In this work the
boosted decision tree method is proposed leading to a significant increase in
the signal over background ratio compared to the standard approaches. The
improved sensitivity is also demonstrated through a comparative analysis of a
set of benchmark astrophysical sources.Comment: 10 pages, 8 figures, 3 tables, accepted for publication in
Astroparticle Physic
The Monoceros very-high-energy gamma-ray source
The H.E.S.S. telescope array has observed the complex Monoceros Loop
SNR/Rosette Nebula region which contains unidentified high energy EGRET sources
and potential very-high-energy (VHE) gamma-ray source. We announce the
discovery of a new point-like VHE gamma-ray sources, HESS J0632+057. It is
located close to the rim of the Monoceros SNR and has no clear counterpart at
other wavelengths. Data from the NANTEN telescope have been used to investigate
hadronic interactions with nearby molecular clouds. We found no evidence for a
clear association. The VHE gamma-ray emission is possibly associated with the
lower energy gamma-ray source 3EG J0634+0521, a weak X-ray source 1RXS
J063258.3+054857 and the Be-star MWC 148.Comment: 4 pages, 4 figures, Contribution to the 30th ICRC, Merida Mexico,
July 200
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
Improved sensitivity of H.E.S.S.-II through the fifth telescope focus system
The Imaging Atmospheric Cherenkov Telescope (IACT) works by imaging the very
short flash of Cherenkov radiation generated by the cascade of relativistic
charged particles produced when a TeV gamma ray strikes the atmosphere. This
energetic air shower is initiated at an altitude of 10-30 km depending on the
energy and the arrival direction of the primary gamma ray. Whether the best
image of the shower is obtained by focusing the telescope at infinity and
measuring the Cherenkov photon angles or focusing on the central region of the
shower is a not obvious question. This is particularly true for large size IACT
for which the depth of the field is much smaller. We address this issue in
particular with the fifth telescope (CT5) of the High Energy Stereoscopic
System (H.E.S.S.); a 28 m dish large size telescope recently entered in
operation and sensitive to an energy threshold of tens of GeVs. CT5 is equipped
with a focus system, its working principle and the expected effect of focusing
depth on the telescope sensitivity at low energies (50-200 GeV) is discussed.Comment: In Proceedings of the 33rd International Cosmic Ray Conference
(ICRC2013), Rio de Janeiro (Brazil
The electromagnetic calorimeter of the AMS-02 experiment
The electromagnetic calorimeter (ECAL) of the AMS-02 experiment is a
3-dimensional sampling calorimeter, made of lead and scintillating fibers. The
detector allows for a high granularity, with 18 samplings in the longitudinal
direction, and 72 sampling in the lateral direction. The ECAL primary goal is
to measure the energy of cosmic rays up to few TeV, however, thanks to the fine
grained structure, it can also provide the separation of positrons from
protons, in the GeV to TeV region. A direct measurement of high energy photons
with accurate energy and direction determination can also be provided.Comment: Proceedings of SF2A conference 201
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
NectarCAM : a camera for the medium size telescopes of the Cherenkov Telescope Array
NectarCAM is a camera proposed for the medium-sized telescopes of the
Cherenkov Telescope Array (CTA) covering the central energy range of ~100 GeV
to ~30 TeV. It has a modular design and is based on the NECTAr chip, at the
heart of which is a GHz sampling Switched Capacitor Array and a 12-bit Analog
to Digital converter. The camera will be equipped with 265 7-photomultiplier
modules, covering a field of view of 8 degrees. Each module includes the
photomultiplier bases, high voltage supply, pre-amplifier, trigger, readout and
Ethernet transceiver. The recorded events last between a few nanoseconds and
tens of nanoseconds. The camera trigger will be flexible so as to minimize the
read-out dead-time of the NECTAr chips. NectarCAM is designed to sustain a data
rate of more than 4 kHz with less than 5\% dead time. The camera concept, the
design and tests of the various subcomponents and results of thermal and
electrical prototypes are presented. The design includes the mechanical
structure, cooling of the electronics, read-out, clock distribution, slow
control, data-acquisition, triggering, monitoring and services.Comment: In Proceedings of the 34th International Cosmic Ray Conference
(ICRC2015), The Hague, The Netherlands. All CTA contributions at
arXiv:1508.0589
Understanding hadronic gamma-ray emission from supernova remnants
We aim to test the plausibility of a theoretical framework in which the
gamma-ray emission detected from supernova remnants may be of hadronic origin,
i.e., due to the decay of neutral pions produced in nuclear collisions
involving relativistic nuclei. In particular, we investigate the effects
induced by magnetic field amplification on the expected particle spectra,
outlining a phenomenological scenario consistent with both the underlying
Physics and the larger and larger amount of observational data provided by the
present generation of gamma experiments, which seem to indicate rather steep
spectra for the accelerated particles. In addition, in order to study to study
how pre-supernova winds might affect the expected emission in this class of
sources, the time-dependent gamma-ray luminosity of a remnant with a massive
progenitor is worked out. Solid points and limitations of the proposed scenario
are finally discussed in a critical way.Comment: 30 pages, 5 figures; Several comments, references and a figure added.
Some typos correcte
H.E.S.S. observations of gamma-ray bursts in 2003-2007
Very-high-energy (VHE; >~100 GeV) gamma-rays are expected from gamma-ray
bursts (GRBs) in some scenarios. Exploring this photon energy regime is
necessary for understanding the energetics and properties of GRBs. GRBs have
been one of the prime targets for the H.E.S.S. experiment, which makes use of
four Imaging Atmospheric Cherenkov Telescopes (IACTs) to detect VHE gamma-rays.
Dedicated observations of 32 GRB positions were made in the years 2003-2007 and
a search for VHE gamma-ray counterparts of these GRBs was made. Depending on
the visibility and observing conditions, the observations mostly start minutes
to hours after the burst and typically last two hours. Results from
observations of 22 GRB positions are presented and evidence of a VHE signal was
found neither in observations of any individual GRBs, nor from stacking data
from subsets of GRBs with higher expected VHE flux according to a
model-independent ranking scheme. Upper limits for the VHE gamma-ray flux from
the GRB positions were derived. For those GRBs with measured redshifts,
differential upper limits at the energy threshold after correcting for
absorption due to extra-galactic background light are also presented.Comment: 9 pages, 4 tables, 3 figure
Discovery of VHE gamma-rays from the high-frequency-peaked BL Lac object RGB J0152+017
Aims: The BL Lac object RGB J0152+017 (z=0.080) was predicted to be a very
high-energy (VHE; > 100 GeV) gamma-ray source, due to its high X-ray and radio
fluxes. Our aim is to understand the radiative processes by investigating the
observed emission and its production mechanism using the High Energy
Stereoscopic System (H.E.S.S.) experiment. Methods: We report recent
observations of the BL Lac source RGB J0152+017 made in late October and
November 2007 with the H.E.S.S. array consisting of four imaging atmospheric
Cherenkov telescopes. Contemporaneous observations were made in X-rays by the
Swift and RXTE satellites, in the optical band with the ATOM telescope, and in
the radio band with the Nancay Radio Telescope. Results: A signal of 173
gamma-ray photons corresponding to a statistical significance of 6.6 sigma was
found in the data. The energy spectrum of the source can be described by a
powerlaw with a spectral index of 2.95+/-0.36stat+/-0.20syst. The integral flux
above 300 GeV corresponds to ~2% of the flux of the Crab nebula. The source
spectral energy distribution (SED) can be described using a two-component
non-thermal synchrotron self-Compton (SSC) leptonic model, except in the
optical band, which is dominated by a thermal host galaxy component. The
parameters that are found are very close to those found in similar SSC studies
in TeV blazars. Conclusions: RGB J0152+017 is discovered as a source of VHE
gamma-rays by H.E.S.S. The location of its synchrotron peak, as derived from
the SED in Swift data, allows clearly classification it as a
high-frequency-peaked BL Lac (HBL).Comment: Accepted for publication in A&A Letters (5 pages, 4 figures
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