137 research outputs found
Status of Identification of VHE gamma-ray sources
With the recent advances made by Cherenkov telescopes such as H.E.S.S., the
field of very high-energy (VHE) gamma-ray astronomy has recently entered a new
era in which for the first time populations of Galactic sources such as e.g.
Pulsar wind nebulae (PWNe) or Supernova remnants (SNRs) can be studied.
However, while some of the new sources can be associated by positional
coincidence as well as by consistent multi-wavelength data to a known
counterpart at other wavelengths, most of the sources remain not finally
identified. In the following, the population of Galactic H.E.S.S. sources will
be used to demonstrate the status of the identifications, to classify them into
categories according to this status and to point out outstanding problems.Comment: To appear in Astrophysics and Space Science (Proceedings of "The
multimessenger approach to unidentified gamma-ray sources
Detection potential to point-like neutrino sources with the NEMO-km3 telescope
The NEMO Collaboration is conducting an R&D activity towards the construction
of a Mediterranean km3 neutrino telescope. In this work, we present the results
of Monte Carlo simulation studies on the capability of the proposed NEMO
telescope to detect and identify point-like sources of high energy muon
neutrinos.Comment: To be published on BCN06 proceedings (Barcelona, July 4-7, 2006
High Energy Processes in Pulsar Wind Nebulae
Young pulsars produce relativistic winds which interact with matter ejected
during the supernova explosion and the surrounding interstellar gas. Particles
are accelerated to very high energies somewhere in the pulsar winds or at the
shocks produced in collisions of the winds with the surrounding medium. As a
result of interactions of relativistic leptons with the magnetic field and low
energy radiation (of synchrotron origin, thermal, or microwave background), the
non-thermal radiation is produced with the lowest possible energies up to
100 TeV. The high energy (TeV) gamma-ray emission has been originally
observed from the Crab Nebula and recently from several other objects. Recent
observations by the HESS Cherenkov telescopes allow to study for the first time
morphology of the sources of high energy emission, showing unexpected spectral
features. They might be also interpreted as due to acceleration of hadrons.
However, theory of particle acceleration in the PWNe and models for production
of radiation are still at their early stage of development since it becomes
clear that realistic modeling of these objects should include their time
evolution and three-dimensional geometry. In this paper we concentrate on the
attempts to create a model for the high energy processes inside the PWNe which
includes existence not only relativistic leptons but also hadrons inside the
nebula. Such model should also take into account evolution of the nebula in
time. Possible high energy expectations based on such a model are discussed in
the context of new observations.Comment: 9 pages, 1 figure, Proc. Multimessenger approach to high energy
gamma-ray source
Gamma rays from molecular clouds
It is believed that the observed diffuse gamma ray emission from the galactic
plane is the result of interactions between cosmic rays and the interstellar
gas. Such emission can be amplified if cosmic rays penetrate into dense
molecular clouds. The propagation of cosmic rays inside a molecular cloud has
been studied assuming an arbitrary energy and space dependent diffusion
coefficient. If the diffusion coefficient inside the cloud is significantly
smaller compared to the average one derived for the galactic disk, the observed
gamma ray spectrum appears harder than the cosmic ray spectrum, mainly due to
the slower penetration of the low energy particles towards the core of the
cloud. This may produce a great variety of gamma ray spectra.Comment: Proceeding of "The multi messenger approach to high energy gamma ray
sources", Barcelona, June 200
Associations of Very High Energy Gamma-Ray Sources Discovered by H.E.S.S. with Pulsar Wind Nebulae
The H.E.S.S. array of imaging Cherenkov telescopes has discovered a number of
previously unknown gamma-ray sources in the very high energy (VHE) domain above
100 GeV. The good angular resolution of H.E.S.S. (~0.1 degree per event), as
well as its sensitivity (a few percent of the Crab Nebula flux) and wide
5-degree field of view, allow a much better constrained search for counterparts
in comparison to previous instruments. In several cases, the association of the
VHE sources revealed by H.E.S.S. with pulsar wind nebulae (PWNe) is supported
by a combination of positional and morphological evidence, multi-wavelength
observations, and plausible PWN model parameters. These include the plerions in
the composite supernova remnants G 0.9+0.1 and MSH 15-52, the recently
discovered Vela X nebula, two new sources in the Kookaburra complex, and the
association of HESS J1825-137 with PSR B1823-13. The properties of these
better-established associations are reviewed. A number of other sources
discovered by H.E.S.S. are located near high spin-down power pulsars, but the
evidence for association is less complete. These possible associations are also
discussed, in the context of the available multi-wavelength data and plausible
PWN scenarios.Comment: 5 pages, to appear in Astrophysics and Space Science (proceedings of
"The Multi-Messenger Approach to High-Energy Gamma-Ray Sources"
Implications of H.E.S.S. observations of pulsar wind nebulae
In this review paper on pulsar wind nebulae (PWN) we discuss the properties
of such nebulae within the context of containment against cross-field diffusion
(versus normal advection), the effect of reverse shocks on the evolution of
offset ``Vela-like'' PWN, constraints on maximum particle energetics, magnetic
field strength estimates based on spectral and spatial properties, and the
implication of such field estimates on the composition of the wind. A
significant part of the discussion is based on the High Energy Stereoscopic
System ({\it H.E.S.S.} or {\it HESS}) detection of the two evolved pulsar wind
nebulae Vela X (cocoon) and HESS J1825-137. In the case of Vela X (cocoon) we
also review evidence of a hadronic versus a leptonic interpretation, showing
that a leptonic interpretation is favored for the {\it HESS} signal. The
constraints discussed in this review paper sets a general framework for the
interpretation of a number of offset, filled-center nebulae seen by {\it HESS}.
These sources are found along the galactic plane with galactic latitudes
, where significant amounts of molecular gas is found. In these
regions, we find that the interstellar medium is inhomogeneous, which has an
effect on the morphology of supernova shock expansion. One consequence of this
effect is the formation of offset pulsar wind nebulae as observed.Comment: to appear in Springer Lecture Notes on Neutron Stars and Pulsars: 40
years after their discovery, eds. W. Becke
On the Potential of the Imaging Atmospheric Cherenkov Technique for Study of the Mass Composition of Primary Cosmic Radiation in the Energy Region above 30 TeV
We suggest a new approach to study the cosmis ray (CR) mass composition in
the energy region from 30 TeV/nucleus up to the "knee" region, i.e. up to a few
PeV/nucleus, using an array of imaging atmospheric Cherenkov telescopes (IACTs)
of a special architecture. This array consists of telescopes with a relatively
small mirror size (~10 square meters) separated from each other by large
distances (~500 meters) and equipped by multichannel cameras with a modest
pixel size (0.3-0.5 degree) and a sufficiently large viewing angle (6-7
degree).
Compared to traditional IACT systems (like HEGRA, HESS or VERITAS) the IACT
array considered here could provide a very large detection area (several square
kilometers or more). At the same time, it allows an accurate measurement of the
energy of CR induced air showers (the energy resolution ranges within 25-35%)
and an effective separation of air showers created by different nuclei.
Particularly, it is possible to enrich air showers belonging to the nucleus
group assigned for selection up to ~90% purity at a detection efficiency of
15-20% of such showers.Comment: 28 pages, 12 figures, accepted for publication in Nucl. Instr. Met
A low density of the Extragalactic Background Light revealed by the H.E.S.S. spectra of the BLLac objects 1ES 1101-232 and H 2356-309
The unexpectedly hard spectra measured by HESS for the BLLacs 1ES 1101-232 and H 2356-309 has allowed an upper limit on the Extragalactic Background Light (EBL) to be derived in the optical/near-infrared range, which is very close to the lower limit given by the resolved galaxy counts. This result seems to exclude a large contribution to the EBL from other sources (e.g. Population III stars) and indicates that the intergalactic space is more transparent to gamma-rays than previously thought. A brief discussion of EBL absorption effects on blazar spectra and further observational tests to check this conclusion are presented, including the selection of new candidates for observations with Cherenkov telescopes
5@5 - a 5 GeV energy threshold array of imaging atmospheric Cherenkov telescopes at 5 km altitude
We discuss the concept and the performance of a powerful future ground-based
astronomical instrument - a stereoscopic array of several large imaging
atmospheric Cherenkov telescopes installed at a very high mountain elevation of
about 5 km a.s.l. or more - for the study of the gamma-ray sky at energies from
several GeV to 100 GeV.Comment: 33 pages, 25 figures, the revised version accepted for publication in
Astroparticle Physic
Similarities and Differences between Relativistic Electron-Photon Cascades Developed in Matter, Photon Gas and Magnetic Field
We investigate properties of astrophysical electromagnetic cascades developed
in matter, photon gas and magnetic fields, and discuss similarities and
differences between characteristics of electron-photon showers developed in
these 3 substances.Comment: 36 pages, 18 figures, submitted to Astroparticle Physic
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