1,052 research outputs found
Anisotropies in the diffuse gamma-ray background measured by the Fermi-LAT
The small angular scale fluctuations of the (on large scale) isotropic gamma-ray background (IGRB) carry information about the presence of unresolved source classes. A guaranteed contribution to the IGRB is expected from the unresolved gamma-ray AGN while other extragalactic sources, Galactic gamma-ray source populations and dark matter Galactic and extragalactic structures (and sub-structures) are candidate contributors.
The IGRB was measured with unprecedented precision by the Large Area Telescope (LAT) on-board of the Fermi gamma-ray observatory, and these data were used for measuring the IGRB angular power spectrum (APS). Detailed Monte Carlo simulations of Fermi-LAT all-sky observations were performed to provide a reference against which to compare the results obtained for the real data set. The Monte Carlo simulations are also a method for performing those detailed studies of the APS contributions of single source populations, which are required in order to identify the actual IGRB contributors.
We present preliminary results of an anisotropy search in the IGRB. At angular scales <2° (e.g., above multipole 155), angular power above the photon noise level is detected, at energies between 1 and 10 GeV in each energy bin, with statistical significance between 7.2 and 4.1Ï. The obtained energy dependences point to the presence of one or more unclustered source populations with the components having an average photon index Î=2.40±0.07
Anisotropies in the diffuse gamma-ray background measured by Fermi LAT
The small angular scale fluctuations of the (on large scale) isotropic
gamma-ray background (IGRB) carry information about the presence of unresolved
source classes. A guaranteed contribution to the IGRB is expected from the
unresolved gamma-ray AGN while other extragalactic sources, Galactic gamma-ray
source populations and dark matter Galactic and extragalactic structures (and
sub-structures) are candidate contributors. The IGRB was measured with
unprecedented precision by the Large Area Telescope (LAT) on-board of the Fermi
gamma-ray observatory, and these data were used for measuring the IGRB angular
power spectrum (APS). Detailed Monte Carlo simulations of Fermi-LAT all-sky
observations were performed to provide a reference against which to compare the
results obtained for the real data set. The Monte Carlo simulations are also a
method for performing those detailed studies of the APS contributions of single
source populations, which are required in order to identify the actual IGRB
contributors. We present preliminary results of an anisotropy search in the
IGRB. At angular scales <2deg (e.g. above multipole 155), angular power above
the photon noise level is detected, at energies between 1 and 10 GeV in each
energy bin, with statistical significance between 7.2 and 4.1 sigmas. The
energy not dependence of the fluctuation anisotropy is pointing to the presence
of one or more unclustered source populations, while the energy dependence of
the intensity anisotropy is consistent with source populations having average
photon index 2.40\pm0.07.Comment: 6 pages, Proceedings of the RICAP 2011 Conference, submitted to NIM
Equivalence between local Fermi gas and shell models in inclusive muon capture from nuclei
Motivated by recent studies of inclusive neutrino nucleus processes and muon
capture within a correlated local Fermi gas model (LFG), we discuss the
relevance of nuclear finite size effects in these reactions at low energy, in
particular for muon capture. To disentangle these effects from others coming
from the reaction dynamics we employ here a simple uncorrelated shell model
that embodies the typical finite size content of the problem. The integrated
decay widths of muon atoms calculated with this shell model are then compared
for several nuclei with those obtained within the uncorrelated LFG, using in
both models exactly the same theoretical ingredients and parameters. We find
that the two predictions are in quite good agreement, within 1--7%, when the
shell model density and the correct energy balance is used as input in the LFG
calculation. The present study indicates that, despite the low excitation
energies involved in the reaction, integrated inclusive observables, like the
total muon capture width, are quite independent of the fine details of the
nuclear wave functions.Comment: 11 pages, 8 figures. Final version to be published in EPJ
Jastrow-type calculations of one-nucleon removal reactions on open - shell nuclei
Single-particle overlap functions and spectroscopic factors are calculated on
the basis of Jastrow-type one-body density matrices of open-shell nuclei
constructed by using a factor cluster expansion. The calculations use the
relationship between the overlap functions corresponding to bound states of the
-particle system and the one-body density matrix for the ground state of
the -particle system. In this work we extend our previous analyses of
reactions on closed-shell nuclei by using the resulting overlap functions for
the description of the cross sections of reactions on the open -
shell nuclei Mg, Si and S and of S
reaction. The relative role of both shell structure and short-range
correlations incorporated in the correlation approach on the spectroscopic
factors and the reaction cross sections is pointed out.Comment: 11 pages, 5 figures, to be published in Phys. Rev.
Meson Exchange Currents in (e,e'p) recoil polarization observables
A study of the effects of meson-exchange currents and isobar configurations
in reactions is presented. We use a distorted wave
impulse approximation (DWIA) model where final-state interactions are treated
through a phenomenological optical potential. The model includes relativistic
corrections in the kinematics and in the electromagnetic one- and two-body
currents. The full set of polarized response functions is analyzed, as well as
the transferred polarization asymmetry. Results are presented for proton
knock-out from closed-shell nuclei, for moderate to high momentum transfer.Comment: 44 pages, 18 figures. Added physical arguments explaining the
dominance of OB over MEC, and a summary of differences with previous MEC
calculations. To be published in PR
Internal alignment and position resolution of the silicon tracker of DAMPE determined with orbit data
The DArk Matter Particle Explorer (DAMPE) is a space-borne particle detector
designed to probe electrons and gamma-rays in the few GeV to 10 TeV energy
range, as well as cosmic-ray proton and nuclei components between 10 GeV and
100 TeV. The silicon-tungsten tracker-converter is a crucial component of
DAMPE. It allows the direction of incoming photons converting into
electron-positron pairs to be estimated, and the trajectory and charge (Z) of
cosmic-ray particles to be identified. It consists of 768 silicon micro-strip
sensors assembled in 6 double layers with a total active area of 6.6 m.
Silicon planes are interleaved with three layers of tungsten plates, resulting
in about one radiation length of material in the tracker. Internal alignment
parameters of the tracker have been determined on orbit, with non-showering
protons and helium nuclei. We describe the alignment procedure and present the
position resolution and alignment stability measurements
Identification of particles with Lorentz factor up to with Transition Radiation Detectors based on micro-strip silicon detectors
This work is dedicated to the study of a technique for hadron identification
in the TeV momentum range, based on the simultaneous measurement of the
energies and of the emission angles of the Transition Radiation (TR) X-rays
with respect to the radiating particles. A detector setup has been built and
tested with particles in a wide range of Lorentz factors (from about to
about crossing different types of radiators. The measured
double-differential (in energy and angle) spectra of the TR photons are in a
reasonably good agreement with TR simulation predictions.Comment: 31 pages, 12 figures, paper published on Nuclear Instruments &
Methods
On possible interpretations of the high energy electron-positron spectrum measured by the Fermi Large Area Telescope
The Fermi-LAT experiment recently reported high precision measurements of the
spectrum of cosmic-ray electrons-plus-positrons (CRE) between 20 GeV and 1 TeV.
The spectrum shows no prominent spectral features, and is significantly harder
than that inferred from several previous experiments. Here we discuss several
interpretations of the Fermi results based either on a single large scale
Galactic CRE component or by invoking additional electron-positron primary
sources, e.g. nearby pulsars or particle Dark Matter annihilation. We show that
while the reported Fermi-LAT data alone can be interpreted in terms of a single
component scenario, when combined with other complementary experimental
results, specifically the CRE spectrum measured by H.E.S.S. and especially the
positron fraction reported by PAMELA between 1 and 100 GeV, that class of
models fails to provide a consistent interpretation. Rather, we find that
several combinations of parameters, involving both the pulsar and dark matter
scenarios, allow a consistent description of those results. We also briefly
discuss the possibility of discriminating between the pulsar and dark matter
interpretations by looking for a possible anisotropy in the CRE flux.Comment: 29 pages, 12 figures. Final version accepted for publication in
Astroparticle Physic
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