399 research outputs found
Hypernovae and light dark matter as possible Galactic positron sources
The electron-positron annihilation source in the Galactic center region has
recently been observed with INTEGRAL/SPI, which shows that this 511 keV source
is strong and its extension is consistent with the Galactic bulge geometry. The
positron production rate, estimated to more than 10 per second, is very
high and raises a challenging question about the nature of the Galactic
positron source. Commonly considered astrophysical positron injectors, namely
type Ia supernovae are rare events and fall short to explain the observed
positron production rate. In this paper, we study the possibility of Galactic
positron production by hypernovae events, exemplified by the recently observed
SN2003dh/GRB030329, an asymmetric explosion of a Wolf-Rayet star associated
with a gamma-ray burst. In these kinds of events, the ejected material becomes
quickly transparent to positrons, which spread out in the interstellar medium.
Non radioactive processes, such as decays of heavy dark matter particles
(neutralinos) predicted by most extensions of the standard model of particle
physics, could also produce positrons as byproducts. However they are expected
to be accompanied by a large flux of high-energy gamma-rays, which were not
observed by EGRET and ground based Tcherenkov experiments. In this context we
explore the possibility of direct positron production by annihilation of light
dark matter particles.Comment: 8 pages, 0 figures, 35th COSPAR, accepted in July 2005 by Elsevier
Science for publication in "Advances in Space Research
The spectral catalogue of INTEGRAL gamma-ray bursts: results of the joint IBIS/SPI spectral analysis
We present the updated INTEGRAL catalogue of gamma-ray bursts (GRBs) observed
between December 2002 and February 2012. The catalogue contains the spectral
parameters for 59 GRBs localized by the INTEGRAL Burst Alert System (IBAS). We
used the data from the two main instruments on board the INTEGRAL satellite:
the spectrometer SPI (SPectrometer on INTEGRAL) nominally covering the energy
range 18 keV - 8 MeV, and the imager IBIS (the Imager on Board the INTEGRAL
Satellite) operating in the range from 15 keV to 10 MeV. For the spectral
analysis we applied a new data extraction technique, developed in order to
explore the energy regions of highest sensitivity for both instruments, SPI and
IBIS. It allowed us to perform analysis of the GRB spectra over a broad energy
range and to determine the bursts' spectral peak energies. The spectral
analysis was performed on the whole sample of GRBs triggered by IBAS, including
all the events observed in period December 2002 - February 2012. The catalogue
contains the trigger times, burst coordinates, positional errors, durations and
peak fluxes for 28 unpublished GRBs observed between September 2008 and
February 2012. The light curves in 20 - 200 keV energy band of these events
were derived using IBIS data. We compare the prompt emission properties of the
INTEGRAL GRB sample with the BATSE and Fermi samples.Comment: 16 pages, 40 figures, accepted for publication in Astronomy &
Astrophysic
Integral and Light Dark Matter
The nature of Dark Matter remains one of the outstanding questions of modern
astrophysics. The success of the Cold Dark Matter cosmological model argues
strongly in favor of a major component of the dark matter being in the form of
elementary particles, not yet discovered. Based on earlier theoretical
considerations, a possible link between the recent SPI/INTEGRAL measurement of
an intense and extended emission of 511 keV photons (positron annihilation)
from the central Galaxy, and this mysterious component of the Universe, has
been established advocating the existence of a light dark matter particle at
variance with the neutralino, in general considered as very heavy. We show that
it can explain the 511 keV emission mapped with SPI/INTEGRAL without
overproducing undesirable signals like high energy gamma-rays arising from
decays, and radio synchrotron photons emitted by high energy
positrons circulating in magnetic fields. Combining the annihilation line
constraint with the cosmological one (i.e. that the relic LDM energy density
reaches about 23% of the density of the Universe), one can restrict the main
properties of the light dark matter particle. Its mass should lie between 1 and
100 MeV, and the required annihilation cross section, velocity dependent,
should be significantly larger than for weak interactions, and may be induced
by the virtual production of a new light neutral spin 1 boson . On
astrophysical grounds, the best target to validate the LDM proposal seems to be
the observation by SPI/INTEGRAL and future gamma ray telescopes of the
annihilation line from the Sagittarius dwarf galaxy and the Palomar-13 globular
cluster, thought to be dominated by dark matter.Comment: 7 pages, 0 figures. To appear in the Proceedings of the 5th INTEGRAL
Workshop: "The INTEGRAL Universe", February 16-20, 2004, Munich, German
ORIGAMIX, a CdTe-based spectro-imager development for nuclear applications
The Astrophysics Division of CEA Saclay has a long history in the development
of CdTe based pixelated detection planes for X and gamma-ray astronomy, with
time-resolved imaging and spectrometric capabilities. The last generation,
named Caliste HD, is an all-in-one modular instrument that fulfills
requirements for space applications. Its full-custom front-end electronics is
designed to work over a large energy range from 2 keV to 1 MeV with excellent
spectroscopic performances, in particular between 10 and 100 keV (0.56 keV FWHM
and 0.67 keV FWHM at 13.9 and 59.5 keV). In the frame of the ORIGAMIX project,
a consortium based on research laboratories and industrials has been settled in
order to develop a new generation of gamma camera. The aim is to develop a
system based on the Caliste architecture for post-accidental interventions or
homeland security, but integrating new properties (advanced spectrometry,
hybrid working mode) and suitable for industry. A first prototype was designed
and tested to acquire feedback for further developments. In this study, we
particularly focused on spectrometric performances with high energies and high
fluxes. Therefore, our device was exposed to energies up to 700 keV (133Ba,
137Cs) and we measured the evolution of energy resolution (0.96 keV at 80 keV,
2.18 keV at 356 keV, 3.33 keV at 662 keV). Detection efficiency decreases after
150 keV, as Compton effect becomes dominant. However, CALISTE is also designed
to handle multiple events, enabling Compton scattering reconstruction, which
can drastically improve detection efficiencies and dynamic range for higher
energies up to 1408 keV (22Na, 60Co, 152Eu) within a 1-mm thick detector. In
particular, such spectrometric performances obtained with 152Eu and 60Co were
never measured before with this kind of detector.Comment: Nuclear Instruments and Methods in Physics Research Section A:
Accelerators, Spectrometers, Detectors and Associated Equipment. Available
online 9 January 2015, ISSN 0168-9002
(http://www.sciencedirect.com/science/article/pii/S0168900215000133).
Keywords: CdTe; X-ray; Gamma-ray; Spectrometry; Charge-sharing; Astrophysics
Instrumentation; Nuclear Instrumentation; Gamma-ray camera
Continuum gamma-ray emission from light dark matter positrons and electrons
The annihilation of light dark matter was recently advocated as a possible
explanation of the large positron injection rate at the Galactic center deduced
from observations by the SPI spectrometer aboard INTEGRAL. The modelling of
internal Bremsstrahlung and in-flight annihilation radiations associated to
this process drastically reduced the mass range of this light dark matter
particle. We estimate critically the various energy losses and radiations
involved in the propagation of the positron before its annihilation --in-
flight or at rest.
Using a simple model with mono-energetic positrons injected and confined to
the Galactic bulge, we compute energy losses and gamma-ray radiations caused by
ionization, Bremsstrahlung interactions as well as in-flight and at rest
annihilation and compare these predictions to the available observations, for
various injection energies.
Confronting the predictions with observations by the CGRO/EGRET,
CGRO/COMPTEL, INTEGRAL/SPI and INTEGRAL/IBIS/ISGRI instruments, we deduce a
mass upper bound of 3 to 7.5 MeV/c^2 for the hypothetical light dark matter
particle. The most restrictive limit is in agreement with the value previously
found by Beacom and Yuksel and was obtained under similar assumptions, while
the 7.5 MeV/c^2 value corresponds to more conservative choices and to a
partially ionized propagation medium. We stress how the limit depends on the
degree of ionization of the propagation medium and how its precision could be
improved by a better appraisal of data uncertainties.Comment: Revision of the section dedicated to the observational
interpretation. 11 pages, 8 figures, 1 table. Accepted for publication in
Phys. Rev.
SVOM pointing strategy: how to optimize the redshift measurements?
The Sino-French SVOM mission (Space-based multi-band astronomical Variable
Objects Monitor) has been designed to detect all known types of gamma-ray
bursts (GRBs) and to provide fast and reliable GRB positions. In this study we
present the SVOM pointing strategy which should ensure the largest number of
localized bursts allowing a redshift measurement. The redshift measurement can
only be performed by large telescopes located on Earth. The best scientific
return will be achieved if we are able to combine constraints from both space
segment (platform and payload) and ground telescopes (visibility).Comment: Proceedings of Gamma-Ray Bursts 2007 conference, Santa Fe, USA, 5-9
November 2007. Published in AIP conf. proc. 1000, 585-588 (2008
Monte-Carlo simulations of the background of the coded-mask camera for X- and Gamma-rays on-board the Chinese-French GRB mission SVOM
For several decades now, wide-field coded mask cameras have been used with
success to localise Gamma-ray bursts (GRBs). In these instruments, the event
count rate is dominated by the photon background due to their large field of
view and large effective area. It is therefore essential to estimate the
instrument background expected in orbit during the early phases of the
instrument design in order to optimise the scientific performances of the
mission. We present here a detailed study of the instrument background and
sensitivity of the coded-mask camera for X- and Gamma-rays (CXG) to be used in
the detection and localisation of high-redshift GRBs on-board the international
GRB mission SVOM. To compute the background spectrum, a Monte-Carlo approach
was used to simulate the primary and secondary interactions between particles
from the main components of the space environment that SVOM will encounter
along its Low Earth Orbit (LEO) (with an altitude of 600 km and an inclination
of ~ 30 deg) and the body of the CXG. We consider the detailed mass model of
the CXG in its latest design. According to our results, i) the design of the
passive shield of the camera ensures that in the 4-50 keV imaging band the
cosmic X-Gamma-ray background is dominant whilst the internal background should
start to become dominant above 70-90 keV; ii) the current camera design ensures
that the CXG camera will be more sensitive to high-redshift GRBs than the Swift
Burst Alert Telescope thanks to a low-energy threshold of 4 keV.Comment: 16 pages, 10 figures (1 colour), accepted for publication in Nuclear
Instruments and Methods in Physics Research: Section
The ECLAIRs micro-satellite mission for gamma-ray burst multi-wavelength observations
Gamma-ray bursts (GRB), at least those with a duration longer than a few
seconds are the most energetic events in the Universe and occur at cosmological
distances. The ECLAIRs micro-satellite, to be launched in 2009, will provide
multi-wavelength observations of GRB, to study their astrophysics and to use
them as cosmological probes. Furthermore in 2009 ECLAIRs is expected to be the
only space borne instrument capable of providing a GRB trigger in near
real-time with sufficient localization accuracy for GRB follow-up observations
with the powerful ground based spectroscopic telescopes available by then. A
"Phase A study" of the ECLAIRs project has recently been launched by the French
Space Agency CNES, aiming at a detailed mission design and selection for flight
in 2006. The ECLAIRs mission is based on a CNES micro-satellite of the
"Myriade" family and dedicated ground-based optical telescopes. The satellite
payload combines a 2 sr field-of-view coded aperture mask gamma-camera using
6400 CdTe pixels for GRB detection and localization with 10 arcmin precision in
the 4 to 50 keV energy band, together with a soft X-ray camera for onboard
position refinement to 1 arcmin. The ground-based optical robotic telescopes
will detect the GRB prompt/early afterglow emission and localize the event to
arcsec accuracy, for spectroscopic follow-up observations.Comment: 7 pages, 1 figure, proceedings of the conference "New Developments in
Photodetection", Beaune (France), June 25005. Submitted to NIM-A (Elsevier
Science
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