255 research outputs found
Nucleosynthesis results from INTEGRAL
Since its launch in October 2002, ESA's INTEGRAL observatory has enabled
significant advances to be made in the study of Galactic nucleosynthesis. In
particular, the imaging Ge spectrometer SPI combines for the first time the
diagnostic powers of high resolution gamma-ray line spectroscopy and moderate
spatial resolution. This review summarizes the major nucleosynthesis results
obtained with INTEGRAL so far. Positron annihilation in our Galaxy is being
studied in unprecented detail. SPI observations yield the first sky maps in
both the 511 keV annihilation line and the positronium continuum emission, and
the most accurate spectrum at 511 keV to date, thereby imposing new constraints
on the source(s) of Galactic positrons which still remain(s) unidentified. For
the first time, the imprint of Galactic rotation on the centroid and shape of
the 1809 keV gamma-ray line due to the decay of 26Al has been seen, confirming
the Galactic origin of this emission. SPI also provided the most accurate
determination of the gamma-ray line flux due to the decay of 60Fe. The combined
results for 26Al and 60Fe have important implications for nucleosynthesis in
massive stars, in particular Wolf-Rayet stars. Both IBIS and SPI are searching
the Galactic plane for young supernova remnants emitting the gamma-ray lines
associated with radioactive 44Ti. None have been found so far, which raises
important questions concerning the production of 44Ti in supernovae, the
Galactic supernova rate, and the Galaxy's chemical evolution.Comment: Invited review, to be published in the proceedings of IAU 230,
"Populations of High Energy Sources in Galaxies", Aug. 14-19, 2005, Dublin,
eds. E.J.A. Meurs and G. Fabbian
Properties of the energetic particle distributions during the October 28, 2003 solar flare from INTEGRAL/SPI observations
Analysis of spectra obtained with the gamma-ray spectrometer SPI onboard
INTEGRAL of the GOES X17-class flare on October 28, 2003 is presented. In the
energy range 600 keV - 8 MeV three prominent narrow lines at 2.223, 4.4 and 6.1
MeV, resulting from nuclear interactions of accelerated ions within the solar
atmosphere could be observed. Time profiles of the three lines and the
underlying continuum indicate distinct phases with several emission peaks and
varying continuum-to-line ratio for several minutes before a smoother decay
phase sets in. Due to the high-resolution Ge detectors of SPI and the
exceptional intensity of the flare, detailed studies of the 4.4 and 6.1 MeV
line shapes was possible for the first time. Comparison with calculated line
shapes using a thick target interaction model and several energetic particle
angular distributions indicates that the nuclear interactions were induced by
downward-directed particle beams with alpha-to-proton ratios of the order of
0.1. There are also indications that the 4.4 MeV to 6.1 MeV line fluence ratio
changed between the beginning and the decay phase of the flare, possibly due to
a temporal evolution of the energetic particle alpha-to-proton ratio.Comment: 24 pages, 10 figures, accepted for publication by A&
The first giant flare from SGR 1806-20: observations with the INTEGRAL SPI Anti-Coincidence Shield
A giant flare from the Soft Gamma-ray Repeater SGR 1806-20 has been detected
by several satellites on 2004 December 27. This tremendous outburst, the first
one observed from this source, was a hundred times more powerful than the two
previous giant flares from SGR 0525-66 and SGR 1900+14. We report the results
obtained for this event with the Anticoincidence Shield of the SPI spectrometer
on board the INTEGRAL satellite, which provides a high-statistics light curve
at E>~80 keV. The flare started with a very strong pulse, which saturated the
detector for ~0.7 s, and whose backscattered radiation from the Moon was
detected 2.8 s later. This was followed by a ~400 s long tail modulated at the
neutron star rotation period of 7.56 s. The tail fluence corresponds to an
energy in photons above 3 keV of 1.6x10^44 (d/15 kpc)^2 erg. This is of the
same order of the energy emitted in the pulsating tails of the two giant flares
seen from other soft repeaters, despite the hundredfold larger overall emitted
energy of the SGR 1806-20 giant flare. Long lasting (~1 hour) hard X-ray
emission, decaying in time as t^-0.85, and likely associated to the SGR 1806-20
giant flare afterglow has also been detected.Comment: revised version - Accepted for publication on The Astrophysical
Journal Letter
MGGPOD: a Monte Carlo Suite for Modeling Instrumental Line and Continuum Backgrounds in Gamma-Ray Astronomy
Intense and complex instrumental backgrounds, against which the much smaller
signals from celestial sources have to be discerned, are a notorious problem
for low and intermediate energy gamma-ray astronomy (~50 keV - 10 MeV).
Therefore a detailed qualitative and quantitative understanding of instrumental
line and continuum backgrounds is crucial for most stages of gamma-ray
astronomy missions, ranging from the design and development of new
instrumentation through performance prediction to data reduction. We have
developed MGGPOD, a user-friendly suite of Monte Carlo codes built around the
widely used GEANT (Version 3.21) package, to simulate ab initio the physical
processes relevant for the production of instrumental backgrounds. These
include the build-up and delayed decay of radioactive isotopes as well as the
prompt de-excitation of excited nuclei, both of which give rise to a plethora
of instrumental gamma-ray background lines in addition to continuum
backgrounds. The MGGPOD package and documentation are publicly available for
download from http://sigma-2.cesr.fr/spi/MGGPOD/.
We demonstrate the capabilities of the MGGPOD suite by modeling high
resolution gamma-ray spectra recorded by the Transient Gamma-Ray Spectrometer
(TGRS) on board Wind during 1995. The TGRS is a Ge spectrometer operating in
the 40 keV to 8 MeV range. Due to its fine energy resolution, these spectra
reveal the complex instrumental background in formidable detail, particularly
the many prompt and delayed gamma-ray lines. We evaluate the successes and
failures of the MGGPOD package in reproducing TGRS data, and provide
identifications for the numerous instrumental lines.Comment: 60 pages, 13 figures, 7 tables, accepted for publication in ApJ
Detecting Dark Matter Annihilation with CMB Polarization : Signatures and Experimental Prospects
Dark matter (DM) annihilation during hydrogen recombination (z ~ 1000) will
alter the recombination history of the Universe, and affect the observed CMB
temperature and polarization fluctuations. Unlike other astrophysical probes of
DM, this is free of the significant uncertainties in modelling galactic
physics, and provides a method to detect and constrain the cosmological
abundances of these particles. We parametrize the effect of DM annihilation as
an injection of ionizing energy at a rate e_{dm}, and argue that this simple
"on the spot'' modification is a good approximation to the complicated
interaction of the annihilation products with the photon-electron plasma.
Generic models of DM do not change the redshift of recombination, but change
the residual ionization after recombination. This broadens the surface of last
scattering, suppressing the temperature fluctuations and enhancing the
polarization fluctuations. We use the temperature and polarization angular
power spectra to measure these deviations from the standard recombination
history, and therefore, indirectly probe DM annihilation. (abridged)Comment: 13 pages, 8 figures, submitted to PR
Diffuse MeV Gamma-rays and Galactic 511 keV Line from Decaying WIMP Dark Matter
The origin of both the diffuse high-latitude MeV gamma-ray emission and the
511 keV line flux from the Galactic bulge are uncertain. Previous studies have
invoked dark matter physics to independently explain these observations, though
as yet none has been able to explain both of these emissions within the
well-motivated framework of Weakly-Interacting Massive Particles (WIMPs). Here
we use an unstable WIMP dark matter model to show that it is in fact possible
to simultaneously reconcile both of these observations, and in the process show
a remarkable coincidence: decaying dark matter with MeV mass splittings can
explain both observations if positrons and photons are produced with similar
branching fractions. We illustrate this idea with an unstable branon, which is
a standard WIMP dark matter candidate appearing in brane world models with
large extra dimensions. We show that because branons decay via three-body final
states, they are additionally unconstrained by searches for Galactic MeV
gamma-ray lines. As a result, such unstable long-lifetime dark matter particles
provide novel and distinct signatures that can be tested by future observations
of MeV gamma-rays.Comment: 19 pages, 4 figure
Searching for the 511 keV annihilation line from galactic compact objects with the IBIS gamma ray telescope
The first detection of a gamma ray line with an energy of about 500 keV from
the center our Galaxy dates back to the early seventies. Thanks to the
astrophysical application of high spectral resolution detectors, it was soon
clear that this radiation was due to the 511 keV photons generated by
electron-positron annihilation. Even though the physical process are known, the
astrophysical origin of this radiation is still a mystery. The spectrometer SPI
aboard the INTEGRAL gamma-ray satellite has been used to produce the first
all-sky map in light of the 511 keV annihilation, but no direct evidence for
511 keV galactic compact objects has been found [...] We present the first deep
IBIS 511 keV all-sky map, obtained by applying standard analysis to about 5
years of data. Possible 511 keV signals are also searched over hour-day-month
timescales. The IBIS sensitivity at 511 keV depends on the detector quantum
efficiency at this energy and on the background. Both these quantities were
estimated in this work. We find no evidence of Galactic 511 keV point sources.
With an exposure of 10 Ms in the center of the Galaxy, we estimate a flux 2 sigma upper limit. A similar limit
is given in a wide area in the Galactic center region with similar exposures.
The IBIS 511 keV flux upper limits for microquasars and supernova remnants
detected in the hard X domain () are also reported. Our results
are consistent with a diffuse annihilation scenario. If positrons
are generated in compact objects, we expect that a significant fraction of them
propagate in the interstellar medium before there are annihilated away from
their birth places.Comment: To appear in A&
The COMPTEL instrumental line background
The instrumental line background of the Compton telescope COMPTEL onboard the
Compton Gamma-Ray Observatory is due to the activation and/or decay of many
isotopes. The major components of this background can be attributed to eight
individual isotopes, namely 2D, 22Na, 24Na, 28Al, 40K, 52Mn, 57Ni, and 208Tl.
The identification of instrumental lines with specific isotopes is based on the
line energies as well as on the variation of the event rate with time,
cosmic-ray intensity, and deposited radiation dose during passages through the
South-Atlantic Anomaly. The characteristic variation of the event rate due to a
specific isotope depends on its life-time, orbital parameters such as the
altitude of the satellite above Earth, and the solar cycle. A detailed
understanding of the background contributions from instrumental lines is
crucial at MeV energies for measuring the cosmic diffuse gamma-ray background
and for observing gamma-ray line emission in the interstellar medium or from
supernovae and their remnants. Procedures to determine the event rate from each
background isotope are described, and their average activity in spacecraft
materials over the first seven years of the mission is estimated.Comment: accepted for publication in A&A, 22 pages, 21 figure
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