127 research outputs found
Lithium Production in Companions of Accreting X-Ray Binaries by Neutron Spallation of C,N,O Elements
We examine the processes which could lead to the observed enhancement of Li
and possibly other light elements (Be, B) in the companions of a number of
X-ray novae. We conclude that one of the most promising mechanisms is the
spallation of CNO elements on the surface of the companion induced by the
neutron flux produced in the hot accretion flow onto the compact object. Direct
production of the observed Li and its deposition onto the dwarf companion seem
less likely, mainly because of the possibility of its destruction in the
production region itself and difficulties in its deposition associated with the
configuration of the companion's magnetic field. We discuss other potential
observables of the above scenario.Comment: 23 pages Latex, of which 5 pages of tables, to appear in the
Astrophysical Journal, Vol 512, Feb 10 issu
Neutron capture and 2.2 MeV emission in the atmosphere of the secondary of an X-ray binary
We consider the production of 2.22 MeV radiation resulting from the capture
of neutrons in the atmosphere of the secondary in an X-ray binary system, where
the neutrons are produced in the accretion disk around the compact primary star
and radiated in all directions. We have considered several accretion disk
models (ADAF, ADIOS, SLE, Uniform-Temperature) and a varity of parameters
(accretion rate, mass of the compact object, mass, temperature and composition
of the secondary star, distance between the two objects, etc.). The neutron
rates are calculated by a network of nuclear reactions in the accretion disk,
and this is handled by a reaction-rate formulation taking into account the
structure equations given by each accretion model. The processes undergone by
the neutrons in the atmosphere of the companion star are studied in great
detail, including thermalization, elastic and inelastic scatterings,
absorption, escape from the surface, decay, and capture by protons. The
radiative transfer of the 2.22 MeV photons is treated separately, taking into
consideration the composition and density of the star's atmosphere. The final
flux of the 2.22 MeV radiation that can be detected from earth is calculated
taking into account the distance to the source, the direction of observation
with respect to the binary system frame, and the rotation of the source, as
this can lead to an observable periodicity in the flux. We produce phasograms
of the 2.22 MeV intensity as well as spectra of the line, where rotational
Doppler shift effects can lead to changes in the spectra that are measurable by
INTEGRAL's spectrometer (SPI).Comment: 13 pages, 10 figures, A&A in pres
Microquasars as sources of positron annihilation radiation
We consider the production of positrons in microquasars, i.e. X-ray binary
systems that exhibit jets frequently, but not continuously. We estimate the
production rate of positrons in microquasars, both by simple energy
considerations and in the framework of various proposed models. We then
evaluate the collective emissivity of the annihilation radiation produced by
Galactic microquasars and we find that it might constitute a substantial
contribution to the annihilation flux measured by INTEGRAL/SPI. We also discuss
the possible spatial distribution of Galactic microquasars, on the basis of the
(scarce) available data and the resulting morphology of the flux received on
Earth. Finally, we consider nearby 'misaligned' microquasars, with jets
occasionally hitting the atmosphere of the companion star; these would
represent interesting point sources, for which we determine the annihilation
flux and the corresponding light curve, as well as the line's spectral profile.
We discuss the possibility of detection of such point sources by future
instruments.Comment: 13 pages, 7 figures, accepted in A&
The 511 keV emission from positron annihilation in the Galaxy
The first gamma-ray line originating from outside the solar system that was
ever detected is the 511 keV emission from positron annihilation in the Galaxy.
Despite 30 years of intense theoretical and observational investigation, the
main sources of positrons have not been identified up to now. Observations in
the 1990's with OSSE/CGRO showed that the emission is strongly concentrated
towards the Galactic bulge. In the 2000's, the SPI instrument aboard ESA's
INTEGRAL gamma-ray observatory allowed scientists to measure that emission
across the entire Galaxy, revealing that the bulge/disk luminosity ratio is
larger than observed in any other wavelength. This mapping prompted a number of
novel explanations, including rather "exotic ones (e.g. dark matter
annihilation). However, conventional astrophysical sources, like type Ia
supernovae, microquasars or X-ray binaries, are still plausible candidates for
a large fraction of the observed total 511 keV emission of the bulge. A closer
study of the subject reveals new layers of complexity, since positrons may
propagate far away from their production sites, making it difficult to infer
the underlying source distribution from the observed map of 511 keV emission.
However, contrary to the rather well understood propagation of high energy
(>GeV) particles of Galactic cosmic rays, understanding the propagation of low
energy (~MeV) positrons in the turbulent, magnetized interstellar medium, still
remains a formidable challenge. We review the spectral and imaging properties
of the observed 511 keV emission and we critically discuss candidate positron
sources and models of positron propagation in the Galaxy.Comment: 62 pages, 35 figures. Review paper to appear in Reviews of Modern
Physic
Using Neural Networks to Perform Rapid High-Dimensional Kilonova Parameter Inference
On the 17th of August, 2017 came the simultaneous detections of GW170817, a gravitational wave that originated from the coalescence of two neutron stars, along with the gamma-ray burst GRB170817A, and the kilonova counterpart AT2017gfo. Since then, there has been much excitement surrounding the study of neutron star mergers, both observationally, using a variety of tools, and theoretically, with the development of complex models describing the gravitational-wave and electromagnetic signals. In this work, we improve upon our pipeline to infer kilonova properties from observed light-curves by employing a Neural-Network framework that reduces execution time and handles much larger simulation sets than previously possible. In particular, we use the radiative transfer code POSSIS to construct 5-dimensional kilonova grids where we employ different functional forms for the angular dependence of the dynamical ejecta component. We find that incorporating an angular dependence improves the fit to the AT2017gfo light-curves by up to ~50% when quantified in terms of the weighted Mean Square Error
The all-sky distribution of 511 keV electron-positron annihilation emission
We present a map of 511 keV electron-positron annihilation emission, based on
data accumulated with the SPI spectrometer aboard ESA's INTEGRAL gamma-ray
observatory, that covers approximately 95% of the celestial sphere. 511 keV
line emission is significantly detected towards the galactic bulge region and,
at a very low level, from the galactic disk. The bulge emission is highly
symmetric and is centred on the galactic centre with an extension of 8 deg. The
emission is equally well described by models that represent the stellar bulge
or halo populations. The disk morphology is only weakly constrained by the
present data, being compatible with both the distribution of young and old
stellar populations. The 511 keV line flux from the bulge and disk components
is 1.05e-3 ph cm-2 s-1 and 0.7e-3 ph cm-2 s-1, respectively, corresponding to a
bulge-to-disk flux ratio in the range 1-3. Assuming a positronium fraction of
0.93 this translates into annihilation rates of 1.5e43 s-1 and 3e42 s-1,
respectively. The ratio of the bulge luminosity to that of the disk is in the
range 3-9. We find no evidence for a point-like source in addition to the
diffuse emission, down to a typical flux limit of 1e-4 ph cm-2 s-1. We also
find no evidence for the positive latitude enhancement that has been reported
from OSSE measurements; our 3 sigma upper flux limit for this feature is 1.5e-4
ph cm-2 s-1. The disk emission can be attributed to the beta+ decay of the
radioactive species 26Al and 44Ti. The bulge emission arises from a different
source which has only a weak or no disk component. We suggest that Type Ia
supernovae and/or low-mass X-ray binaries are the prime candidates for the
source of the galactic bulge positrons. Light dark matter annihilation could
also explain the observed 511 keV bulge emission characteristics.Comment: accepted for publication in A&
SPI observations of positron annihilation radiation from the 4th galactic quadrant: sky distribution
During its first year in orbit the INTEGRAL observatory performed deep
exposures of the Galactic Center region and scanning observations of the
Galactic plane. We report on the status of our analysis of the positron
annihilation radiation from the 4th Galactic quadrant with the spectrometer
SPI, focusing on the sky distribution of the 511 keV line emission. The
analysis methods are described; current constraints and limits on the Galactic
bulge emission and the bulge-to-disk ratio are presented.Comment: 4 pages, 2 figures, accepted for publication in the proceedings of
the 5th INTEGRAL worksho
Spectral analysis of the Galactic e+e- annihilation emission
We present a spectral analysis of the e+e- annihilation emission from the
Galactic Centre region based on the first year of measurements made with the
spectrometer SPI of the INTEGRAL mission. We have found that the annihilation
spectrum can be modelled by the sum of a narrow and a broad 511 keV line plus
an ortho-Ps continuum. The broad line is detected with a flux of
(0.35+/-0.11)e-3 s-1 cm-2. The measured width of 5.4+/-1.2 keV FWHM is in
agreement with the expected broadening of 511 keV photons emitted in the
annihilation of Ps that are formed by the charge exchange process of slowing
down positrons with H atoms. The flux of the narrow line is (0.72+/-0.12)e-3
s-1 cm-2 and its width is 1.3+/-0.4 keV FWHM. The measured ortho-Ps continuum
flux yields a fraction of Ps of (96.7+/-2.2)%. To derive in what phase of the
interstellar medium positrons annihilate, we have fitted annihilation models
calculated for each phase to the data. We have found that 49(+2,-23)% of the
annihilation emission comes from the warm neutral phase and 51(+3,-2)% from the
warm ionized phase. While we may not exclude that less than 23% of the emission
might come from cold gas, we have constrained the fraction of annihilation
emission from molecular clouds and hot gas to be less than 8% and 0.5%,
respectively. We have compared our knowledge of the interstellar medium in the
bulge and the propagation of positrons with our results and found that they are
in good agreement if the sources are diffusively distributed and if the initial
kinetic energy of positrons is lower than a few MeV. Despite its large filling
factor, the lack of annihilation emission from the hot gas is due to its low
density, which allows positrons to escape this phase.Comment: 12 pages, 6 figures, accepted in A&
Efficient FPGA implementation of high-throughput mixed radix multipath delay commutator FFT processor for MIMO-OFDM
This article presents and evaluates pipelined architecture designs for an improved high-frequency Fast Fourier
Transform (FFT) processor implemented on Field Programmable Gate Arrays (FPGA) for Multiple Input Multiple Output
Orthogonal Frequency Division Multiplexing (MIMO-OFDM). The architecture presented is a Mixed-Radix Multipath Delay
Commutator. The presented parallel architecture utilizes fewer hardware resources compared to Radix-2 architecture,
while maintaining simple control and butterfly structures inherent to Radix-2 implementations. The high-frequency
design presented allows enhancing system throughput without requiring additional parallel data paths common in
other current approaches, the presented design can process two and four independent data streams in parallel
and is suitable for scaling to any power of two FFT size N. FPGA implementation of the architecture demonstrated
significant resource efficiency and high-throughput in comparison to relevant current approaches within
literature. The proposed architecture designs were realized with Xilinx System Generator (XSG) and evaluated
on both Virtex-5 and Virtex-7 FPGA devices. Post place and route results demonstrated maximum frequency
values over 400 MHz and 470 MHz for Virtex-5 and Virtex-7 FPGA devices respectively
Data-driven expectations for electromagnetic counterpart searches based on LIGO/Virgo public alerts
Searches for electromagnetic counterparts of gravitational-wave signals have redoubled since the first detection in 2017 of a binary neutron star merger with a gamma-ray burst, optical/infrared kilonova, and panchromatic afterglow. Yet, one LIGO/Virgo observing run later, there has not yet been a second, secure identification of an electromagnetic counterpart. This is not surprising given that the localization uncertainties of events in LIGO and Virgo's third observing run, O3, were much larger than predicted. We explain this by showing that improvements in data analysis that now allow LIGO/Virgo to detect weaker and hence more poorly localized events have increased the overall number of detections, of which well-localized, gold-plated events make up a smaller proportion overall. We present simulations of the next two LIGO/Virgo/KAGRA observing runs, O4 and O5, that are grounded in the statistics of O3 public alerts. To illustrate the significant impact that the updated predictions can have, we study the follow-up strategy for the Zwicky Transient Facility. Realistic and timely forecasting of gravitational-wave localization accuracy is paramount given the large commitments of telescope time and the need to prioritize which events are followed up. We include a data release of our simulated localizations as a public proposal planning resource for astronomers
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