1,648 research outputs found
Gamma-ray variability from wind clumping in HMXBs with jets
In the subclass of high-mass X-ray binaries known as "microquasars",
relativistic hadrons in the jets launched by the compact object can interact
with cold protons from the star's radiatively driven wind, producing pions that
then quickly decay into gamma rays. Since the resulting gamma-ray emissivity
depends on the target density, the detection of rapid variability in
microquasars with GLAST and the new generation of Cherenkov imaging arrays
could be used to probe the clumped structure of the stellar wind. We show here
that the fluctuation in gamma rays can be modeled using a "porosity length"
formalism, usually applied to characterize clumping effects. In particular, for
a porosity length defined by h=l/f, i.e. as the ratio of the characteristic
size l of clumps to their volume filling factor f, we find that the relative
fluctuation in gamma-ray emission in a binary with orbital separation a scales
as sqrt(h/pi a) in the "thin-jet" limit, and is reduced by a factor 1/sqrt(1 +
phi a/(2 l)) for a jet with a finite opening angle phi. For a thin jet and
quite moderate porosity length h ~ 0.03 a, this implies a ca. 10 % variation in
the gamma-ray emission. Moreover, the illumination of individual large clumps
might result in isolated flares, as has been recently observed in some massive
gamma-ray binaries.Comment: Accepted for publication in ApJ; 5 pages, 1 figur
Exact-exchange density-functional calculations for noble-gas solids
The electronic structure of noble-gas solids is calculated within density
functional theory's exact-exchange method (EXX) and compared with the results
from the local-density approximation (LDA). It is shown that the EXX method
does not reproduce the fundamental energy gaps as well as has been reported for
semiconductors. However, the EXX-Kohn-Sham energy gaps for these materials
reproduce about 80 % of the experimental optical gaps. The structural
properties of noble-gas solids are described by the EXX method as poorly as by
the LDA one. This is due to missing Van der Waals interactions in both, LDA and
EXX functionals.Comment: 4 Fig
Critical density of a soliton gas
We quantify the notion of a dense soliton gas by establishing an upper bound
for the integrated density of states of the quantum-mechanical Schr\"odinger
operator associated with the KdV soliton gas dynamics. As a by-product of our
derivation we find the speed of sound in the soliton gas with Gaussian spectral
distribution function.Comment: 7 page
Strain-Induced Conduction Band Spin Splitting in GaAs from First Principles Calculations
We use a recently developed self-consistent GW approximation to present first
principles calculations of the conduction band spin splitting in GaAs under
[110] strain. The spin orbit interaction is taken into account as a
perturbation to the scalar relativistic hamiltonian. These are the first
calculations of conduction band spin splitting under deformation based on a
quasiparticle approach; and because the self-consistent GW scheme accurately
reproduces the relevant band parameters, it is expected to be a reliable
predictor of spin splittings. We also discuss the spin relaxation time under
[110] strain and show that it exhibits an in-plane anisotropy, which can be
exploited to obtain the magnitude and sign of the conduction band spin
splitting experimentally.Comment: 8 pages, 4 figures, 1 tabl
Positivity of Lyapunov exponents for a continuous matrix-valued Anderson model
We study a continuous matrix-valued Anderson-type model. Both leading
Lyapunov exponents of this model are proved to be positive and distinct for all
ernergies in except those in a discrete set, which leads to
absence of absolutely continuous spectrum in . This result is an
improvement of a previous result with Stolz. The methods, based upon a result
by Breuillard and Gelander on dense subgroups in semisimple Lie groups, and a
criterion by Goldsheid and Margulis, allow for singular Bernoulli
distributions
Hadronic gamma-ray emission from windy microquasars
The jets of microquasars with high-mass stellar companions are exposed to the
dense matter field of the stellar wind. We present estimates of the gamma-ray
emission expected from the jet-wind hadronic interaction and we discuss the
detectability of the phenomenon at high energies. The proposed mechanism could
explain some of the unidentified gamma-ray sources detected by EGRET instrument
on the galactic plane.Comment: Accepted for publication in Astronomy and Astrophysics as a Letter to
the Edito
Jet-Induced Nucleosynthesis in Misaligned Microquasars
The jet axes and the orbital planes of microquasar systems are usually
assumed to be approximately perpendicular, eventhough this is not currently an
observational requirement. On the contrary, in one of the few systems where the
relative orientations are well-constrained, V4641Sgr, the jet axis is known to
lie not more than ~36 degrees from the binary plane. Such a jet, lying close to
the binary plane, and traveling at a significant fraction of the speed of light
may periodically impact the secondary star initiating nuclear reactions on its
surface. The integrated yield of such nuclear reactions over the age of the
binary system (less the radiative mass loss) will detectably alter the
elemental abundances of the companion star. This scenario may explain the
anomalously high Li enhancements (roughly ~20-200 times the sun's photospheric
value; or, equivalently, 0.1-1 times the average solar system value) seen in
the companions of some black-hole X-ray binary systems. (Such enhancements are
puzzling since Li nuclei are exceedingly fragile - being easily destroyed in
the interiors of stars - and Li would be expected to be depleted rather than
enhanced there.) Gamma-ray line signatures of the proposed process could
include the 2.22 MeV neutron capture line as well as the 0.478 MeV 7Li*
de-excitation line, both of which may be discernable with the INTEGRAL
satellite if produced in an optically thin region during a large outburst. For
very energetic jets, a relatively narrow neutral pion gamma-decay signature at
67.5 MeV could also be measurable with the GLAST satellite. We argue that about
10-20% of all microquasar systems ought to be sufficiently misaligned as to be
undergoing the proposed jet-secondary impacts.Comment: ApJ, accepted. Includes referee's suggestions and some minor
clarifications over previous versio
Generalized Lyapunov Exponent and Transmission Statistics in One-dimensional Gaussian Correlated Potentials
Distribution of the transmission coefficient T of a long system with a
correlated Gaussian disorder is studied analytically and numerically in terms
of the generalized Lyapunov exponent (LE) and the cumulants of lnT. The effect
of the disorder correlations on these quantities is considered in weak,
moderate and strong disorder for different models of correlation. Scaling
relations between the cumulants of lnT are obtained. The cumulants are treated
analytically within the semiclassical approximation in strong disorder, and
numerically for an arbitrary strength of the disorder. A small correlation
scale approximation is developed for calculation of the generalized LE in a
general correlated disorder. An essential effect of the disorder correlations
on the transmission statistics is found. In particular, obtained relations
between the cumulants and between them and the generalized LE show that, beyond
weak disorder, transmission fluctuations and deviation of their distribution
from the log-normal form (in a long but finite system) are greatly enhanced due
to the disorder correlations. Parametric dependence of these effects upon the
correlation scale is presented.Comment: 18 pages, 11 figure
Mott Transition of MnO under Pressure: Comparison of Correlated Band Theories
The electronic structure, magnetic moment, and volume collapse of MnO under
pressure are obtained from four different correlated band theory methods; local
density approximation + Hubbard U (LDA+U), pseudopotential self-interaction
correction (pseudo-SIC), the hybrid functional (combined local exchange plus
Hartree-Fock exchange), and the local spin density SIC (SIC-LSD) method. Each
method treats correlation among the five Mn 3d orbitals (per spin), including
their hybridization with three O orbitals in the valence bands and their
changes with pressure. The focus is on comparison of the methods for rocksalt
MnO (neglecting the observed transition to the NiAs structure in the 90-100 GPa
range). Each method predicts a first-order volume collapse, but with variation
in the predicted volume and critical pressure. Accompanying the volume collapse
is a moment collapse, which for all methods is from high-spin to low-spin (5/2
to 1/2), not to nonmagnetic as the simplest scenario would have. The specific
manner in which the transition occurs varies considerably among the methods:
pseudo-SIC and SIC-LSD give insulator-to-metal, while LDA+U gives
insulator-to-insulator and the hybrid method gives an insulator-to-semimetal
transition. Projected densities of states above and below the transition are
presented for each of the methods and used to analyze the character of each
transition. In some cases the rhombohedral symmetry of the
antiferromagnetically ordered phase clearly influences the character of the
transition.Comment: 14 pages, 9 figures. A 7 institute collaboration, Updated versio
INTEGRAL observations of SS433, a supercritically accreting microquasar with hard spectrum
Observations of SS433 by INTEGRAL carried out in March -- May 2003 are
presented. SS433 is evidently detected on the INTEGRAL images of the
corresponding sky region in the energy bands 25-50 and 50-100 keV. The
precessional variability of the hard X-ray flux is clearly seen. The X-ray
eclipse caused by the binary orbital motion is also detected. A possible origin
of the hard continuum is briefly discussed.Comment: 5 pages, 6 figures. Accepted to A&A INTEGRAL special volum
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