1,148 research outputs found
Identification of four RXTE Slew Survey sources with nearby luminous active galactic nuclei
Based on RXTE scans and observations with the SWIFT/XRT telescope and
INTEGRAL observatory, we report the identification of four X-ray sources
discovered during the RXTE Slew Survey of the |b|>10deg sky with nearby (z ~
0.017-0.098) luminous (log L_2-10keV ~ 42.7-44 erg/s) active galactic nuclei.
Two of the objects exhibit heavily intrinsically absorbed X-ray spectra
(NHL~10^23 cm^-2).Comment: 4 pages, 2 figures. Sibmitted to Astronomy and Astrophysics Letter
Constraints on the collimated X-ray emission of SS 433 from the reflection on molecular clouds
We calculate X-ray signal that should arise due to reflection of the putative
collimated X-ray emission of the Galactic supercritical accretor SS 433 on
molecular clouds in its vicinity. The molecular gas distribution in the region
of interest has been constructed based on the data of the BU-FCRAO GRS in
CO emission line, while the collimated emission was
assumed to be aligned with the direction of the relativistic jets, which are
continuously launched by the system. We consider all the available
observations covering the regions possibly containing the reflection signal and
put constraints on the apparent face-on luminosity of SS 433 above 4 keV. No
signatures of the predicted signal have been found in the analysed regions down
to a 4-8 keV surface brightness level of erg/s/cm/deg.
This translates into the limit on the apparent face-on 2-10 keV luminosity of
SS 433 erg/s, provided that the considered
clouds do fall inside the illumination cone of the collimated emission. This,
however, might not be the case due to persisting uncertainty in the
line-of-sight distances to SS 433 (4.5-5.5 kpc) and to the
considered molecular clouds. For half-opening angle of the collimation cone
larger than or comparable to the amplitude of the jets' precession
(), the stringent upper limit quoted above is most relevant if
kpc, provided that the kinematic distances to the considered
molecular clouds are sufficiently accurate. Dropping the last assumption, a
more conservative constraint is erg/s for
kpc (and yet worse outside this range). We conclude that
SS 433 is not likely to belong to the brightest ultraluminous X-ray sources if
it could be observed face-on, unless its X-ray emission is highly collimated.
(Abridged)Comment: Astronomy Letters, in press; 16 pages, 8 figure
Optical transparency modes in anisotropic media
The modes of nonlinear propagation of the two-component electromagnetic
pulses through optically uniaxial media containing resonant particles are
studied. The features of their manifestation in the "dense" media and in the
media with expressed positive and negative birefringences are discussed. It is
shown that exponentially and rationally decreasing solutions of the system of
material and wave equations allow us also to describe the propagation of the
self-induced transparency pulses in isotropic media in the case, when the
direct electric dipole-dipole interaction between the resonant particles is
taken into account.Comment: LaTeX, 11 pages, 4 figures, International Conference on Coherent and
Nonlinear Optics (ICONO 2005
Diffusion of Elements in the Interstellar Medium in Early-Type Galaxies
We consider the role of diffusion in the redistribution of elements in the
hot interstellar medium (ISM) of early-type galaxies. It is well known that
gravitational sedimentation can affect significantly the abundances of helium
and heavy elements in the intracluster gas of massive galaxy clusters. The
self-similarity of the temperature profiles and tight mass--temperature
relation of relaxed cool-core clusters suggest that the maximum effect of
sedimentation take place in the most massive virialized objects in the
Universe. However, Chandra and XMM-Newton observations demonstrate more complex
scaling relations between the masses of early-type galaxies and other
parameters, such as the ISM temperature and gas mass fraction. An important
fact is that early-type galaxies can show both decreasing and increasing radial
temperature profiles. We have calculated the diffusion based on the observed
gas density and temperature distributions for 13 early-type galaxies that
belonging to the different environments and cover a wide range of X-ray
luminosities. To estimate the maximum effect of sedimentation and thermal
diffusion, we have solved the full set of Burgers' equations for a
non-magnetized ISM plasma. The results obtained demonstrate a considerable
increase of the He/H ratio within one effective radius for all galaxies of our
sample. For galaxies with a flat or declining radial temperature profile the
average increase of the helium abundance is 60\% in one billion years of
diffusion. The revealed effect can introduce a significant bias in the metal
abundance measurements based on X-ray spectroscopy and can affect the evolution
of stars that could be formed from a gas with a high helium abundance.Comment: 15 pages, 9 figures, 3 tables, published in Astronomy Letters, 2017,
Volume 43, Issue 5, pp.285-30
Inverse Compton scattering in mildly relativistic plasma
We investigated the effect of inverse Compton scattering in mildly
relativistic static and moving plasmas with low optical depth using Monte Carlo
simulations, and calculated the Sunyaev-Zel'dovich effect in the cosmic
background radiation. Our semi-analytic method is based on a separation of
photon diffusion in frequency and real space. We use Monte Carlo simulation to
derive the intensity and frequency of the scattered photons for a monochromatic
incoming radiation. The outgoing spectrum is determined by integrating over the
spectrum of the incoming radiation using the intensity to determine the correct
weight. This method makes it possible to study the emerging radiation as a
function of frequency and direction. As a first application we have studied the
effects of finite optical depth and gas infall on the Sunyaev-Zel'dovich effect
(not possible with the extended Kompaneets equation) and discuss the parameter
range in which the Boltzmann equation and its expansions can be used. For high
temperature clusters ( keV) relativistic corrections based
on a fifth order expansion of the extended Kompaneets equation seriously
underestimate the Sunyaev-Zel'dovich effect at high frequencies. The
contribution from plasma infall is less important for reasonable velocities. We
give a convenient analytical expression for the dependence of the cross-over
frequency on temperature, optical depth, and gas infall speed. Optical depth
effects are often more important than relativistic corrections, and should be
taken into account for high-precision work, but are smaller than the typical
kinematic effect from cluster radial velocities.Comment: LateX, 30 pages and 11 figures. Accepted for publication in the
Astrophysical Journa
Convergent series for lattice models with polynomial interactions
The standard perturbative weak-coupling expansions in lattice models are
asymptotic. The reason for this is hidden in the incorrect interchange of the
summation and integration. However, substituting the Gaussian initial
approximation of the perturbative expansions by a certain interacting model or
regularizing original lattice integrals, one can construct desired convergent
series. In this paper we develop methods, which are based on the joint and
separate utilization of the regularization and new initial approximation. We
prove, that the convergent series exist and can be expressed as the re-summed
standard perturbation theory for any model on the finite lattice with the
polynomial interaction of even degree. We discuss properties of such series and
make them applicable to practical computations. The workability of the methods
is demonstrated on the example of the lattice -model. We calculate the
operator using the convergent series, the comparison
of the results with the Borel re-summation and Monte Carlo simulations shows a
good agreement between all these methods.Comment: 25 pages, 14 figure
Evidence of widespread degradation of gene control regions in hominid genomes
Although sequences containing regulatory elements located close to protein-coding genes are often only weakly conserved during evolution, comparisons of rodent genomes have implied that these sequences are subject to some selective constraints. Evolutionary conservation is particularly apparent upstream of coding sequences and in first introns, regions that are enriched for regulatory elements. By comparing the human and chimpanzee genomes, we show here that there is almost no evidence for conservation in these regions in hominids. Furthermore, we show that gene expression is diverging more rapidly in hominids than in murids per unit of neutral sequence divergence. By combining data on polymorphism levels in human noncoding DNA and the corresponding human¿chimpanzee divergence, we show that the proportion of adaptive substitutions in these regions in hominids is very low. It therefore seems likely that the lack of conservation and increased rate of gene expression divergence are caused by a reduction in the effectiveness of natural selection against deleterious mutations because of the low effective population sizes of hominids. This has resulted in the accumulation of a large number of deleterious mutations in sequences containing gene control elements and hence a widespread degradation of the genome during the evolution of humans and chimpanzees
Impact of thermal diffusion and other abundance anomalies on cosmological uses of galaxy clusters
Depending on the topology of the magnetic field and characteristics of
turbulent motions, diffusion can significantly affect the distribution of
elements, in particular helium, in the intracluster medium (ICM). As has been
noted previously, an incorrect assumption about the helium abundance will lead
to an error in the iron abundance determined from X-ray spectroscopy. The
corresponding effect on the temperature measurement is negligibly small. An
incorrectly assumed helium abundance will also lead to a systematic error in
angular distance measurements based on X-ray and Sunyaev-Zeldovich (SZ)
observations of clusters of galaxies. Its magnitude is further amplified by the
associated error in the metal abundance determination, the impact being larger
at lower ICM temperatures. Overall, a factor of 2-5 error in the helium
abundance will lead to an ~ 10-25 % error in the angular distance.
We solve the full set of Burgers equations for a multi-component intracluster
plasma to determine the maximal effect of diffusion on the interpretation of
X-ray and microwave observations of clusters of galaxies. For an isothermal
cluster, gravitational sedimentation can lead to up to a factor of ~ 5-10
enhancements of helium and metal abundances in the cluster center on a ~ 3-7
Gyr timescale. In cool-core clusters on the contrary, thermal diffusion can
counteract gravitational sedimentation and effectively remove helium and metals
from the cluster inner core. In either case, a significant, up to ~ 40 %, error
in the metal abundances determined by means of X-ray spectroscopy is possible.
The angular distance determined from X-ray and SZ data can be underestimated by
up to ~ 10-25 %.Comment: 11 pages, 10 figure
Hard X-ray emission of the Earth's atmosphere: Monte Carlo simulations
We perform Monte Carlo simulations of cosmic ray-induced hard X-ray radiation
from the Earth's atmosphere. We find that the shape of the spectrum emergent
from the atmosphere in the energy range 25-300 keV is mainly determined by
Compton scatterings and photoabsorption, and is almost insensitive to the
incident cosmic-ray spectrum. We provide a fitting formula for the hard X-ray
surface brightness of the atmosphere as would be measured by a satellite-born
instrument, as a function of energy, solar modulation level, geomagnetic cutoff
rigidity and zenith angle. A recent measurement by the INTEGRAL observatory of
the atmospheric hard X-ray flux during the occultation of the cosmic X-ray
background by the Earth agrees with our prediction within 10%. This suggests
that Earth observations could be used for in-orbit calibration of future hard
X-ray telescopes. We also demonstrate that the hard X-ray spectra generated by
cosmic rays in the crusts of the Moon, Mars and Mercury should be significantly
different from that emitted by the Earth's atmosphere.Comment: 12 pages, 16 figures, MNRAS accepte
Hybrid LQG-Neural Controller for Inverted Pendulum System
The paper presents a hybrid system controller, incorporating a neural and an
LQG controller. The neural controller has been optimized by genetic algorithms
directly on the inverted pendulum system. The failure free optimization process
stipulated a relatively small region of the asymptotic stability of the neural
controller, which is concentrated around the regulation point. The presented
hybrid controller combines benefits of a genetically optimized neural
controller and an LQG controller in a single system controller. High quality of
the regulation process is achieved through utilization of the neural
controller, while stability of the system during transient processes and a wide
range of operation are assured through application of the LQG controller. The
hybrid controller has been validated by applying it to a simulation model of an
inherently unstable system of inverted pendulum
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