6,764 research outputs found
X-ray Polarization Signatures of Compton Scattering in Magnetic Cataclysmic Variables
Compton scattering within the accretion column of magnetic cataclysmic
variables (mCVs) can induce a net polarization in the X-ray emission. We
investigate this process using Monte Carlo simulations and find that
significant polarization can arise as a result of the stratified flow structure
in the shock-ionized column. We find that the degree of linear polarization can
reach levels up to ~8% for systems with high accretion rates and low
white-dwarf masses, when viewed at large inclination angles with respect to the
accretion column axis. These levels are substantially higher than previously
predicted estimates using an accretion column model with uniform density and
temperature. We also find that for systems with a relatively low-mass white
dwarf accreting at a high accretion rate, the polarization properties may be
insensitive to the magnetic field, since most of the scattering occurs at the
base of the accretion column where the density structure is determined mainly
by bremsstrahlung cooling instead of cyclotron cooling.Comment: 7 pages, 8 figures, accepted by MNRA
Monte-Carlo simulations of thermal/nonthermal radiation from a neutron-star magnetospheric accretion shell
We discuss the space-and-time-dependent Monte Carlo code we have developed to
simulate the relativistic radiation output from compact astrophysical objects,
coupled to a Fokker-Planck code to determine the self-consistent lepton
populations. We have applied this code to model the emission from a magnetized
neutron star accretion shell near the Alfven radius, reprocessing the radiation
from the neutron sar surface. We explore the parameter space defined by the
accretion rate, stellar surface field and the level of wave turbulence in the
shell. Our results are relevant to the emission from atoll sources, soft-X-ray
transient X-ray binaries containing weakly magnetized neutron stars, and to
recently suggested models of accretion-powered emission from anomalous X-ray
pulsars.Comment: 24 pages, including 7 figures; uses epsf.sty. final version, accepted
for publication in ApJ. Extended introduction and discussio
Accretion column disruption in GX 1+4
Daily observations of the binary X-ray pulsar GX 1+4 were made with the Rossi
X-ray Timing Explorer (RXTE) satellite between 1997 May 16-20 as part of a
four-month monitoring program. On May 17 the sharp dips normally observed in
the lightcurve were all but absent, resulting in a pulse fraction f_p approx.
0.5 instead of the more typical value of approx. 0.8 measured before and after.
Also observed was a dramatic hardening of the 2-40 keV phase-averaged spectrum.
The power-law photon index was 1.16 +/- 0.02, whereas values of 1.6-2.0 are
more typical. In terms of a Comptonization continuum component, the optical
depth for scattering was tau approx 19, with 4-6 the usual range for RXTE
spectra (Galloway 2000). Pulse-phase spectrosopy indicates that tau is
decreased relative to the phase-averaged value around the primary minimum,
where an increase is normally observed. The reduced depth of the dip is
interpreted as disruption of the accretion column, and the accompanying
spectral variation suggests a substantially different accretion regime than is
usual for this source.Comment: 10 pages, 2 figures, minor abstract typo and wording of final
paragraph correcte
The profile of a narrow line after single scattering by Maxwellian electrons: relativistic corrections to the kernel of the integral kinetic equation
The frequency distribution of photons in frequency that results from single
Compton scattering of monochromatic radiation on thermal electrons is derived
in the mildly relativistic limit. Algebraic expressions are given for (1) the
photon redistribution function, K(nu,Omega -> nu',Omega'), and (2) the spectrum
produced in the case of isotropic incident radiation, P(nu -> nu'). The former
is a good approximation for electron temperatures kT_e < 25 keV and photon
energies hnu < 50 keV, and the latter is applicable when hnu(hnu/m_ec^2) < kT_e
< 25 keV, hnu < 50 keV. Both formulae can be used for describing the profiles
of X-ray and low-frequency lines upon scattering in hot, optically thin
plasmas, such as present in clusters of galaxies, in the coronae of accretion
disks in X-ray binaries and AGNs, during supernova explosions, etc. Both
formulae can also be employed as the kernels of the corresponding integral
kinetic equations (direction-dependent and isotropic) in the general problem of
Comptonization on thermal electrons. The K(nu,Omega -> nu',Omega') kernel, in
particular, is applicable to the problem of induced Compton interaction of
anisotropic low-frequency radiation of high brightness temperature with free
electrons in the vicinity of powerful radiosources and masers.
Fokker-Planck-type expansion (up to fourth order) of the integral kinetic
equation with the P(nu -> nu') kernel derived here leads to a generalization of
the Kompaneets equation. We further present (1) a simpler kernel that is
necessary and sufficient to derive the Kompaneets equation and (2) an
expression for the angular function for Compton scattering in a hot plasma,
which includes temperature and photon energy corrections to the Rayleigh
angular function.Comment: 29 pages, 17 figures, accepted for publication in ApJ, uses
emulateapj.sty, corrects misprints in previous astro-ph versio
Backward asymmetry of the Compton scattering by an isotropic distribution of relativistic electrons: astrophysical implications
The angular distribution of low-frequency radiation after single scattering
by an isotropic distribution of relativistic electrons considerably differs
from the Rayleigh angular function. In particular, the scattering by an
ensemble of ultra-relativistic electrons obeys the law p=1-cos(alpha), where
alpha is the scattering angle; hence photons are preferentially scattered
backwards. We discuss some consequences of this fact for astrophysical
problems. We show that a hot electron-scattering atmosphere is more reflective
than a cold one: the fraction of incident photons which become reflected having
suffered a single scattering event can be larger by up to 50 per cent in the
former case. This should affect the photon exchange between cold accretion
disks and hot coronae or ADAF flows in the vicinity of relativistic compact
objects; as well as the rate of cooling (through multiple inverse-Compton
scattering of seed photons supplied from outside) of optically thick clouds of
relativistic electrons in compact radiosources. The forward-backward scattering
asymmetry also causes spatial diffusion of photons to proceed slower in hot
plasma than in cold one, which is important for the shapes of Comptonization
spectra and the time delays between soft and hard radiations coming from
variable X-ray sources.Comment: 20 pages, 3 figures, to appear in Astronomy Letters, added reference
Heating of gas inside radio sources to mildly relativistic temperatures via induced Compton scattering
Measured values of the brightness temperature of low-frequency synchrotron
radiation emitted by powerful extragalactic sources reach 10^11--10^12 K. If
some amount of nonrelativistic ionized gas is present within such sources, it
should be heated as a result of induced Compton scattering of the radiation. If
this heating is counteracted by cooling due to inverse Compton scattering of
the same radio radiation, then the plasma can be heated up to mildly
relativistic temperatures kT~10--100 keV. The stationary electron velocity
distribution can be either relativistic Maxwellian or quasi-Maxwellian (with
the high-velocity tail suppressed), depending on the efficiency of Coulomb
collisions and other relaxation processes. We derive several easy-to-use
approximate expressions for the induced Compton heating rate of mildly
relativistic electrons in an isotropic radiation field, as well as for the
stationary distribution function and temperature of electrons. We also give
analytic expressions for the kernel of the integral kinetic equation (one as a
function of the scattering angle and another for the case of an isotropic
radiation field), which describes the redistribution of photons in frequency
caused by induced Compton scattering in thermal plasma. These expressions can
be used in the parameter range hnu<< kT<~ 0.1mc^2 (the formulae earlier
published in Sazonov, Sunyaev, 2000 are less accurate).Comment: 22 pages, 7 figures, submitted to Astronomy Letter
Effects of broadening and electron overheating in tunnel structures based on metallic clusters
We study the influence of energy levels broadening and electron subsystem
overheating in island electrode (cluster) on current-voltage characteristics of
three-electrode structure. A calculation scheme for broadening effect in
one-dimensional case is suggested. Estimation of broadening is performed for
electron levels in disc-like and spherical gold clusters. Within the
two-temperature model of metallic cluster and by using a size dependence of the
Debye frequency the effective electron temperature as a function of bias
voltage is found approximately. We suggest that the effects of broadening and
electron overheating are responsible for the strong smoothing of
current-voltage curves, which is observed experimentally at low temperatures in
structures based on clusters consisting of accountable number of atoms.Comment: 8 pages, 5 figure
Vertical Structure of the Outer Accretion Disk in Persistent Low-Mass X-Ray Binaries
We have investigated the influence of X-ray irradiation on the vertical
structure of the outer accretion disk in low-mass X-ray binaries by performing
a self-consistent calculation of the vertical structure and X-ray radiation
transfer in the disk. Penetrating deep into the disk, the field of scattered
X-ray photons with energy \,keV exerts a significant influence on
the vertical structure of the accretion disk at a distance
\,cm from the neutron star. At a distance \,cm,
where the total surface density in the disk reaches
\,g\,cm, X-ray heating affects all layers of an
optically thick disk. The X-ray heating effect is enhanced significantly in the
presence of an extended atmospheric layer with a temperature
\,K above the accretion disk. We have derived
simple analytic formulas for the disk heating by scattered X-ray photons using
an approximate solution of the transfer equation by the Sobolev method. This
approximation has a \,% accuracy in the range of X-ray photon
energies \,keV.Comment: 19 pages, 8 figures, published in Astronomy Letter
Neuroprotective effects of a 40% ethanol extract of the black walnut bark (Juglans nigra L.)
Neuroprotection is a promising area of adjuvant therapy of ischemic brain lesions. At the same time, among potentially effective neuroprotectors, herbal remedies are distinguished due to their high efficiency and safety of use. In this work, some aspects of the neuroprotective effect of 40% ethanol extract of black walnut bark were investigated in comparison with its major component juglon
Pharmacological characteristics of intranasal dosage forms containing Ginkgo biloba extracts
To evaluate specific pharmacological activity of the developed dosage forms of ginkgo bilob
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