87 research outputs found
Electron-positron pairs in hot plasma of accretion column in bright X-ray pulsars
The luminosity of X-ray pulsars powered by accretion onto magnetized neutron
stars covers a wide range over a few orders of magnitude. The brightest X-ray
pulsars recently discovered as pulsating ultraluminous X-ray sources reach
accretion luminosity above which exceeds the
Eddington value more than by a factor of ten. Most of the energy is released
within small regions in the vicinity of magnetic poles of accreting neutron
star - in accretion columns. Because of the extreme energy release within a
small volume accretion columns of bright X-ray pulsars are ones of the hottest
places in the Universe, where the internal temperature can exceed 100 keV.
Under these conditions, the processes of creation and annihilation of
electron-positron pairs can be influential but have been largely neglected in
theoretical models of accretion columns. In this letter, we investigate
properties of a gas of electron-positron pairs under physical conditions
typical for accretion columns. We argue that the process of pairs creation can
crucially influence both the dynamics of the accretion process and internal
structure of accretion column limiting its internal temperature, dropping the
local Eddington flux and increasing the gas pressure.Comment: 5 pages, 5 figures, accepted for publication in MNRAS Letter
Analytical Study on the Sunyaev-Zeldovich Effect for Clusters of Galaxies. II. comparison of covariant formalisms
We study a covariant formalism for the Sunyaev-Zeldovich effects developed in
the previous papers by the present authors, and derive analytic expressions for
the redistribution functions in the Thomson approximation. We also explore
another covariant formalism recently developed by Poutanen and Vurm. We show
that the two formalisms are mathematically equivalent in the Thomson
approximation which is fully valid for the cosmic microwave background photon
energies. The present finding will establish a theoretical foundation for the
analysis of the Sunyaev-Zeldovich effects for the clusters of galaxies.Comment: Accepted version, 7 pages, 1 figure, accepted by Physical Review D
for publicatio
Compton scattering S-matrix and cross section in strong magnetic field
Compton scattering of polarized radiation in a strong magnetic field is
considered. The recipe for calculation of the scattering matrix elements, the
differential and total cross sections based on quantum electrodynamic (QED)
second order perturbation theory is presented for the case of arbitrary initial
and final Landau level, electron momentum along the field and photon momentum.
Photon polarization and electron spin state are taken into account. The correct
dependence of natural Landau level width on the electron spin state is taken
into account in general case of arbitrary initial photon momentum for the first
time. A number of steps in calculations were simplified analytically making the
presented recipe easy-to-use. The redistribution functions over the photon
energy, momentum and polarization states are presented and discussed. The paper
generalizes already known results and offers a basis for accurate calculation
of radiation transfer in strong -field, for example, in strongly magnetized
neutron stars.Comment: 26 pages, 12 figures, accepted for publication in Phys. Rev.
Theory of Compton scattering by anisotropic electrons
Compton scattering plays an important role in various astrophysical objects
such as accreting black holes and neutron stars, pulsars, and relativistic
jets, clusters of galaxies as well as the early Universe. In most of the
calculations it is assumed that the electrons have isotropic angular
distribution in some frame. However, there are situations where the anisotropy
may be significant due to the bulk motions, or anisotropic cooling by
synchrotron radiation, or anisotropic source of seed soft photons. We develop
here an analytical theory of Compton scattering by anisotropic distribution of
electrons that can simplify significantly the calculations. Assuming that the
electron angular distribution can be represented by a second order polynomial
over cosine of some angle (dipole and quadrupole anisotropy), we integrate the
exact Klein-Nishina cross-section over the angles. Exact analytical and
approximate formulae valid for any photon and electron energies are derived for
the redistribution functions describing Compton scattering of photons with
arbitrary angular distribution by anisotropic electrons. The analytical
expressions for the corresponding photon scattering cross-section on such
electrons as well as the mean energy of scattered photons, its dispersion and
radiation pressure force are also derived. We applied the developed formalism
to the accurate calculations of the thermal and kinematic Sunyaev-Zeldovich
effects for arbitrary electron distributions.Comment: 23 pages, 12 figures, ApJ Supplement Series, in pres
On the Origin of Polarization near the Lyman Edge in Quasars
Optical/UV radiation from accretion disks in quasars is likely to be partly
scattered by a hot plasma enveloping the disk. We investigate whether the
scattering may produce the steep rises in polarization observed blueward of the
Lyman limit in some quasars. We suggest and assess two models. In the first
model, primary disk radiation with a Lyman edge in absorption passes through a
static ionized "skin" covering the disk, which has a temperature about 3 keV
and a Thomson optical depth about unity. Electron scattering in the skin smears
out the edge and produces a steep rise in polarization at lambda < 912 A. In
the second model, the scattering occurs in a hot coronal plasma outflowing from
the disk with a mildly relativistic velocity. We find that the second model
better explains the data. The ability of the models to fit the observed rises
in polarization is illustrated with the quasar PG 1630+377.Comment: submitted to ApJ Letter
Timing properties of ULX pulsars: optically thick envelopes and outflows
It has recently been discovered that a fraction of ultra-luminous X-ray
sources (ULXs) exhibit X-ray pulsations, and are therefore powered by
super-Eddington accretion onto magnetized neutron stars (NSs). For typical ULX
mass accretion rates (), the inner parts of
the accretion disc are expected to be in the supercritical regime, meaning that
some material is lost in a wind launched from the disc surface, while the rest
forms an optically thick envelope around the NS as it follows magnetic field
lines from the inner disc radius to the magnetic poles of the star. The
envelope hides the central object from a distant observer and defines key
observational properties of ULX pulsars: their energy spectrum, polarization,
and timing features. The optical thickness of the envelope is affected by the
mass losses from the disc. We calculate the mass loss rate due to the wind in
ULX pulsars, accounting for the NS magnetic field strength and advection
processes in the disc. We argue that the detection of strong outflows from ULX
pulsars can be considered evidence of a relatively weak dipole component of the
NS magnetic field. We estimate the influence of mass losses on the optical
thickness of the envelope and analyze how the envelope affects broadband
aperiodic variability in ULXs. We show that brightness fluctuations at high
Fourier frequencies can be strongly suppressed by multiple scatterings in the
envelope and that the strength of suppression is determined by the mass
accretion rate and geometrical size of the magnetosphere.Comment: 12 pages, 11 figures, accepted for publication in MNRA
Sources of Radiation in the Early Universe: The Equation of Radiative Transfer and Optical Distances
We have derived the radiative-transfer equation for a point source with a
specified intensity and spectrum, originating in the early Universe between the
epochs of annihilation and recombination, at redshifts z_\s =10^8\div 10^4.
The direct radiation of the source is separated from the diffuse radiation it
produces. Optical distances from the source for Thomson scattering and
bremsstrahlung absorption at the maximum of the thermal background radiation
are calculated as a function of the redshift z.The distances grow sharply with
decreasing z, approaching asymptotic values, the absorption distance increasing
more slowly and reaching their limiting values at lower z. For the adopted z
values, the optical parameters of the Universe can be described in a flat model
with dusty material and radiation, and radiative transfer can be treated in a
grey approximation.Comment: 14 pages, 2 figure
Relativistic Kinetic Equation for Induced Compton Scattering of Polarized Radiation
The relativistic kinetic equations describing time evolution and space
dependence of the density matrices of polarized photons and electrons
interacting via Compton scattering are deduced from the quantum Liouville
equation. The induced scattering and exclusion principle are taken into
account. The Bogoliubov method is used in the frame of quantum electrodynamics.
The equation for polarized radiation scattered by unpolarized electrons is
considered as a particular case and is reformulated in terms of the Stokes
parameters. The expressions for the scattering amplitudes and cross-sections
are derived simultaneously.Comment: 19 pages; Astronomy & Astrophysics, in pres
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