13 research outputs found
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.
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
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
Properties of the transient X-ray pulsar Swift J1816.7--1613 and its optical companion
We present results of investigation of the poorly studied X-ray pulsar Swift
J1816.7--1613 during its transition from the type I outburst to the quiescent
state. Our studies are based on the data obtained from X-ray observatories
\textit{Swift}, \textit{NuSTAR} and \textit{Chandra} alongside with the latest
IR data from UKIDSS/GPS and \textit{Spitzer}/GLIMPSE surveys. The aim of the
work is to determine parameters of the system: the strength of the neutron star
magnetic field and the distance to the source, which are required for the
interpretation of the source behaviour in the framework of physically motivated
models. No cyclotron absorption line was detected in the broad-band energy
spectrum. However, the timing analysis hints at the typical for the X-ray
pulsars magnetic field from a few to a few G.
We also estimated type of the IR-companion as a B0-2e star located at distance
of 7--13~kpc.Comment: 8 pages, 7 figures, accepted for publication in A&
Relativistic kinetic equation for Compton scattering of polarized radiation in strong magnetic field
We derive the relativistic kinetic equation for Compton scattering of
polarized radiation in strong magnetic field using the Bogolyubov method. The
induced scattering and the Pauli exclusion principle are taken into account.
The electron polarization is also considered in the general form of the kinetic
equation. The special forms of the equation for the cases of the non-polarized
electrons, the rarefied electron gas and the two polarization mode description
of radiation are found. The derived equations are valid for any photon and
electron energies and the magnetic field strength below about 10^{16} G. These
equations provide the basis for formulation of the equation for polarized
radiation transport in atmospheres and magnetospheres of strongly magnetized
neutron stars.Comment: 23 pages, accepted for publication in Phys. Rev.
Pulsating ULXs: large pulsed fraction excludes strong beaming
The recent discovery of pulsating ultraluminous X-ray sources (ULXs) shows that the apparent luminosity of accreting neutron stars can exceed the Eddington luminosity by a factor of 100s. The relation between the actual and apparent luminosity is a key ingredient in theoretical models of ULXs, but it is still under debate. A typical feature of the discovered pulsating ULXs is a large pulsed fraction (PF). Using Monte Carlo simulations, we consider a simple geometry of accretion flow and test the possibility of simultaneous presence of a large luminosity amplification due the geometrical beaming and a high PF. We argue that these factors largely exclude each other and only a negligible fraction of strongly beamed ULX pulsars can show PF above 10 per cent. Discrepancy between this conclusion and current observations indicates that pulsating ULXs are not strongly beamed and their apparent luminosity is close to the actual one