153 research outputs found
Propagating mass accretion rate fluctuations in X-ray binaries under the influence of viscous diffusion
Many statistical properties of X-ray aperiodic variability from accreting
compact objects can be explained by the propagating fluctuations model applied
to the accretion disc. The mass accretion rate fluctuations originate from
variability of viscosity, which arises at every radius and causes local
fluctuations of the density. The fluctuations diffuse through the disc and
result in local variability of the mass accretion rate, which modulates the
X-ray flux from the inner disc in the case of black holes, or from the surface
in the case of neutron stars. A key role in the theoretical explanation of fast
variability belongs to the description of the diffusion process. The
propagation and evolution of the fluctuations is described by the diffusion
equation, which can be solved by the method of Green functions. We implement
Green functions in order to accurately describe the propagation of fluctuations
in the disc. For the first time we consider both forward and backward
propagation. We show that (i) viscous diffusion efficiently suppress
variability at time scales shorter than the viscous time, (ii) local
fluctuations of viscosity affect the mass accretion rate variability both in
the inner and the outer parts of accretion disc, (iii) propagating fluctuations
give rise not only to hard time lags as previously shown, but also produce soft
lags at high frequency similar to those routinely attributed to reprocessing,
(iv) deviation from the linear rms-flux relation is predicted for the case of
very large initial perturbations. Our model naturally predicts bumpy power
spectra.Comment: 20 pages, 17 figures, accepted for publication in MNRA
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
Optically thick envelopes around ULXs powered by accreating neutron stars
Magnetized neutron stars power at least some ultra-luminous X-ray sources.
The accretion flow in these cases is interrupted at the magnetospheric radius
and then reaches the surface of a neutron star following magnetic field lines.
Accreting matter moving along magnetic field lines forms the accretion envelope
around the central object. We show that, in case of high mass accretion rates
the envelope becomes closed and optically
thick, which influences the dynamics of the accretion flow and the
observational manifestation of the neutron star hidden behind the envelope.
Particularly, the optically thick accretion envelope results in a multi-color
black-body spectrum originating from the magnetospheric surface. The spectrum
and photon energy flux vary with the viewing angle, which gives rise to
pulsations characterized by high pulsed fraction and typically smooth pulse
profiles. The reprocessing of radiation due to interaction with the envelope
leads to the disappearance of cyclotron scattering features from the spectrum.
We speculate that the super-orbital variability of ultra-luminous X-ray sources
powered by accreting neutron stars can be attributed to precession of the
neutron star due to interaction of magnetic dipole with the accretion disc.Comment: 8 pages, 6 figures, accepted for publication in MNRA
Stable accretion from a cold disc in highly magnetized neutron stars
The aim of this paper is to investigate the transition of a strongly
magnetized neutron star into the accretion regime with very low accretion rate.
For this purpose we monitored the Be-transient X-ray pulsar GRO J1008-57
throughout a full orbital cycle. The current observational campaign was
performed with the Swift/XRT telescope in the soft X-ray band (0.5-10 keV)
between two subsequent Type I outbursts in January and September 2016. The
expected transition to the propeller regime was not observed. However, the
transitions between different regimes of accretion were detected. In
particular, after an outburst the source entered a stable accretion state
characterised by the accretion rate of ~10^14-10^15 g/s. We associate this
state with accretion from a cold (low-ionised) disc of temperature below ~6500
K. We argue that a transition to such accretion regime should be observed in
all X-ray pulsars with certain combination of the rotation frequency and
magnetic field strength. The proposed model of accretion from a cold disc is
able to explain several puzzling observational properties of X-ray pulsars.Comment: 8 pages, 3 figures, 1 table, accepted by A&
Cyclotron emission, absorption, and the two faces of X-ray pulsar A 0535+262
Deep NuSTAR observation of X-ray pulsar A 0535+262, performed at a very low
luminosity of erg s, revealed the presence of two
spectral components. We argue that the high-energy component is associated with
cyclotron emission from recombination of electrons collisionally excited to the
upper Landau levels. The cyclotron line energy of keV
was measured at the luminosity of almost an order of magnitude lower than what
was achieved before. The data firmly exclude a positive correlation of the
cyclotron energy with the mass accretion rate in this source.Comment: 5 pages, 3 figures, accepted by MNRAS Letter
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