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 $B$-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 $10^{40}\,{\rm erg\,s^{-1}}$ 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 $\gtrsim 10^{19}\,{\rm g\,s^{-1}}$ 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

Coupling of radiation and magnetospheric accretion flow in ULX pulsars: radiation pressure and photon escape time

The accretion flow within the magnetospheric radius of bright X-ray pulsars can form an optically thick envelope, concealing the central neutron star from the distant observer. Most photons are emitted at the surface of a neutron star and leave the system after multiple reflections by the accretion material covering the magnetosphere. Reflections cause momentum to be transferred between photons and the accretion flow, which contributes to the radiative force and should thus influence the dynamics of accretion. We employ Monte Carlo simulations and estimate the acceleration along magnetic field lines due to the radiative force as well as the radiation pressure across magnetic field lines. We demonstrate that the radiative acceleration can exceed gravitational acceleration along the field lines, and similarly, radiation pressure can exceed magnetic field pressure. Multiple reflections of X-ray photons back into the envelope tend to amplify both radiative force along the field lines and radiative pressure. We analyze the average photon escape time from the magnetosphere of a star and show that its absolute value is weakly dependent on the magnetic field strength of a star and roughly linearly dependent on the mass accretion rate being $\sim 0.1\,{\rm s}$ at $\dot{M}\sim 10^{20}\,{\rm g\,s^{-1}}$. At high mass accretion rates, the escape time can be longer than free-fall time from the inner disc radius.Comment: accepted for publication MNRAS, 9 pages, 6 figure

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 ($\gtrsim 10^{19}\,{\rm g\,s^{-1}}$), 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