X-ray Pulsar Accretion Research Articles

Bright X-ray pulsars: how outflows influence beaming, pulsations and pulse phase lags

ABSTRACT Extreme accretion in X-ray pulsars (XRPs) results in radiation-driven outflows launched from the inner parts of the accretion disc. The outflows affect the apparent luminosity of the XRPs and their pulsations through the geometrical beaming. We model processes of geometrical beaming and pulse formation using Monte Carlo simulations. We confirm our earlier statement that strong amplification of luminosity due to the collimation of X-ray photons is inconsistent with a large pulsed fraction. Accounting for relativistic aberration due to possibly high outflow velocity (∼0.2c) does not affect this conclusion. We demonstrate that the beaming causes phase lags of pulsations. Within the opening angle of the accretion cavity formed by the outflows, phase lags tend to be sensitive to observers viewing angles. Variations in outflow geometry and corresponding changes of the phase lags might influence the detectability of pulsation in bright X-ray pulsars and ULXs. We speculate that the strong geometrical beaming is associated with large radiation pressure on the walls of accretion cavity due to multiple photons reflections. We expect that the mass-loss rate limits geometrical beaming: strong beaming becomes possible only under sufficiently large fractional mass-loss rate from the disc.

Квазисферическая дозвуковая аккреция на рентгеновские пульсары

Квазисферическая дозвуковая аккреция на рентгеновские пульсары

Signs of magnetic accretion in X-ray pulsars

The spin-down mechanism of accreting neutron stars is discussedwith an application to one of the best studied X-ray pulsars GX301-2. We show that the maximum possible spin-down torque applied to a neutron star from the accretion flow can be evaluated as K (t)sd = µ2/(r m r cor)3/2. The spin-down rate of the neutron star in GX301-2 can be explained provided the magnetospheric radius of the neutron star is smaller than its canonical value. We calculate the magnetospheric radius considering the mass-transfer in the binary system in the frame of the magnetic accretion scenario suggested by V.F. Shvartsman. The spin-down rate of the neutron star expected within this approach is in a good agreement with that derived from observations of GX301-2.

Theory of quasi-spherical accretion in X-ray pulsars

A theoretical model for quasi-spherical subsonic accretion onto slowly rotating magnetized neutron stars is constructed. In this model the accreting matter subsonically settles down onto the rotating magnetosphere forming an extended quasi-static shell. This shell mediates the angular momentum removal from the rotating neutron star magnetosphere during spin-down episodes by large-scale convective motions. The accretion rate through the shell is determined by the ability of the plasma to enter the magnetosphere. The settling regime of accretion can be realized for moderate accretion rates $\dot M< \dot M_*\simeq 4\times 10^{16}$ g/s. At higher accretion rates a free-fall gap above the neutron star magnetosphere appears due to rapid Compton cooling, and accretion becomes highly non-stationary. From observations of the spin-up/spin-down rates (the angular rotation frequency derivative $\dot \omega^*$, and $\partial\dot\omega^*/\partial\dot M$ near the torque reversal) of X-ray pulsars with known orbital periods, it is possible to determine the main dimensionless parameters of the model, as well as to estimate the magnetic field of the neutron star. We illustrate the model by determining these parameters for three wind-fed X-ray pulsars GX 301-2, Vela X-1, and GX 1+4. The model explains both the spin-up/spin-down of the pulsar frequency on large time-scales and the irregular short-term frequency fluctuations, which can correlate or anti-correlate with the X-ray flux fluctuations in different systems. It is shown that in real pulsars an almost iso-angular-momentum rotation law with $\omega \sim 1/R^2$, due to strongly anisotropic radial turbulent motions sustained by large-scale convection, is preferred.

Two Modes of Disc Accretion in X-ray Pulsars?

In this paper I compare the properties of the Be/X-ray transients to those of disc accreting X-ray pulsars that show torque reversals without significant changes in the X-ray fluxes. I find that it is unlikely that the cause of the torque reversal is a radiation-induced warp in the accretion disc. The preferred explanations of the phenomenon is that either there is an intrinsic difference in the properties of the neutron stars between the two groups of objects, or that the torque reversals appear in systems in which the accretion disc is formed from a stellar wind.