Аннотация
We investigate aperiodic X-ray flux variability in accreting highly
magnetized neutron stars - X-ray pulsars (XRPs). The X-ray variability is
largely determined by mass accretion rate fluctuations at the NS surface, which
replicate accretion rate fluctuations at the inner radius of the accretion
disc. The variability at the inner radius is due to fluctuations arising all
over the disc and propagating inwards under the influence of viscous diffusion.
The inner radius varies with mean mass accretion rate and can be estimated from
the known magnetic field strength and accretion luminosity of XRPs.
Observations of transient XRPs covering several orders of magnitude in
luminosity give a unique opportunity to study effects arising due to the
changes of the inner disc radius. We investigate the process of viscous
diffusion in XRP accretion discs and construct new analytical solutions of the
diffusion equation applicable for thin accretion discs truncated both from
inside and outside. Our solutions are the most general ones derived in the
approximation of Newtonian mechanics. We argue that the break observed at high
frequencies in the power density spectra of XRPs corresponds to the minimal
time scale of the dynamo process, which is responsible for the initial
fluctuations. Comparing data from the bright X-ray transient A 0535+26 with our
model, we conclude that the time scale of initial variability in the accretion
disc is a few times longer than the local Keplerian time scale.
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