Quenching of the accretion disk strong aperiodic variability at the magnetospheric boundary

TL;DR

This study analyzes the power density spectra of X-ray flux variability in accreting binary systems, revealing how the magnetospheric boundary influences the observed aperiodic variability and its relation to the neutron star's spin.

Contribution

It provides a detailed explanation of the PDS break at the magnetospheric boundary using the perturbation propagation model, linking variability features to accretion disk and magnetosphere interactions.

Findings

PDS shows a break at the neutron star spin frequency in corotating pulsars.

The break frequency varies with the accretion rate in transient pulsars.

Above the break, the PDS steepens to an f^{-2} law.

Abstract

We study power density spectra (PDS) of X-ray flux variability in binary systems where the accretion flow is truncated by the magnetosphere. PDS of accreting X-ray pulsars where the neutron star is close to the corotation with the accretion disk at the magnetospheric boundary, have a distinct break/cutoff at the neutron star spin frequency. This break can naturally be explained in the "perturbation propagation" model, which assumes that at any given radius in the accretion disk stochastic perturbations are introduced to the flow with frequencies characteristic for this radius. These perturbations are then advected to the region of main energy release leading to a self-similar variability of X-ray flux P~f^{-1...-1.5}. The break in the PDS is then a natural manifestation of the transition from the disk to magnetospheric flow at the frequency characteristic for the accretion disk truncation radius (magnetospheric radius). The proximity of the PDS break frequency to the spin frequency in corotating pulsars strongly suggests that the typical variability time scale in accretion disks is close to the Keplerian one. In transient accreting X-ray pulsars characterized by large variations of the mass accretion rate during outbursts, the PDS break frequency follows the variations of the X-ray flux, reflecting the change of the magnetosphere size with the accretion rate. Above the break frequency the PDS steepens to ~f^{-2} law which holds over a broad frequency range. These results suggest that strong f^{-1...-1.5} aperiodic variability which is ubiquitous in accretion disks is not characteristic for magnetospheric flows.