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

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 almost corotating 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 by the “perturbation propagation” model, which assumes that at any given radius in the accretion disk stochastic perturbations are introduced in flow with frequencies that are characteristic of this radius. These perturbations are then advected to the region of main energy release leading to a self-similar variability in the X-ray flux Pproptof -1...-1.5. The break in the PDS is then a natural manifestation of the transition from the disk to the magnetospheric flow at the frequency characteristic of 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 timescale in accretion disks is close to the Keplerian one. In transient accreting X-ray pulsars characterized by large variations in the mass accretion rate during outbursts, the PDS break frequency follows the variations in the X-ray flux, reflecting the change in the magnetosphere size with the accretion rate. Above the break frequency, the PDS steepens to close 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 of magnetospheric flows.