Constraints on the magnetic field structure in accreting compact objects from aperiodic variability

TL;DR

This study links the aperiodic variability in accreting neutron stars and intermediate polars to their magnetic field structures, revealing that complex magnetic fields with multipole components influence accretion dynamics and observed properties.

Contribution

It demonstrates that observed break frequencies can be used to estimate magnetic field structures, including multipole components, in accreting compact objects, providing new insights into their magnetospheres.

Findings

Break frequencies align with estimated inner disc radii.

Magnetic field strength from dipole assumption is lower than cyclotron estimates.

Multipole magnetic fields explain discrepancies and high critical luminosity.

Abstract

We investigate the aperiodic variability for a relatively large sample of accreting neutron stars and intermediate polars, focusing on the properties of the characteristic break commonly observed in power spectra of accreting objects. In particular, we investigate the relation of the break frequency and the magnetic field strength, both of which are connected to the size of the magnetosphere. We find that for the majority of objects in our sample the measured break frequency values indeed agree with estimated inner radii of the accretion disc, which allows to use observed break frequencies to independently assess the magnetic field strength and structure in accreting compact objects. As a special case, we focus on Hercules X-1 which is a persistent, medium-luminosity X-ray pulsar accreting from its low-mass companion. In the literature, it has been suggested that the complex pulse profiles, the spin-up behaviour and the luminosity-correlation of the cyclotron energy seen in Her X-1 can be explained with a complex magnetic field structure of the neutron star. Here, we connect the measured break frequency to the magnetospheric radius and show that the magnetic field strength derived assuming a dipole configuration is nearly an order of magnitude smaller than the magnetic field strength corresponding to the cyclotron energy. Accordingly, this discrepancy can be explained with the magnetic field having strong multipole components. The multipolar structure would also increase the accreting area on the neutron star surface, explaining why the critical luminosity for accretion column formation is puzzlingly high in this source.