The Moving Hotspots model for kHz QPOs in accreting neutron stars

TL;DR

This paper introduces a model for kHz QPOs in accreting neutron stars based on the movement of hotspots caused by different accretion processes, explaining observed QPO features.

Contribution

It proposes a novel moving hotspots model that links accretion flow instabilities and funnel flows to specific kHz QPOs in neutron stars.

Findings

Hotspot motion explains QPO signatures in power spectra.

Model reproduces properties of observed kHz QPOs.

Different hotspot types correspond to lower and upper kHz QPOs.

Abstract

3D MHD simulation of accretion onto neutron stars have shown in the last few years that the footprint (hotspot) of the accretion flow changes with time. Two different kinds of accretion, namely the funnel flow and the equatorial accretion produced by instabilities at the inner disk, produce different kinds of motion of the hotspot. The funnel flow produces hotspots that move around the magnetic pole, while instabilities produce other hotspots that appear randomly and move along the equator or slightly above. The angular velocities of the two hotspots are different, the equatorial one being higher and both close to the Keplerian velocity in the inner region. Modeling of the lightcurves of these hotspots with Monte Carlo simulations show that the signatures produced in power specra by them, if observed, are QPOs plus low frequency components. Their frequencies, general behavior and features describe correctly most of the properties of kHz QPOs, if we assume the funnel flow hotspots as the origin of the lower kHz QPO and instabilities as the origin of the upper kHz QPO.