QPO emission from moving hot spots on the surface of neutron stars: a model

TL;DR

This paper models how moving hot spots on neutron stars, caused by accretion instabilities, can produce high-frequency QPOs observed in lightcurves, using 3D simulations and Monte Carlo methods.

Contribution

It introduces a new model linking hot spot movements due to instabilities with observed QPO features, expanding beyond fixed hot spot assumptions.

Findings

Hot spot motion can generate high-frequency QPOs.

Rayleigh-Taylor instabilities cause hot spot wandering.

Model reproduces key features of observed kHz QPOs.

Abstract

We present recent results of 3D magnetohydrodynamic simulations of neutron stars with small misalignment angles, as regards the features in lightcurves produced by regular movements of the hot spots during accretion onto the star. In particular, we show that the variation of position of the hot spot created by the infalling matter, as observed in 3D simulations, can produce high frequency Quasi Periodic Oscillations with frequencies associated with the inner zone of the disk. Previously reported simulations showed that the usual assumption of a fixed hot spot near the polar region is valid only for misalignment angles relatively large. Otherwise, two phenomena challenge the assumption: one is the presence of Rayleigh-Taylor instabilities at the disk-magnetospheric boundary, which produce tongues of accreting matter that can reach the star almost anywhere between the equator and the polar region; the other one is the motion of the hot spot around the magnetic pole during stable accretion. In this paper we start by showing that both phenomena are capable of producing short-term oscillations in the lightcurves. We then use Monte Carlo techniques to produce model lightcurves based on the features of the movements observed, and we show that the main features of kHz QPOs can be reproduced. Finally, we show the behavior of the frequencies of the moving spots as the mass accretion rate changes, and propose a mechanism for the production of double QPO peaks.