3D MHD Simulations of accreting neutron stars: evidence of QPO emission from the surface

TL;DR

This study uses 3D MHD simulations to demonstrate that moving hot spots on accreting neutron stars can produce observable high-frequency QPOs, with frequencies linked to accretion rates and sometimes appearing as double QPOs.

Contribution

The paper provides new 3D MHD simulation results showing the formation of moving hot spots and their connection to QPO emission, including the occurrence of double QPOs.

Findings

Hot spots move during accretion, producing high-frequency QPOs.

QPO frequencies correlate with mass accretion rate.

Double QPOs can occur with similar correlations.

Abstract

3D Magnetohydrodynamic simulations show that when matter accretes onto neutron stars, in particular if the misalignment angle is small, it does not constantly fall at a fixed spot. Instead, the location at which matter reaches the star moves. These moving hot spots can be produced both during stable accretion, where matter falls near the magnetic poles of the star, and unstable accretion, characterized by the presence of several tongues of matter which fall on the star near the equator, due to Rayleigh-Taylor instabilities. Precise modeling with Monte Carlo simulations shows that those movements could be observed as high frequency Quasi Periodic Oscillations. We performed a number of new simulation runs with a much wider set of parameters, focusing on neutron stars with a small misalignment angle. In most cases we observe oscillations whose frequency is correlated with the mass accretion rate $\dot{M}$. Moreover, in some cases double QPOs appear, each of them showing the same correlation with $\dot{M}$.