Magnetized SASI: its mechanism and possible connection to some QPOs in XRBs

TL;DR

This paper investigates the mechanisms behind standing accretion shock instability (SASI) in accretion flows onto compact objects, linking it to observed quasi-periodic oscillations (QPOs) in X-ray binaries, and explores the influence of magnetic fields on these phenomena.

Contribution

It demonstrates that the advective-acoustic mechanism explains SASI better than the purely acoustic one and connects shock oscillations to high-frequency QPOs, including the role of magnetic fields in low-frequency modulations.

Findings

Advective-acoustic mechanism aligns with observed oscillation timescales.

Shock oscillations can explain high-frequency QPOs in XRBs.

Magnetic fields introduce low-frequency modulations related to hHz QPOs.

Abstract

The presence of a surface at the inner boundary, such as in a neutron star or a white dwarf, allows the existence of a standing shock in steady spherical accretion.The standing shock can become unstable in 2D or 3D; this is called the {\em standing accretion shock instability} (SASI).Two mechanisms -- advective-acoustic and purely acoustic -- have been proposed to explain SASI. Using axisymmetric hydrodynamic (HD) and magnetohydrodynamic (MHD) simulations, we find that the advective-acoustic mechanism better matches the observed oscillation timescales in our simulations. The global shock oscillations present in the accretion flow can explain many observed high frequency ($\gtrsim 100$ Hz) quasi-periodic oscillations (QPOs) in X-ray binaries (XRBs). The presence of a moderately strong magnetic field adds more features to the shock oscillation pattern, giving rise to low frequency modulation in the computed light curve. This low frequency modulation can be responsible for $\sim 100$ Hz QPOs (known as hHz QPOs). We propose that the appearance of hHz QPO determines the separation of twin peak QPOs of higher frequencies.