Magnetized transonic accretion disks

TL;DR

This study uses magnetohydrodynamic simulations to analyze the stability and oscillations of shock fronts in transonic accretion disks around black holes, linking shock dynamics to observable quasi-periodic oscillations.

Contribution

It introduces a simulation framework for magnetized transonic accretion flows, exploring shock stability and oscillations, and connects these to observed QPOs in black hole systems.

Findings

Magnetic fields induce shock oscillations and luminosity variability.

Shock position determines QPO frequency, with closer shocks producing higher frequencies.

Simulations demonstrate the stability of shock solutions under magnetic influences.

Abstract

Theoretical studies of transonic accretion onto black holes reveal a wide range of possible solutions, broadly classified into smooth flows and flows featuring shocks. Accretion solutions that involve the formation of shocks are particularly intriguing, as they are expected to naturally produce observable variability features. However, despite their theoretical significance, time-dependent studies exploring the stability and evolution of such shocked solutions remain relatively scarce. To address this gap, we perform simulations of transonic accretion flows around a black hole in ideal magneto-hydrodynamic framework. Our simulations are initialized using boundary conditions derived from semi-analytical hydrodynamical models, allowing us to explore the stability of these flows under varying magnetic field strengths. The presence of magnetic fields modifies the dynamics of the accretion flow through magnetic pressure, and the resulting force imbalance induces oscillations in the position of shock front. Our results show that variations in the emitted luminosity arising from shock oscillations appear as quasi-periodic oscillations (QPOs), a characteristic feature commonly observed in accreting black holes. We find that the QPO frequency is determined by the radial position of the shock front: oscillations occurring closer to the black hole produce frequencies of tens of hertz, whereas shocks located farther out yield sub-hertz frequencies