Magnetized transonic accretion disks

TL;DR

This work investigates magnetized transonic accretion onto a non-rotating black hole by performing ideal MHD simulations with boundary conditions sourced from semi-analytical hydrodynamic shocks. Magnetic fields disrupt and oscillate the shock front, producing luminosity variability that naturally yields quasi-periodic oscillations (QPOs) whose frequencies scale with the shock radius: inner shocks yield tens of Hz while outer shocks yield sub-Hz values. In 3D, intrinsic non-axisymmetric structures emerge, compatible with SASI-like behavior, and the inferred effective viscosity from Maxwell and Reynolds stresses is modest (\alpha_{ss} ~ 10^{-2}). The results link magnetic-field strength to shock stability and QPO phenomenology, providing a framework to interpret BHXRB/AGN variability and outlining directions for future GRMHD and radiative-cooling extensions to connect to spectra and timing.

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

Figures (11)

• Figure S1: Top row: Density snapshots of the non-magnetized run M0. Arrows indicate the fluid velocity. Bottom row: Pressure snapshots with white contours indicating shock and sonic surfaces. Units of space and time are in terms of $r_g$ and $t_g$ correspondingly.
• Figure S2: Density contours and distribution of plasma $\beta$ for model $\texttt{M5}$.
• Figure S3: Temporal variation of the shock front position for different simulation runs, as indicated in the labels.
• Figure S4: Same as Fig. \ref{['fig:beta1e5']} but for model M4.
• Figure S5: Same as Fig. \ref{['fig:beta1e5']} but for model M3.