Low-angular-momentum Black Hole Accretion: First General Relativistic Magnetohydrodynamic Evidence of Standing Shocks

Abstract Understanding the dynamics of low-angular-momentum accretion flow around black holes (BHs) is essential for probing extreme plasma behavior in strong gravity, where shock formation can naturally produce variability signatures. In this paper, we perform general relativistic magnetohydrodynamic (GRMHD) simulations of low angular momentum accretion flows onto a BH with different BH spins to investigate the accretion dynamics near the central BH region. The simulation results show the standard and normal evolution regime in all cases. In particular, we report the formation and persistence of standing shocks in low-angular-momentum accretion flows using multidimensional (2D and 3D) GRMHD simulations for the first time. Previous studies did not detect such stable standing shock structures, making our findings a significant advancement in this field. The finding of shock dynamics can be further associated with some radiation features, such as flares observed in Sgr A * and quasiperiodic oscillation signals detected in some X-ray binaries and active galactic nuclei.