Properties of GRMHD accretion flows around black holes in steady-state limit (Samik Mitra,18/7/2022)
Channel: Academy of Athens
Date: 18/07/2022

Samik Mitra
Prime Minister's Research Scholar, Department of Physics, IIT Guwahati, India

Abstract:

Magnetic fields play a pivotal role in driving the accretion flows around black holes (BHs). However, the underlying disk dynamics and their correlation with the magnetic field remain inconclusive. Hence, the study of magneto-hydrodynamics (MHD) in a general relativistic (GR) framework is inevitable to explain the behaviors of all the fluid and magneto-fluid variables around a BH. Therefore, we present a novel approach to study the global structure of axisymmetric GRMHD accretion flows in the steady-state. Here, we adapt the ideal MHD condition, and relativistic equation of state (REoS) to solve the governing equations and obtain all possible transonic global accretion solutions for the first time. Interestingly, the inner critical point solutions seem magnetically more preferable than flows containing the outer critical points. Further, we find that the toroidal magnetic field (b^\phi) dominates over the radial (b^r) component in the equatorial plane, suggesting that the toroidal magnetic fields play a decisive role in governing the disk dynamics. We additionally infer that the disk remains mostly gas-dominated over the mid-plane, however as the accreting matter reaches close to the horizon, magnetic fields become more active as plasma-β ∼ 1. Further, the presence of Maxwell's stress plays an important role in transporting the angular momentum of the flow. Finally, we compute the best fit for the flow variables, namely, density (ρ) ∝ r ^−(n+1/2) , p_gas ∝ r ^−(n+7/6) , p_mag ∝ r ^−(n+5/2) , which closely matches with the self-similar solutions for n ∼ 1, though slightly steeper. Overall, we emphasize that the present formalism can act as a potential source to provide steady-state seed solutions to realistic GRMHD simulations.

Talk presented at the Research Center for Astronomy and Applied Mathematics of the Academy of Athens