Thin Accretion disks in GR-MHD simulations

TL;DR

This paper reviews recent GR-MHD simulations of thin accretion disks around black holes, focusing on their dynamical evolution, jet formation, magnetic reconnection, and potential observational signatures, without considering thermodynamics or radiation effects.

Contribution

It introduces a thin disk GR-MHD simulation approach that emphasizes dynamical evolution, magnetic flux generation, and jet launching mechanisms, expanding beyond traditional MAD and SANE models.

Findings

Keplerian rotation enables magneto-centrifugal outflows

Magnetic reconnection produces plasmoids ejected from the disk

Simulated data can be used to derive X-ray spectra and variability

Abstract

We review some recent results of general relativistic magnetohydrodynamic (GR-MHD) simulations considering the evolution of geometrically thin disks around a central black hole. Thin disk GR-MHD simulations complement the widely used MAD (Magnetically Arrested Disk) or SANE (Standard And Normal Evolution) approaches of evolving from an initial disk torus. In particular, we discuss the dynamical evolution of the disk, its role in the formation of disk winds or jets, the impact of disk resistivity, and its potential role in generating magnetic flux by an internal disk dynamo. The main characteristics of a thin disk in our approach are the Keplerian rotation of the disk material, which allows to launch disk outflows by the Blandford-Payne magneto-centrifugal effect, in addition to the Blandford-Znajek-driven spine jet from the black hole ergosphere. Thus, for this approach, we neglect disk thermodynamics and radiative effects, concentrating predominantly on the dynamical evolution of the system. Resistive MHD further allows the investigation of physical reconnection and also dynamo action. Magnetic reconnection may generate magnetic islands of plasmoids that are ejected from the disk along with the outflow. We also discussed potential applications of thin disk in explaining the decaying phase of an outburst in black hole X-ray binaries (BH-XRBs). Post-processing of radiation using the simulated dynamical data allows to derive spectra or fluxes, e.g., in the X-ray band, and to derive potential variability characteristics.