Effects of resistivity on standing shocks in low angular momentum flows around black holes

TL;DR

This study investigates how resistivity influences shock behavior and luminosity variations in low angular momentum accretion flows around black holes, using resistive MHD simulations relevant to Sgr A* observations.

Contribution

It demonstrates the impact of different resistivity levels on shock dynamics, luminosity oscillations, and flow steadiness in black hole accretion models, linking to observed flares.

Findings

Low resistivity causes quasi-periodic luminosity oscillations.

High resistivity suppresses magnetic activity and stabilizes the flow.

Steady shocks form more outward with higher resistivity.

Abstract

We study two dimensional low angular momentum flow around the black hole using the resistive magnetohydrodynamic module of PLUTO code. Simulations have been performed for the flows with parameters of specific angular momentum, specific energy, and magnetic field which may be expected for the flow around Sgr A*. For flows with lower resistivity $\eta=10^{-6}$ and $0.01$, the luminosity and the shock location on the equator vary quasi-periodically. The power density spectra of luminosity variation show the peak frequencies which correspond to the periods of $5 \times 10^5$, $1.4 \times 10^5$, and $5 \times 10^4$ seconds, respectively. These quasi-periodic oscillations (QPOs) occur due to the interaction between the outer oscillating standing shock and the inner weak shocks occurring at the innermost hot blob. While for cases with higher resistivity $\eta=0.1$ and 1.0, the high resistivity considerably suppresses the magnetic activity such as the MHD turbulence and the flows tend to be steady and symmetric to the equator. The steady standing shock is formed more outward compared with the hydrodynamical flow. The low angular momentum flow model with the above flow parameters and with low resistivity has a possibility for the explanation of the long-term flares with $\sim$ one per day and $\sim 5 - 10$ days of Sgr A* in the latest observations by Chandra, Swift, and XMM-Newton monitoring of Sgr A*.