Possible fractal nature of accretion flows in MAD and SANE simulations: Implications to GRS 1915+105

TL;DR

This study applies nonlinear timeseries analysis to GRMHD simulations of black hole accretion flows, revealing fractal and dynamic differences between MAD and SANE states, with implications for understanding GRS 1915+105.

Contribution

It introduces the use of Higuchi fractal dimension, Hurst Index, and spectral slope to characterize accretion disk and jet dynamics in MAD and SANE simulations.

Findings

MADs show higher fractal dimension and lower Hurst Index than SANEs.

Fractal dimension in MAD decreases with black hole spin, indicating more ordered jets.

Spectral properties of GRS 1915+105 suggest it is more MAD-like, consistent with simulation results.

Abstract

The general relativistic magnetohydrodynamic (GRMHD) simulations are widely used to study accretion disk and jet dynamics around a black hole. Despite strong observational evidences for intrinsically nonlinear behavior, the interpretations of GRMHD simulation results, more precisely the underlying timeseries, have not been well-explored by nonlinear timeseries analysis. In this work, we characterize the jet and disk dynamics of different GRMHD simulated flows using the nonlinear timeseries analysis. As diagnostic tools, we consider Higuchi fractal dimension (HFD), Hurst Index (H) and spectral slope. We implement them for two model disk frameworks: magnetically arrested disk (MAD) and standard and normal evolution (SANE), across a range of black hole spins with the Kerr parameter spanning from -0.9375 to 0.9375. We simulate the disk/jet systems by two well-documented codes: HARMPI and BHAC, and obtain, respectively, low and high temporally resolved timeseries data. For both jet and disk dynamics, MADs are characterized by higher HFD, lower H and flatter spectral slopes than SANEs. High HFD in MAD could be due to its intermittent variability and indicates that it has lesser long-range temporal correlations than SANE. Moreover, HFD in MAD decreases with spin magnitude owing to increase in collimated, hence ordered, jets. However, in SANE, it increases with spin for positive ones due to interplay of winds and jets. Extending our analysis to observations, we attempt to segregate the classes of black hole: GRS 1915+105, into MAD- and SANE-like clusters based on their spectral properties extracted from X-ray data. The mean HFD of MAD-like cluster is higher than SANE-like cluster, thus, corroborating with the simulation results. Our work highlights the role of nonlinear timeseries analysis to understand the underlying dynamics of accretion flows and their connection to magnetic regulation.