Broadband variability

Long-term X-ray observations have shown that black-hole X-ray binaries exhibit extreme, aperiodic variability on time scales of a few milliseconds to seconds. The observed light-curve displays various characteristic features which indicate that the underlying variability process is stochastic in nature and is thought to be intrinsic to the accretion process. A general idea is that this variability is driven by the inward propagating fluctuations of mass accretion rate on viscous timescales, although much remains puzzling about the physical process driving the fluctuations. To confirm this, we performed a GRMHD simulation of black hole accretion low with simultaneous analysis of other GRMHD simulations from the literature.  With long-duration simulations (over 30s) assembled from numerical codes, this work is likely the most comprehensive investigation of variability in GRMHD simulations to date (Bollimpalli et. al 2021). The characteristics of the generated synthetic light curves are in broad agreement with X-ray observations. We found that the accretion rate from simulations does show evidence for inward propagating fluctuations by featuring a) Strong radial coherence below the viscous frequency, b) Positive time lags of variability at smaller radii compared to larger radii, c) Linear rms-flux relation derived from the power spectra and d) Log-normal distribution. Now, unlike the observations, we find the time lags are not frequency-dependent, and that the power spectra are dominated by the high-frequency variability above the radial epicyclic frequency. These are likely the results of non-dissipative fluctuations (such as p-modes), and suggest that the accretion rate, after all, may not be a good proxy for luminosity for these studies.