MAXI J1820+070 X-ray spectral-timing reveals the nature of the accretion flow in black hole binaries

Tenyo Kawamura, Chris Done, Magnus Axelsson, Tadayuki Takahashi

Published 2022-09-29Version 1

Black hole X-ray binaries in the low/hard states display significant broad-band (stochastic) variability on short time-scales (0.01-100 seconds), with a complex pattern of lags in correlated variability seen in different energy bands. This behaviour is generally interpreted in a model where slow fluctuations stirred up at large radii propagate down through the accretion flow, modulating faster fluctuations stirred up at smaller radii. Coupling this scenario with a radially-stratified emission property opens the way to measure the propagation time-scale from data and hence directly test models of the accretion flow structure. Our previous spectral-timing model could fit the NICER (0.5-10 keV) data from the brightest recent black hole transient, MAXI J1820+070. Here we use new data from Insight-HXMT to explore the variability up to higher energies. We have to extend the model so that the spectrum emitted at each radius changes shape in response to a fluctuation (pivoting) rather than just changing normalisation. This extension gives the strong suppression of fractional variability as a function of energy seen in the data. We find that the derived propagation time-scale is slower than predicted by models with maximum magnetic flux on the horizon (MAD flows), despite this system showing a strong jet. Our model jointly fits the spectrum and broad-band variability up to 50 keV, so the QPO can most easily be explained as an extrinsic modulation of the flow, such as produced in Lense-Thirring precession rather than arising in an additional spectral-timing component such as the jet.