Kavli Affiliate: Tadayuki Takahashi
| First 5 Authors: Tenyo Kawamura, Chris Done, Magnus Axelsson, Tadayuki Takahashi,
| Summary:
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.
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