  The geometry of the accretion flow around stellar-mass black holes can change
on timescales of days to months. When a black hole emerges from quiescence
(that is, it "turns on" after accreting material from its companion) it has a
very hard (high-energy) X-ray spectrum produced by a hot corona positioned
above its accretion disk, and then transitions to a soft (lower-energy)
spectrum dominated by emission from the geometrically thin accretion disk,
which extends to the innermost stable circular orbit. Much debate persists over
how this transition occurs and whether it is driven largely by a reduction in
the truncation radius of the disk or by a reduction in the spatial extent of
the corona. Observations of X-ray reverberation lags in supermassive black-hole
systems suggest that the corona is compact and that the disk extends nearly to
the central black hole. Observations of stellar-mass black holes, however,
reveal equivalent (mass-scaled) reverberation lags that are much larger,
leading to the suggestion that the accretion disk in the hard X-ray state of
stellar-mass black holes is truncated at a few hundreds of gravitational radii
from the black hole. Here we report X-ray observations of the black-hole
transient MAXI J1820+070. We find that the reverberation time lags between the
continuum-emitting corona and the irradiated accretion disk are 6 to 20 times
shorter than previously seen. The timescale of the reverberation lags shortens
by an order of magnitude over a period of weeks, whereas the shape of the
broadened iron K emission line remains remarkably constant. This suggests a
reduction in the spatial extent of the corona, rather than a change in the
inner edge of the accretion disk.
