Author(s): E. Kara, Dept. of Astronomy, University of Maryland, College Park, Maryland, UNITED STATES|E. Kara, NASA Goddard Space Flight Center, Greenbelt, Maryland, UNITED STATES|
Institution(s): 1. Dept. of Astronomy, University of Maryland, College Park, MD, United States. 2. NASA Goddard Space Flight Center, Greenbelt, MD, United States.
Contributing team(s): NICER Observatory Science Working Group
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 it has a very hard X-ray spectrum produced by a hot corona, and then transitions to a soft spectrum dominated by emission from a geometrically thin accretion disc extending to the innermost stable circular orbit. Much debate, however, persists over how this transition occurs, whether it is driven largely by a reduction in the truncation radius of the disc or 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 disc extends in close 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 disc in the hard state of stellar mass black holes is truncated out to hundreds of gravitational radii. Here we report X-ray observations of the new black hole transient MAXI J1820+070. We find that the reverberation time lags between the continuum-emitting corona and the irradiated accretion disc are 6–20 times shorter than previously seen. The timescale of the reverberation lags shortens by an order of magnitude over a period of weeks, while 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 disc.