Insight-HXMT observations of jet-like corona in a black hole X-ray
  binary MAXI J1820+070
  A black hole X-ray binary produces hard X-ray radiation from its corona and
disk when the accreting matter heats up. During an outburst, the disk and
corona co-evolves with each other. However, such an evolution is still unclear
in both its geometry and dynamics. Here we report the unusual decrease of the
reflection fraction in MAXI J1820+070, which is the ratio of the coronal
intensity illuminating the disk to the coronal intensity reaching the observer,
as the corona is observed to contrast during the decay phase. We postulate a
jet-like corona model, in which the corona can be understood as a standing
shock where the material flowing through. In this dynamical scenario, the
decrease of the reflection fraction is a signature of the corona's bulk
velocity. Our findings suggest that as the corona is observed to get closer to
the black hole, the coronal material might be outflowing faster.
