  The Galactic black hole X-ray binary MAXI J1820+070 had a bright outburst in
2018 when it became the second brightest X-ray source in the Sky. It was too
bright for X-ray CCD instruments such as XMM-Newton and Chandra, but was well
observed by photon-counting instruments such as NICER and NuSTAR. We report
here on the discovery of an excess emission component during the soft state. It
is best modelled with a blackbody spectrum in addition to the regular disk
emission, modelled either as diskbb or kerrbb. Its temperature varies from
about 0.9 to 1.1 keV which is about 30 to 80 per cent higher than the inner
disc temperature of diskbb. Its flux varies between 4 and 12 percent of the
disc flux. Simulations of magnetised accretion discs have predicted the
possibility of excess emission associated with a non-zero torque at the
Innermost Stable Circular Orbit (ISCO) about the black hole, which from other
NuSTAR studies lies at about 5 gravitational radii or about 60 km (for a black
hole mass is 8 M). In this case the emitting region at the ISCO has a width
varying between 1.3 and 4.6 km and would encompass the start of the plunge
region where matter begins to fall freely into the black hole.
