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The Galactic black hole X-ray binary MAXI J1820+070 had a bright outburst in2018 when it became the second brightest X-ray source in the Sky. It was toobright for X-ray CCD instruments such as XMM-Newton and Chandra, but was wellobserved by photon-counting instruments such as NICER and NuSTAR. We reporthere on the discovery of an excess emission component during the soft state. Itis best modelled with a blackbody spectrum in addition to the regular diskemission, modelled either as diskbb or kerrbb. Its temperature varies fromabout 0.9 to 1.1 keV which is about 30 to 80 per cent higher than the innerdisc temperature of diskbb. Its flux varies between 4 and 12 percent of thedisc flux. Simulations of magnetised accretion discs have predicted thepossibility of excess emission associated with a non-zero torque at theInnermost Stable Circular Orbit (ISCO) about the black hole, which from otherNuSTAR studies lies at about 5 gravitational radii or about 60 km (for a blackhole mass is 8 M). In this case the emitting region at the ISCO has a widthvarying between 1.3 and 4.6 km and would encompass the start of the plungeregion where matter begins to fall freely into the black hole.

The Soft State of the Black Hole Transient Source MAXI J1820+070: Emission from the Edge of the Plunge Region?