Plunging region emission in the X-ray binary MAXI J0637$-$430

On the second of November 2019 the black hole X-ray binary MAXI J0637$-$430 went into outburst, at the start of which it was observed in a thermal disc-dominated'' state. High photon energy (extending above 10 keV) observations taken by the NuSTAR telescope reveal that this thermal spectrum can not be fit by conventional two-component (disc plus corona) approaches which ignore disc emission sourced from within the plunging region of the black hole's spacetime. Instead, these models require a third additional’’ thermal component to reproduce the data. Using new disc solutions which extend classical models into the plunging region we show that this ``additional’’ thermal emission can be explained self-consistently with photons emitted from the accretion flow at radii within the innermost stable circular orbit of the black hole. This represents the second low mass X-ray binary, after MAXI J1820+070, with a detection of plunging region emission, suggesting that signatures of this highly relativistic region may well be widespread but not previously widely appreciated. To allow for a detection of the plunging region, the black hole in MAXI J0637$-$430 must be at most moderately spinning, and we constrain the spin to be $a_{\bullet }<0.86$ at 99.9$\mathrm{\%}$ confidence. We finish by discussing the observational requirements for the robust detection of this region.