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

On 2019 November 2, 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 cannot be fit by conventional two-component (disc plus corona) approaches that ignore disc emission sourced from within the plunging region of the black hole's space-time. Instead, these models require a third 'additional' thermal component to reproduce the data. Using new disc solutions that 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 $a_\bullet \lt 0.86$ at 99.9 ${{\ \rm per\ cent}}$ confidence. We finish by discussing the observational requirements for the robust detection of this region.