2025MNRAS.544.2880M

Emission from within the innermost stable circular orbit (ISCO) of black hole accretion discs has recently been detected in two X-ray binary systems, while a possible discrepancy exists between the inferred spins of gravitational wave and electromagnetically detected black holes. Motivated by these two results we demonstrate, using theoretical calculations and observational data, that the inclusion of intra-ISCO emission results in a black hole with a low spin producing a thermal X-ray spectrum that mimics that produced by a much more rapidly rotating black hole with no emission from within the ISCO. We show this explicitly for the high-mass X-ray binary system M33 X-7, which requires a high spin $a_\bullet = 0.84\pm 0.05$ assuming zero intra-ISCO emission. However, a disc around a Schwarzschild black hole can equally well describe the data, provided that photons emitted from within the ISCO are included and the ISCO stress is in line with results seen in simulations. We then present an analysis of two further soft-state X-ray binaries (MAXI J1820+070 and MAXI J0637-430) that require the presence of intra-ISCO emission at high statistical significance. These two sources sit on the low-spin moderate-stress part of the degeneracy exhibited by M33 X-7, suggesting that when high-quality data are available the high-spin low-stress region of parameter space is ruled out. We discuss how future advances in numerical simulations and data modelling will be essential to determining the spin of X-ray binary black holes, which may well be systematically lower than what current continuum fitting methods suggest.