Revisiting the dynamical case for a massive black hole in IC10 X-1

The relative phasing of the X-ray eclipse ephemeris and optical radial velocity (RV) curve for the X-ray binary IC10 X-1 suggests that the He [λ4686] emission line originates in a shadowed sector of the stellar wind that avoids ionization by X-rays from the compact object. The line attains maximum blueshift when the wind is directly towards us at mid X-ray eclipse, as is also seen in Cygnus X-3. If the RV curve is unrelated to stellar motion, evidence for a massive black hole (BH) evaporates because the mass function of the binary is unknown. The reported X-ray luminosity, spectrum, slow QPO and broad eclipses caused by absorption/scattering in the Wolf–Rayet (WR) wind are all consistent with either a low-stellar-mass BH or a neutron star (NS). For an NS, the centre of mass lies inside the WR envelope whose motion is then far below the observed 370 km s−1 RV amplitude, while the velocity of the compact object is as high as 600 km s−1. The resulting 0.4 per cent Doppler variation of X-ray spectral lines could be confirmed by missions in development. These arguments also apply to other putative BH binaries whose RV and eclipse curves are not yet phase-connected. Theories of BH formation and predicted rates of gravitational wave sources may need revision.