Revisiting the Dynamical Case for a Massive Black Hole in IC10 X-1

TL;DR

This paper reevaluates evidence for a massive black hole in IC10 X-1, suggesting that the observed radial velocity may not indicate stellar motion, which challenges previous claims of a black hole and impacts theories of black hole formation.

Contribution

It provides a new interpretation of the radial velocity data, proposing that the emission line origin and observed velocities do not necessarily imply a massive black hole in IC10 X-1.

Findings

The He emission line may originate in a shadowed wind sector, not the stellar surface.

The observed RV amplitude could be due to wind dynamics, not stellar motion.

Implications for black hole formation theories and gravitational wave source predictions.

Abstract

The relative phasing of the X-ray eclipse ephemeris and optical radial velocity (RV) curve for the X-ray binary IC10 X-1 suggests the He[$\lambda$4686] 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 toward 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 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 WR wind are all consistent with either a low-stellar-mass BH or a NS. For a NS, the centre of mass lies inside the WR envelope whose motion is then far below the observed 370 km/s RV amplitude, while the velocity of the compact object is as high as 600 km/s. The resulting 0.4\% 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.