The Case for Hypercritical Accretion in M33 X-7

TL;DR

This paper argues that the black hole in M33 X-7 likely gained its high spin through hypercritical accretion rather than being born with it, based on binary evolution constraints.

Contribution

It demonstrates that natal spin is unlikely and highlights the necessity of hypercritical accretion to explain the observed system properties.

Findings

Natal spin hypothesis is inconsistent with observed orbital period.

Hypercritical accretion of about 5 solar masses is required to explain the black hole's spin.

Binary disruption would occur if the black hole's spin was natal.

Abstract

The spin parameter of the black hole in M33 X-7 has recently been measured to be a*=0.77+-0.05 (Liu et al. 2008). It has been proposed that the spin of the 15.65 M_sun black hole is natal. We show that this is not a viable evolutionary path given the observed binary orbital period of 3.45 days since the explosion that would produce a black hole with the cited spin parameter and orbital period would disrupt the binary. Furthermore, we show that the system has to be evolved through the hypercritical mass transfer of about 5 M_sun from the secondary star to the black hole.