The need for hypercritical accretion in massive black-hole binaries with large Kerr parameters

TL;DR

This paper investigates the evolutionary scenarios of massive black-hole binaries to explain their high Kerr parameters, concluding that hypercritical accretion is necessary regardless of the specific binary evolution model.

Contribution

It demonstrates that hypercritical accretion is essential to produce the observed high Kerr parameters in massive black-hole binaries across different evolutionary scenarios.

Findings

High Kerr parameters require hypercritical accretion.

Both Case C and Case M models support hypercritical accretion necessity.

Observed black hole spins imply significant mass transfer after formation.

Abstract

Recent measurements of the Kerr parameters of the black holes in M33 X-7 and LMC X-1 yield a*=0.84\pm0.05 and a*=0.90^{+.04}_{-.09} respectively. We study massive binary evolution scenarios that can reproduce such high values for the Kerr parameters. We first discuss a model with Case C mass transfer leading to a common envelope and tidal synchronization of the primary before it collapses into a black hole. We also study a Case M evolution model (which involves tidally-locked, rotationally-mixed, chemically-homogeneous stars in a close binary). Our analysis suggests that, regardless of the specific scenario, the observed Kerr parameters for the black holes in M33 X-7 and LMC X-1 had to be obtained through hypercritical mass accretion.