The Formation of Rapidly Rotating Black Holes in High Mass X-ray Binaries

TL;DR

This paper proposes that rapidly spinning black holes in high mass X-ray binaries can form from failed supernovae with fallback material, which increases the black hole's spin beyond traditional limits, supported by hydrodynamic simulations.

Contribution

It introduces a novel formation scenario for high-spin black holes via failed supernovae and fallback accretion, supported by hydrodynamic simulations.

Findings

High spin parameters ($a \,\gtrsim\, 0.8$) can be achieved through fallback in failed supernovae.

The scenario explains atmospheric metal pollution in HMXRB stellar companions.

Simulations show possible high-spin black hole formation in close binaries.

Abstract

High mass X-ray binaries (HMXRBs) like Cygnus X-1, host some of the most rapidly spinning black holes (BHs) known to date, reaching spin parameters $a \gtrsim 0.84$. However, there are several effects that can severely limit the maximum BH spin parameter that could be obtained from direct collapse, such as tidal synchronization, magnetic core-envelope coupling and mass loss. Here we propose an alternative scenario where the BH is produced by a {\it failed} supernova (SN) explosion that is unable to unbind the stellar progenitor. A large amount of fallback material ensues, whose interaction with the secondary naturally increases its overall angular momentum content, and therefore, the spin of the BH when accreted. Through SPH hydrodynamic simulations, we studied the unsuccessful explosion of a $8M_{\odot }$ pre-SN star in a close binary with a $12M_{\odot}$ companion with an orbital period of $\approx1.2$ days, finding that it is possible to obtain a BH with a high spin parameter $a\gtrsim0.8$ even when the expected spin parameter from direct collapse is $a \lesssim 0.3$. This scenario also naturally explains the atmospheric metal pollution observed in HMXRB stellar companions.