Attempt to explain black hole spin in X-ray binaries with new physics

TL;DR

This paper proposes that some rapidly spinning black holes in X-ray binaries might not be Kerr black holes but instead are ordinary black holes with new physics effects, explaining high spins without requiring significant accretion.

Contribution

It introduces a novel explanation for high black hole spins in X-ray binaries through new physics, challenging the assumption they are Kerr black holes formed from stellar collapse.

Findings

Small matter accretion can mimic high spin in non-Kerr black holes.

Current observations do not exclude non-Kerr black holes as explanations.

The proposed model aligns with existing spin measurements without requiring large accretion.

Abstract

It is widely believed that the spin of black holes in X-ray binaries is mainly natal. A significant spin-up from accretion is not possible. If the secondary has a low mass, the black hole spin cannot change too much even if the black hole swallows the whole stellar companion. If the secondary has a high mass, its lifetime is too short to transfer the necessary amount of matter and spin the black hole up. However, while black holes formed from the collapse of a massive star with Solar metallicity are expected to have low birth spin, current spin measurements show that some black holes in X-ray binaries are rotating very rapidly. Here I show that, if these objects are not the Kerr black holes of general relativity, the accretion of a small amount of matter ($\sim 2$~$M_\odot$) can make them look like very fast-rotating Kerr black holes. Such a possibility is not in contradiction with any observation and it can explain current spin measurements in a very simple way.