Investigating the interaction of a Cosmic String with an Accreting Black Hole

TL;DR

This paper explores how cosmic strings influence the spin evolution and accretion disk properties of black holes, proposing that observable differences in the ISCO can indicate the presence of cosmic strings.

Contribution

It introduces a method to detect cosmic strings by analyzing changes in the ISCO position and accretion behavior of spinning black holes in X-ray binaries.

Findings

Cosmic strings slow down black hole spin-up during accretion.

Absence of transition phase in highly string-attached black holes indicates cosmic strings.

Detectable differences in ISCO position can serve as observational evidence.

Abstract

Rotating black holes when attached to a cosmic string have their rotational energy extracted leading to a change in its spin and mass. The spin of a black hole can be measured using various methods for an accreting black hole in an X-ray binary system. Accretion disks around black holes have an innermost stable circular orbit (ISCO) whose location is directly dependent on spin and mass of the black hole. The orbit's location changes as the black hole's spin changes and hence can be a method to detect the presence of cosmic strings. This study investigates this change and suggests the ejection of accretion material as black hole spin approaches maximum for prograde motion and material falling into the black hole for retrograde motion, regardless of the presence of cosmic string. However, in the presence of cosmic string, the spin-up process due to accretion is found out to be slower, even with high accretion rates and is detectable. There is a transition phase that occurs as the black hole approaches maximum spin, where even small changes in spin result in significant changes in the ISCO's position. Accreting black holes attached to a large string never reach this transition phase and this absence serves as potential evidence for the existence of a cosmic string.