Searching for cosmic strings via black hole spin-down

TL;DR

This paper explores how cosmic strings attached to black holes can extract rotational energy, leading to observable spin-down effects, especially in primordial black holes and supermassive black holes, providing potential observational signatures.

Contribution

It introduces the idea that cosmic strings can significantly spin down black holes, offering a new method to detect or constrain cosmic strings through black hole spin measurements.

Findings

Primordial black holes with mass >30 M_sun should have near-zero spins if cosmic strings with tension >10^{-20} exist.

Cosmic strings can cause observable spin-down effects in supermassive black holes.

Potential observational signatures include altered black hole spins and pulsar timing anomalies.

Abstract

Cosmic strings that are attached to rapidly spinning black holes can extract significant amounts of rotational energy and angular momentum. Here we study the effect on primordial black holes, which are expected to form with one or more cosmic strings attached. Although large primordial black holes are predicted to rapidly spin up due to accretion soon after forming, we argue that cosmic strings will spin them down again. We show that if there are cosmic strings with tension greater than $10^{-20}$, the spins of large primordial black holes of mass greater than $30 M_\odot$ should consequently be observed to be near zero. We also investigate the effect on a supermassive black hole of capturing a cosmic string and the possibility of observing the subsequent spin down by its effect on a pulsar orbiting the black hole.