Polarimetric signatures of the photon ring of a black hole that is pierced by a cosmic axion string

TL;DR

This paper explores how the presence of a cosmic axion string piercing a rotating black hole can create unique polarimetric signatures in the photon ring, offering a potential observational method to detect such exotic objects.

Contribution

It introduces a novel polarization pattern in the black hole photon ring caused by axion-photon coupling and the Aharonov-Bohm effect, linking cosmic strings to black hole imaging.

Findings

Distinct polarization patterns identified in the photon ring.

Measurement of these patterns can reveal the axion-photon coupling coefficient.

Conditions for the formation of black holes pierced by cosmic strings discussed.

Abstract

A black hole image contains a bright ring of photons that have closely circled the black hole on their way from the source to the detector. Here, we study the photon ring of a rotating black hole which is pierced by a global hyper-light axion-type cosmic string. We show that the coupling $\phi F \tilde{F}$ between the axion $\phi$ and the photon can give rise to a unique polarimetric structure of the photon ring. The structure emerges due to an Aharonov-Bohm type effect that leads to a change of the polarization directions of linear polarized photons when they circle the black hole. For several parameter choices, we determine concrete polarization patterns in the ring. Measuring these patterns can provide us with a way of determining the value of the coefficient of the mixed anomaly between electromagnetism and the symmetry that gave rise to the cosmic string. Finally, we briefly review a possible formation mechanism of black holes that are pierced by cosmic strings and discuss under which conditions we can expect such objects to be present as supermassive black holes in the center of galaxies.