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

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 (F) over tilde 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.

Automated summary

The study explores the photon ring and its polarization structure in a black hole image, revealing a unique polarimetric structure due to the coupling between axions and photons. By measuring these patterns, it can provide a way of determining the mixed anomaly coefficient and discuss the conditions for the presence of supermassive black holes in galaxies pierced by cosmic strings.