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

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 𝜙F$$ \overset{\sim }{F} $$ F ~ between the axion 𝜙 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.

Kerr black hole shadows from axion-photon coupling

“…The birefringence effects of the polarization photons have been used to analyze the axion dark matter distribution near M87* black hole [62] and in the protoplanetary disk around a young star [63]. The axion-induced birefringence effect gives arise to the unique polarimetric structure [64] and the electric vector position angle oscillation of linearly polarized photons [65]. Such oscillation has been applied to constrain region of the axion mass and axion-photon coupling parameter space together with the observation data from Event Horizon Telescope [65].…”

Probing axions via light circular polarization and event horizon telescope

“…A cloud of ultralight bosons around BHs can be probed also through its gravitational-wave (GW) signatures [24,25] or through the study of the dynamics of SMBH [26]. Moreover, the axion cloud surrounding a Kerr BH behaves like an optically active medium which impacts on the polarization properties of light rays traveling through it [27][28][29][30][31][32]. Therefore, polarization observations of the emission from the accretion disks of stellar-mass and suppermassive BHs JCAP04(2023)017 can provide a novel way to probe the existence of axion field close to the event horizon of a BH.…”

Photon Ring Astrometry for Superradiant Clouds

“…The prospects to detect such a time delay are less interesting due to the limited time resolution of the photon ring autocorrelation. However, in the presence of an emission source with shorter time correlation lengths, such as a hot spot [130][131][132][133][134][135] or a nearby pulsar [136][137][138][139][140], echoes from those sources could be used as astrometry with a dramatically enhanced time resolution similar to pulsar timing array searches.…”