Birefringence Tomography for Axion Cloud

TL;DR

This paper explores how axion-induced birefringence effects around supermassive black holes can be detected and optimized using future VLBI observations, considering various black hole and accretion flow properties.

Contribution

It provides a detailed analysis of how black hole characteristics and accretion flow features affect axion birefringence signals, guiding future observational strategies.

Findings

Black hole inclination, spin, and magnetic fields influence birefringence signatures.

Finite accretion flow thickness causes washout effects on the signals.

Future VLBI observations can significantly improve axion detection sensitivity.

Abstract

An axion cloud surrounding a supermassive black hole can be naturally produced through the superradiance process. Its existence can be examined by the axion induced birefringence effect. It predicts an oscillation of the electric vector position angle of linearly polarized radiations. Stringent constraints of the existence of the axion in a particular mass window has been obtained based on the recent Event Horizon Telescope measurement on M87$^\star$. The future Very-Long-Baseline Interferometry (VLBI) observations will be able to measure the vicinity of many supermassive black holes, thus it opens the possibility to search for the existence of axions in a wide mass regime. In this paper, we study how different black hole properties and accretion flows influence the signatures of the axion induced birefringence. We include the impacts of black hole inclination angles, spins, magnetic fields, plasma velocity distributions, the thickness of the accretion flows. We pay special attention to characterize the washout effects induced by the finite thickness of the accretion flows and the lensed photons. Based on this study, we give prospects on how to optimize the axion search using future VLBI observations, such as the next-generation Event Horizon Telescope, to further increase the sensitivity.