Placing Limits on Light Axions with EHT Polarimetry via Closure Traces

Abstract

Black holes can amplify incoming bosonic waves via rotational superradiance, inducing bound states of ultralight bosons around them. This phenomenon has the potential to confine the parameter spaces of new bosons. Axions, and axion-like particles (ALPs) are candidates beyond-standard-model particles that can form such clouds around supermassive black holes (SMBHs) and impact the polarization signal in a similar fashion to Faraday rotation via axion-photon coupling. Prior research has used data from the Event Horizon Telescope (EHT) M87 2017 observations to limit the dimensionless axion-photon coupling to previously unexplored regions. With the novel calibration-insensitive quantities: clo sure traces and conjugate closure trace products, it is possible to constrain the existence of axion clouds while avoiding the most dominant sources of systematic uncertainties, e.g., station gains and polarization leakages. I utilize a simple geometric model for the polarization map of M87* to fit the model parameters with both simulated and real datasets to verify the applicability of this method and reach a comparable level of constraint in the accuracy with which an axion cloud may be excluded in M87. Such approach is feasible with application to future M87* and Sgr A* observations by EHT and next-generation EHT (ngEHT) and may provide stronger constraints on axions and ALPs.