Black hole portal into hidden valleys

Abstract

Superradiant instability turns rotating astrophysical black holes into unique probes of light axions. We consider what happens when a light axion is coupled to a strongly coupled hidden gauge sector. In this case superradiance results in an adiabatic increase of a hidden sector $CP$-violating $\ensuremath{\theta}$ parameter in a near horizon region. This may trigger a first order phase transition in the gauge sector. As a result a significant fraction of a black hole mass is released as a cloud of hidden mesons and can be later converted into electromagnetic radiation. This results in a violent electromagnetic burst. The characteristic frequency of such bursts may range from $\ensuremath{\sim}100\text{ }\text{ }\mathrm{eV}$ to $\ensuremath{\sim}100\text{ }\text{ }\mathrm{MeV}$.