Probing Boson Clouds with Supermassive Black Hole Binaries

TL;DR

This paper investigates how supermassive black hole binaries can host boson clouds generated by superradiance, and explores observational signatures through gravitational waves and electromagnetic signals, considering astrophysical evolution effects.

Contribution

It extends previous models to supermassive black hole binaries, analyzing how their evolution influences boson cloud depletion and observational prospects.

Findings

Accelerated orbital evolution due to energy loss channels.

Inefficiency of hyperfine resonant transitions in SMBHBs.

Potential to detect boson cloud ionization effects via multi-messenger observations.

Abstract

Rotating black holes can generate boson clouds via superradiance when the boson's Compton wavelength is comparable to the black hole's size. In binary systems, these clouds can produce distinctive observational imprints. Recent studies accounting for nonlinearities induced by orbital backreaction suggest that if the binary forms at a large separation, resonance transitions can significantly deplete the cloud, minimizing later observational consequences except for very specific orbital inclinations. In this paper, we extend this framework to supermassive black hole binaries (SMBHBs), considering the influence of their astrophysical evolutionary histories. We find that, before entering the gravitational wave (GW) radiation stage, the additional energy loss channels can accelerate orbital evolution. This acceleration makes hyperfine resonant transitions inefficient, allowing a sufficient portion of the cloud to remain for later direct observations. We further discuss the ionization effects and cloud depletion occurring at this stage. Based on these theoretical insights, we explore how multi-messenger observations for SMBHBs can be utilized to detect the ionization effects of boson clouds by examining changes in the orbital period decay rate via electromagnetic measurements and variations in GW strain over a wide frequency band. Our findings reveal a complex dependence on the binary's total mass, mass ratio, and boson mass, emphasizing the significant role of astrophysical evolution histories in detecting boson clouds within binaries.