Signatures of Ultralight Bosons in Compact Binary Inspiral and Outspiral

TL;DR

This paper explores how ultralight boson clouds around spinning black holes influence binary system dynamics, potentially causing observable outspiral effects detectable via pulsar timing.

Contribution

It models the orbit dynamics of binaries with superradiant boson clouds, including effects of mass depletion and dynamical friction, and assesses observational signatures.

Findings

Mass depletion can cause outspiral in binary systems.

Dynamical friction may dominate the inspiral phase.

Outspiral signals could be detectable through pulsar timing.

Abstract

Ultralight bosons are well-motivated particles from various physical and cosmological theories, and can be spontaneously produced during the superradiant process, forming a dense hydrogen-like cloud around the spinning black hole. After the growth saturates, the cloud slowly depletes its mass through gravitational-wave emission. In this work we study the orbit dynamics of a binary system containing such a gravitational atom saturated in various spin-0,1,2 superradiant states, taking into account both the effects of dynamical friction and the cloud mass depletion. We estimate the significance of mass depletion, finding that although dynamical friction could dominate the inspiral phase, it typically does not affect the outspiral phase driven by the mass depletion. Focusing on the large orbit radius, we investigate the condition to observe the outspiral, and the detectability of the cloud via pulsar-timing signal in the case of black hole-pulsar binary.