Evolution of binary systems accompanying axion clouds in extreme mass ratio inspirals

Abstract

Superradiant instability of rotating black holes (BHs) leads to the formation of a cloud of ultralight bosons, such as axions. When the BH with the cloud belongs to a binary system and is in an inspiraling orbit, the resonant transition between the axion's bound states can occur. We study the history of the evolution of the binary system accompanying the cloud composed of the fastest growing mode and its impact on the observational signatures, especially for small mass ratio cases. In this case, the hyperfine resonance, which has a very small resonance frequency, is relevant. Therefore, due to the long timescale, we should take into account the decaying process of axions in the transition destination mode, the backreaction to the orbital motion and the central BH, and gravitational emission from the cloud. We present a formulation to examine the evolution of the system around the resonance and useful expressions for the analysis. As a result, we found the mass of the cloud that can remain after the resonance is, at most, about 10 −5 of the central BH. The maximum remaining cloud mass is achieved when the mass ratio of the binary is q ∼ 10 −3 . In addition, we show that the resonant transition hardly changes the BH mass and spin distribution, while the associated modification of the gravitational wave frequency evolution when the binary passes through the resonance can be a signature of the presence of the cloud.

Dynamical friction in gravitational atoms

“…Studies of later stages of the inspiral, when the gravitational wave signal is stronger and the impact of the cloud's ionization becomes more relevant, have instead focused on quasi-circular orbits [19,20,24,67]. Most work on resonant transitions also made this same simplifying assumption [17,55,[68][69][70][71][72], with only [73] considering nonzero eccentricity, at a time where, however, some physical aspects of the problem were not yet completely understood.…”

Axion cloud decay due to the axion-photon conversion with multi-pole background magnetic fields

“…Then the axion field surrounding the black hole can be amplified and form a cloud. Possible observational signals from BH atoms [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] and the final fate of this instability with or without external factors [28][29][30][31][32][33] have been actively discussed.…”