Dynamical friction in gravitational atoms

Abstract

Due to superradiant instabilities, clouds of ultralight bosons can spontaneously grow around rotating black holes, creating so-called “gravitational atoms”. In this work, we study their dynamical effects on binary systems. We first focus on open orbits, showing that the presence of a cloud can increase the cross section for the dynamical capture of a compact object by more than an order of magnitude. We then consider closed orbits and demonstrate that the backreaction of the cloud's ionization on the orbital motion should be identified as dynamical friction. Finally, we study for the first time eccentric and inclined orbits. We find that, while ionization quickly circularizes the binary, it barely affects the inclination angle. These results enable a more realistic description of the dynamics of gravitational atoms in binaries and pave the way for dedicated searches with future gravitational wave detectors.

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.…”

Resonant dynamics of extreme mass-ratio inspirals in a perturbed Kerr spacetime

“…They generally have a superior power in detecting weak environmental forces because of the large number (10 4 -10 5 ) of orbital cycles in band, so that weak effects may be amplified to achieve detectable gravitational-wave phase shifts. These environmental forces may come from the tidal gravitational field of a third stellar-mass object [17][18][19][20][21], the migration force from an accretion disk, and/or the interaction between the stellarmass black hole and a possible dark-matter cloud [22][23][24][25][26][27][28][29]. From the perspective of testing general relativity, it is interesting to test the Kerr metric as a key prediction of general relativity for rotating black holes, as modified gravity theories may predict different black hole spacetimes (e.g., Einstein-dilaton-Gauss-Bonnet [30], dynamical Chern-Simons gravity [31], and effective field theory extensions of general relativity [32]).…”