Impact of the axion-like self-interactions in gravitational atoms for LISA

Abstract

Ultralight bosons with self-interactions, such as axion-like particles, can form astrophysical Bose--Einstein condensates around stars or compact objects, often referred to as gravitational atoms. In this work, we adopt a recently proposed dynamical formation mechanism for these halos and estimate their impact on extreme- and intermediate-mass-ratio inspirals when present around the primary black hole. We show that, for signal-to-noise ratios $\lesssim 100$, LISA can distinguish gravitational waveforms from binaries embedded in such halo overdensities. Our analysis indicates that LISA can probe boson masses $m_{\mathrm{dm}}\sim 10^{-17}$--$10^{-15}\mathrm{eV}$ and decay constants $f_a\sim 3\times 10^{10}$--$6\times 10^{12}\mathrm{GeV}$ using binaries with total masses $M\sim 10^4$--$10^5{M}_{\odot }$, assuming conservative DM densities consistent with the central values of Navarro--Frenk--White profiles. Allowing for higher background densities and different extreme-mass-ratio configurations further extends the accessible parameter space. Moreover, we find that for a binary configuration with $M\sim 10^4{M}_{\odot }$, ${\rho }_{\mathrm{dm}}=10^{4}GeV/c{\mathrm{m}}^3$, and signal to noise ratio $\text{SNR}\sim 20$, a particle mass of $m_{dm}=3.2\cdot 10^{-15}$ eV and decay constant of $f_a=1.6\cdot 10^{11}$ GeV maximize the dephasing due to dynamical friction, enabling the recovery of the particle parameters at the percent level. These results demonstrate that LISA can place constraints on axion-like particle masses and self-interactions without requiring additional couplings to Standard Model fields.

Comments: 19 pages, 12 figures