Distinguishing boson stars from black holes using gravitational-wave observations

The spinning compact binaries possess non-vanishing spin-induced multipole moments that leave an imprint on the emitted gravitational wave (GW) signal. These spin-induced multipole moments are uniquely determined for Kerr black holes, given their mass and spin. On the other hand, the spin-induced multipole moments of neutron stars and exotic compact objects depend on their internal composition in addition to their mass and spin. Hence, the GW measurement of the spin-induced multipole moments is useful for probing the true nature of the observed compact object. We demonstrate a novel test to distinguish massive boson stars from binary black holes using GW observations of the spin-induced quadrupole moment. Starting with selected GW events detected during the first three observing runs of ground-based GW detectors, we perform a complete Bayesian analysis with an efficient stochastic sampling algorithm to obtain the posteriors on the boson star model parameters. We employ a GW waveform model for inspiraling binary boson stars with quartic self-interactions, which has contributions from its spin-induced quadrupole moment along with higher modes and precession. This detailed analysis constrains the boson star model parameters, such as the mass of the boson ($\mu_B$) and the self-interaction parameter ($\lambda$), for the first time. We find that, GW190412 gives the best bound in the $\mu_B-\lambda$ plane, constraining $\lambda$ approximately between $1\times 10^5-9\times 10^5$ for bosons with masses of $\mathcal{O}$(1 GeV). In the future, with improved detector sensitivity and an increase in the population of binaries, this method can provide interesting constraints on black hole mimicker candidates.