Boson star-black hole binaries: initial data and head-on collisions

TL;DR

This study develops improved initial data for boson star-black hole head-on collisions and analyzes gravitational-wave signals, highlighting higher multipoles as key observables for distinguishing these mergers from black hole binaries.

Contribution

The paper introduces a one-body conformal-factor correction for initial data, reducing artifacts and enabling detailed gravitational-wave analysis of BS-BH collisions.

Findings

Constraint violations are reduced with the new initial data method.

Gravitational-wave energy approaches black hole binary limits for highly compact boson stars.

Higher multipole modes help distinguish BS-BH mergers from pure black hole binaries.

Abstract

We present a numerical-relativity study of comparable-mass boson star-black hole (BS-BH) head-on collisions, focusing on both initial-data construction and gravitational-wave (GW) phenomenology. We show that plain superposition can strongly perturb the BS core, leading to large constraint violations and unphysical radial oscillations. To remedy this problem, we introduce a one-body conformal-factor correction and find that it robustly suppresses these artifacts. Using the improved initial data, we analyze GW emission from equal- and unequal-mass BS-BH binaries and compare with matched BS-BS and BH-BH baselines. For equal masses, the BS-BH radiated energy increases with BS compactness and approaches the BH-BH limit for highly compact stars. For unequal masses, the dominant $(2,0)$ mode often remains close to the BH-BH morphology, whereas the subdominant $(3,0)$ mode provides clear discriminatory power when the BH is the heavier companion. Our results identify higher multipoles as a key observable for distinguishing mixed BS-BH mergers from pure BH binaries.