Inspirals into bosonic dark matter stars and chirp mimickers

Abstract

We investigate extreme-mass-ratio inspirals in which a stellar-mass compact object orbits a supermassive bosonic dark matter star, modeled as a boson star, using fully relativistic perturbative methods. Unlike inspirals around electro-vacuum black holes, these systems can shed scalar matter through dynamical friction which significantly alters the inspiral dynamics. We show that this additional dissipation can induce a chirp-like gravitational-wave signal closely resembling that of black hole binaries, allowing boson stars to act as gravitational-wave chirp mimickers even when they are not ultracompact. The inspiral evolution and resulting waveform depend sensitively on the compactness of the central boson star: highly compact configurations trigger dipolar scalar radiation, leading to a rapid plunge, whereas less compact stars yield smoother inspirals dominated by gravitational and quadrupolar scalar waves. To support waveform modeling, we derive semi-analytical prescriptions for the gravitational and scalar energy fluxes that remain accurate deep into the relativistic regime. Our findings indicate that future space-based detectors such as LISA could distinguish these mimicker signals from true black hole inspirals through measurable phase dephasings induced by scalar dissipation.

Figures (16)

• Figure 1: Schematic illustration of the evolution of an EMRI around a supermassive BS. In the initial stage (left panel), the evolution proceeds similarly to the black hole case, with the inspiral primarily driven by the emission of quadrupolar gravitational waves. As the smaller object enters the BS (middle panel), two effects take place: a resonant interaction that deposits energy into the star’s matter—altering its configuration—and the emission of scalar waves, often referred to as dynamical friction. These processes accelerate the inspiral, causing the smaller object to plunge more rapidly. Finally, depending on the compactness of the BS, two scenarios can occur. If the star is not compact enough for dipolar emission to arise (upper-right panel), the inspiral proceeds slowly toward the center, producing an almost monochromatic signal that depends on the star’s structure. Conversely, if the star is sufficiently compact (bottom-right panel), dipolar emission triggers a rapid plunge of the smaller compact object, abruptly quenching the radiation.
• Figure 2: Orbital frequency of the perturber on a circular geodesic as a function of the radial coordinate $r_p$. The curve corresponds to the most compact BS configuration, with total mass $M\mu=0.633$.
• Figure 3: Threshold for dipolar and quadrupolar scalar emission. The (scaled) left-hand side of Eq. \ref{['ineq:scalar_rad']} is shown as a function of the central value of the scalar field. When this quantity is positive, scalar emission is allowed for a given multipole. Note that while quadrupolar emission is possible for any BS configuration, dipolar emission only occurs for the largest values of the central scalar field, corresponding to highly compact solutions.
• Figure 4: The total mass of the BS as a function of the scalar field frequency. Dipolar emission by an orbiting point particle is absent in the black branch and can be present in the red branch. The dashed branch denotes unstable BS solutions.
• Figure 5: Scalar and gravitational-wave contributions to the rate of change of the particle's binding energy, as function of its radial position. We show the contribution of each part (GW and $\Phi$) as highlighted in Eq. \ref{['eq:flux']}. Left panel: Configuration with $M\mu=0.62448$, for which dipolar emission is absent. Right panel: the most massive configuration, with $M\mu=0.633$, is considered, which allows for dipolar emission for sufficiently low values of $r_p$. The dashed line corresponds to the Newtonian expression.