Implications of GW241011 for rotating exotic compact objects

TL;DR

This work leverages the precise GW241011 constraint on the spin-induced quadrupole moment (SIQM) to test the nature of its primary as an exotic compact object (ECO). It develops a PN-based SIQM framework with a Kerr-like baseline ($\kappa_i=1$) and a generalized parameterization $Q_i = - \kappa_i \chi_i^2 m_i^3$, incorporating corrections up to 2PN/3PN in an IMRPhenomXPHM waveform and performing Bayesian inference for $\kappa_1$ (and $\kappa_2$). The authors compute SIQMs for rotating boson stars across repulsive, solitonic, and axionic potentials, and for exotic fluid stars, deriving constraints on compactness that exclude broad classes of BSs (notably repulsive ones with large couplings) and place lower bounds on $C$ for other ECOs. The results narrow the viable ECO landscape for GW241011, favoring highly compact configurations with $C \gtrsim 0.24$ and highlighting the need for expanded ECO templates and cross-channel observations to fully map beyond-Kerr physics.

Abstract

A number of theoretical proposals have been made for horizonless compact objects with masses and spins similar to those of black holes. While gravitational wave signatures from their mergers can resemble those of black holes, features like the spin-induced quadrupole moment may reveal their distinct nature. Using the tight bounds on the spin-induced quadrupole moment of GW241011, we place gravitational wave constraints on the nature of its primary. We find that large classes of exotic compact objects (including rotating boson stars) cannot explain its nature, however, models of sufficiently large compactness of $C \gtrsim 0.24$ may still be viable contenders.

Figures (8)

• Figure 1: The one-dimensional posterior distribution of $\kappa_1$ obtained from GW241011. The colored lines represent posteriors obtained from the likelihood truncated at different frequencies $f_{\rm cut}$, whereas the black line corresponds to the full inference. The dashed vertical lines represent the 95% CI bounds and the dotted vertical line denotes the corresponding Kerr BH value of $\kappa_1 = 1$.
• Figure 2: Top: Sequences of repulsive BSs in the $\chi-\kappa$ plane (solid lines), with different colors denoting the coupling values, $\lambda / \mu^2$, and dots the maximum mass solution. The dotted lines indicate dynamically unstable solutions according to our relativistic criterion (see the main text). The gray shaded region denotes the $\kappa$--$\chi$ measurement (at 95% credible level). Bottom: Exclusion regions in the $m_\mathrm{b} - \tilde{\Lambda}$ plane, where $m_\mathrm{b}$ is the boson mass in physical units, $\tilde{\Lambda} = \Lambda (\hbar c)$ is dimensionless and $\Lambda$ is the coupling in physical units. Gray denotes the region excluded by GW241011, yellow corresponds to stars with masses incompatible with GW241011, whereas blue contains only unstable solutions. For reference, we also overplot the $\tilde{\Lambda}$ and $m_{\rm b}$ values corresponding to a hypothetical dark matter particle with cross-section per unit mass of $0.1 \,\mathrm{cm}^{2}\,\mathrm{g}^{-1}$ in dashed red line.
• Figure 3: Same as Fig. \ref{['fig:repulsive']}, but for solitonic stars (top), axionic stars (middle), and exotic fluid stars (bottom), with colors corresponding to values of $\sigma$, $f$, and $r$ respectively. Here, however, we do not indicate solutions that suffer from dynamical instabilities.
• Figure S1: The one-dimensional posterior distribution of $\kappa_2$ obtained from GW241011 along with the prior distribution in grey.
• Figure S2: The approximate contact frequency $f_{\rm contact}$ for different compactnesses.