GW190521 as a merger of Proca stars: a potential new vector boson of $8.7 \times 10^{-13}$ eV

TL;DR

This paper proposes that GW190521 could be explained by the merger of Proca stars, offering a potential new candidate for dark matter particles and demonstrating a degeneracy with black hole merger models.

Contribution

It introduces the novel idea that GW190521 may originate from Proca star collisions, expanding the understanding of possible sources of gravitational waves.

Findings

GW190521 is consistent with Proca star merger simulations.

Proca stars with a specific ultra-light vector boson mass fit the data.

The Proca star model is slightly statistically favored over black hole models.

Abstract

Advanced LIGO-Virgo reported a short gravitational-wave signal (GW190521) interpreted as a quasi-circular merger of black holes, one populating the pair-instability supernova gap, forming a remnant black hole of $M_f\sim 142 M_\odot$ at a luminosity distance of $d_L \sim 5.3$ Gpc. With barely visible pre-merger emission, however, GW190521 merits further investigation of the pre-merger dynamics and even of the very nature of the colliding objects. We show that GW190521 is consistent with numerically simulated signals from head-on collisions of two (equal mass and spin) horizonless vector boson stars (aka Proca stars), forming a final black hole with $M_f = 231^{+13}_{-17}\,M_\odot$, located at a distance of $d_L = 571^{+348}_{-181}$ Mpc. The favoured mass for the ultra-light vector boson constituent of the Proca stars is $\mu_{\rm V}= 8.72^{+0.73}_{-0.82}\times10^{-13}$ eV. This provides the first demonstration of close degeneracy between these two theoretical models, for a real gravitational-wave event. Confirmation of the Proca star interpretation, which we find statistically slightly preferred, would provide the first evidence for a long sought dark matter particle.