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

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 $M_f\sim 142M_{\odot }$ 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 $M_f=231_{-17}^{+13}{M}_{\odot }$, located at a distance of $d_L=571_{-181}^{+348}$ Mpc. The favoured mass for the ultra-light vector boson constituent of the Proca stars is ${\mu }_{\mathrm{V}}=8.72_{-0.82}^{+0.73}\times 10^{-13}$ 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.

Comments: 12 pages, 6 figures, Version Accepted for publication in Phys.Rev.Lett, including Supplementary Material