GW190521 as a Merger of Proca Stars: A Potential New Vector Boson of $8.7\times {10}^{-13}\mathrm{eV}$

Advanced LIGO-Virgo have reported a short gravitational-wave signal (GW190521) interpreted as a quasicircular merger of black holes, one at least populating the pair-instability supernova gap, that formed a remnant black hole of $M_f\sim 142M_{\odot }$ at a luminosity distance of $d_L\sim 5.3\mathrm{Gpc}$. 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 $d_L=571_{-181}^{+348}\mathrm{Mpc}$. This provides the first demonstration of close degeneracy between these two theoretical models, for a real gravitational-wave event. The favored mass for the ultralight vector boson constituent of the Proca stars is ${\mu }_{\mathrm{V}}=8.72_{-0.82}^{+0.73}\times {10}^{-13}\mathrm{eV}$. Confirmation of the Proca star interpretation, which we find statistically slightly preferred, would provide the first evidence for a long sought dark matter particle.

Figure 1

Time series and spectrum of GW190521. Left: Whitened strain data of the LIGO Livingston detector at the time of GW190521, together with the best fitting waveforms for a head-on merger of two BHs (green), two equal or unequal mass PSs (red and blue) and for a quasicircular BH merger (black). The time axis is expressed so that the GPS time is equal to $t_{\mathrm{GPS}}=t+1242442965.6069\mathrm{s}$. Right: corresponding waveforms shown in the Fourier domain. Solid lines denote raw waveforms (scaled by a suitable, common factor) while dashed lines show the whitened versions. The vertical line denotes the 20 Hz limit, below which the detector noise increases dramatically. Because of this, a putative inspiral signal from a quasicircular BBH merger (solid black) would be almost invisible to the detector (see dashed gray) and barely distinguishable from PHOC signals (dashed red and blue).

Figure 2

Redshifted final mass and spin of GW190521 according to different waveform models, and directly inferred from a ringdown analysis. The contours delimit 90% credible intervals. For head-on PS and BH mergers (PHOCs and HOCs), we plot the samples colored according to their log likelihood. The horizontal dashed line denotes an experimental limit for the final spin of head-on BH mergers that separates them from head-on PS mergers.

Figure 3

Posterior distribution for the values of the bosonic field associated with GW190521. The left panel shows the oscillation frequency of the bosonic field $\omega /{\mu }_V$. The right panel shows the mass of the ultralight boson ${\mu }_V$. We assume a merger of two equal-mass and equal-spin Proca stars.