New Spin on LIGO-Virgo Binary Black Holes

Gravitational waves from binary black holes have the potential to yield information on both of the intrinsic parameters that characterize the compact objects: their masses and spins. While the component masses are usually resolvable, the component spins have proven difficult to measure. This limitation stems in great part from our choice to inquire about the spins of the most and least massive objects in each binary, a question that becomes ill defined when the masses are equal. In this Letter, we show that one can ask a different question of the data: what are the spins of the objects with the highest and lowest dimensionless spins in the binary? We show that this can significantly improve estimates of the individual spins, especially for binary systems with comparable masses. When applying this parametrization to the first 13 gravitational-wave events detected by the LIGO-Virgo Collaboration (LVC), we find that the highest-spinning object is constrained to have nonzero spin for most sources and to have significant support at the Kerr limit for GW151226 and GW170729. A joint analysis of all the confident binary black hole detections by the LVC finds that, unlike with the traditional parametrization, the distribution of spin magnitude for the highest-spinning object has negligible support at zero spin. Regardless of the parametrization used, the configuration where all of the spins in the population are aligned with the orbital angular momentum is excluded from the 90% credible interval for the first ten events and from the 99% credible interval for all current confident detections.

Figure 1

Comparison corner plot showing the spin magnitudes and tilts recovered for our simulated equal-mass signal using both the mass sorting in green and the spin sorting in blue. The marginalized one-dimensional priors for the spin sorting are shown in gray. Orange lines mark the true value, and the equal-spin diagonal is shown as a dashed line for reference.

Figure 2

Comparison corner plot for the spin magnitudes for the posteriors obtained using the imrphenompv2 waveform for GW151226 and GW170729. The equal-spin diagonal is shown as a dashed line for reference.

Figure 3

PPDs for $p({\chi }_A)$ (top) and $p({\chi }_B)$ (bottom). The blue curves include only events from GWTC-1, while the orange curves include all 44 confident BBH events in GWTC-2. The shaded regions correspond to the 50% and 90% credible intervals, and the PPDs obtained using prior samples for the individual GWTC-2 events are shown in the dashed lines.

Figure 4

Corner plot comparing the inference on the spin tilt hyperparameters using the mass-sorted tilts ${\theta }_1$ and ${\theta }_2$ to the posteriors obtained using the same population model but for the spin-sorted tilts ${\theta }_A$ and ${\theta }_B$ for the GWTC-2 events. The posteriors obtained using prior samples for the individual events are shown in gray.