LIGO Document P1700067-v3

Distinguishing Spin-Aligned and Isotropic Black Hole Populations With Gravitational Waves

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P - Publications
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The first direct detections of gravitational waves (GW) from merging binary black holes (BBH) open a unique window into the BBH formation environment. One promising signature of the formation environment is the angular distribution of the black hole (BH) spins; systems formed through dynamical interactions among already-compact objects are expected to have isotropic spin orientations whereas binaries formed from pairs of stars born together are more likely to have spins preferentially aligned with the binary orbit as a consequence of their joint evolution toward a BBH system. We consider existing GW measurements of $\chieff$, the best-measured combination of spin parameters, in the three likely binary black hole detections GW150914, LVT151012, and GW151226. If binary black hole spin magnitudes extend to high values, as is suggested by observations of black hole X-ray binaries, we show that the data already exhibit a $1.7 \sigma$ ($0.087$ odds ratio) preference for an isotropic angular distribution. By considering the effect of an additional 10 detections drawn from the various models in the suite we show that if all observations come from a single population such an augmented data set would enable at least a $2.9 \sigma$ ($0.0035$ odds ratio) distinction between the isotropic and aligned models for the assumed spin magnitude distributions, and in most cases better than $5\sigma$ ($2.9 \times 10^{-7}$ odds ratio). The existing preference for either an isotropic spin distribution or low spin magnitudes for the observed systems will be confirmed (or overturned) confidently in the near future by subsequent observations.
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As submitted; minor changes from v2 to address P&P comments.

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