The LIGO-Virgo collaboration recently reported the properties of GW190412 , a binary black hole merger with unequal component masses ( mass ratio 0.25 ^ { +0.06 } _ { -0.04 } when using the EOBNR PHM approximant ) and a non-vanishing effective spin aligned with the orbital angular momentum .
They used uninformative priors to infer that the more massive black hole had a dimensionless spin magnitude between 0.17 and 0.59 at 90 % confidence .
We argue that , within the context of isolated binary evolution , it is more natural to assume a priori that the first-born , more massive black hole has a negligible spin , while the spin of the less massive black hole is preferentially aligned with the orbital angular momentum if it is spun up by tides .
Under this astrophysically motivated prior , we conclude that the lower mass black hole had a dimensionless spin component between 0.64 and 0.99 along the orbital angular momentum .