OJ 287 is a quasi-periodic quasar with roughly 12 year optical cycles .
It displays prominent outbursts which are predictable in a binary black hole model .
The model predicted a major optical outburst in December 2015 .
We found that the outburst did occur within the expected time range , peaking on 2015 December 5 at magnitude 12.9 in the optical R -band .
Based on Swift /XRT satellite measurements and optical polarization data , we find that it included a major thermal component .
Its timing provides an accurate estimate for the spin of the primary black hole , \chi = 0.313 \pm 0.01 .
The present outburst also confirms the established general relativistic properties of the system such as the loss of orbital energy to gravitational radiation at the 2 \% accuracy level and it opens up the possibility of testing the black hole no-hair theorem with a 10 \% accuracy during the present decade .