  OJ287 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 +- 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.
