Valtonen, M. J.Zola, S.Ciprini, S.Gopakumar, A.Matsumoto, K.Sadakane, K.Kidger, M.2025-01-272025-01-2720162041-82052041-8213https://doi.org/10.3847/2041-8205/819/2/L37https://hdl.handle.net/20.500.12428/24447OJ 287 is a quasi-periodic quasar with roughly 12 year optical cycles. It displays prominent outbursts that are predictable in a binary black hole model. The model predicted a major optical outburst in 2015 December. 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 10% accuracy during the present decade.eninfo:eu-repo/semantics/openAccessblack hole physicsquasars: generalquasars: individual (OJ 287)quasars: supermassive black holesArticle819210.3847/2041-8205/819/2/L37Q1WOS:0003717883000212-s2.0-84961133222Q1