Valtonen, M. J.M. J.ValtonenZola, S.S.ZolaCiprini, StefanoStefanoCipriniPerri, MatteoMatteoPerriVerrecchia, FrancescoFrancescoVerrecchia2020-09-172020-09-172016-03-01https://hdl.handle.net/20.500.13025/4756The Astrophysical Journal Letters, Volume 819, Issue 2, article id. L37, <NUMPAGES>6</NUMPAGES> pp. (2016).OJ 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.black hole physicsquasars generalquasars individual OJ 287quasars supermassive black holes10.3847/2041-8205/819/2/L37http://adsabs.harvard.edu/abs/2016ApJ...819L..37V2016ApJ...819L..37V