MEASURING THE SPIN OF THE PRIMARY BLACK HOLE IN OJ287

The compact binary system in OJ287 is modeled to contain a spinning primary black hole with an accretion disk and a non-spinning secondary black hole. Using post-Newtonian (PN) accurate equations that include 2.5PN accurate non-spinning contributions, the leading-order general relativistic and classical spin-orbit terms, the orbit of the binary black hole in OJ287 is calculated and as expected it depends on the spin of the primary black hole. Using the orbital solution, the specific times when the orbit of the secondary crosses the accretion disk of the primary are evaluated such that the record of observed outbursts from 1913 up to 2007 is reproduced. The timings of the outbursts are quite sensitive to the spin value. In order to reproduce all the known outbursts, including a newly discovered one in 1957, the Kerr parameter of the primary has to be 0.28 +/- 0.08. The quadrupole-moment contributions to the equations of motion allow us to constrain the "no-hair" parameter to be 1.0 +/- 0.3, where 0.3 is the 1 sigma error. This supports the "black hole no-hair theorem" within the achievable precision. It should be possible to test the present estimate in 2015 when the next outburst is due. The timing of the 2015 outburst is a strong function of the spin: if the spin is 0.36 of the maximal value allowed in general relativity, the outburst begins in early 2015 November, while the same event starts in the end of January 2016 if the spin is 0.2.