Black hole spin from wobbling and rotation of the M87 jet and a sign of a magnetically arrested disc

New long-term Very Long Baseline Array observations of the well-known jet in the M87 radio galaxy at 43 GHz show that the jet experiences a sideways shift with an approximately 8-10 yr quasi-periodicity. Such jet wobbling can be indicative of a relativistic Lense-Thirring precession resulting from a tilted accretion disc. The wobbling period together with up-to-date kinematic data on jet rotation opens up the possibility for estimating angular momentum of the central supermassive black hole. In the case of a test-particle precession, the specific angular momentum is $J/Mc=(2.7\pm1.5)\times10^{14}$ cm, implying moderate dimensionless spin parameters $a=0.5\pm0.3$ and $0.31\pm0.17$ for controversial gas-dynamic and stellar-dynamic black hole masses. However, in the case of a solid-body-like precession, the spin parameter is much smaller for both masses, $0.15\pm0.05$. Rejecting this value on the basis of other independent spin estimations requires the existence of a magnetically arrested disc in M87.