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.
