The Lense-Thirring effect at work in M87$^\ast$

Abstract

Recently, the temporal evolution of the angles characterizing the spatial configuration of the jet in the supermassive black hole M87$^\ast$ was measured exhibiting a precessional pattern around the hole's spin axis. It would be due to the dragging induced by the fact that the hole's external spacetime is described by the Kerr metric. Here, it is shown that the Lense-Thirring orbital precessions of a test particle moving about a rotating massive object, calculated perturbatively to the first post-Newtonian order, are able to fully reproduce all the measured features of the jet axis of M87$^\ast$. In particular, by assuming that the latter is aligned with the angular momentum of the accretion disk, modelled as an effective particle moving along a circular orbit, the condition that the absolute value of the predicted Lense-Thirring precessional frequency of the disk agrees with the measured value of $0.56\pm 0.02$ radians per year of the jet's one is satisfied for a range of physically meaningful values of the hole's spin parameter, close to unity, and of the effective disk radius, of the order of just over a dozen gravitational radii. Relying upon such assumptions and results, it is possible to predict that the angle between the hole's spin axis and the jet's one stays constant over the years amounting to $1.16^\circ$, in agreement with its measured value of $1.25^\circ\pm 0.18^\circ$. Furthermore, also the temporal pattern and the amplitudes of the time series of the jet's angles are reproduced by the aforementioned Lense-Thirring precessional model.