Magnetic field effects on spherical orbit in Kerr-Bertotti-Robinson spacetime: constraints from jet precession of M87*

TL;DR

This paper investigates how magnetic fields influence spherical orbits around Kerr-Bertotti-Robinson black holes and uses M87* jet precession data to constrain the magnetic field strength near the black hole.

Contribution

It develops a Hamiltonian approach to analyze spherical orbits in a non-separable spacetime and derives new bounds on magnetic field strength from jet precession observations.

Findings

Spherical orbits exist only within finite radial ranges under strong magnetic fields.

Upper bounds on magnetic field strength are established for prograde and retrograde orbits.

Jet precession data tightens the magnetic field constraint to B ≲ 0.0145 M^{-1}.

Abstract

The recently reported precession period of about $11.24$ years of the M87* jet provides a sensitive probe of strong field gravity and the electromagnetic environment in the immediate vicinity of supermassive black holes. In this work, we study the precession of the spherical orbit in the Kerr-Bertotti-Robinson geometry describing a rotating black hole immersed in a uniform electromagnetic field. Although the timelike geodesics is non-separable, we develop a Hamiltonian approach to investigate the spherical orbits. For sufficiently strong magnetic fields, the study shows that the spherical orbits can only exist within a finite radial range for given orbital inclination. Requiring the existence of the spherical orbits, we obtain an upper bound of the magnetic field, i.e., $B<0.33 M^{-1}$ for prograde and $B<0.0165 M^{-1}$ for retrograde motion. Furthermore, imposing the observed jet precession period, we obtain a significantly tighter constraint, $B\lesssim 0.0145 M^{-1}$, providing a new constrain on the magnetic field of M87* independent of the shadow. Our results provide unified constraints on the parameters of the KBR black hole and demonstrate that the jet precession offers a robust and complementary probe of magnetized black holes in the strong gravity regime.