External magnetic-field effects on dipolar-particle orbits and critical collisions in Kerr--Bertotti--Robinson spacetime
Strong magnetic fields affect particle dynamics through two distinct channels: gravitational backreaction deforms the spacetime, while direct coupling to an intrinsic magnetic moment depends on the relative orientation of the field and the dipole. We disentangle these effects by studying equatorial orbits and near-horizon collisions of electrically neutral magnetized particles in the exact Kerr--Bertotti--Robinson spacetime. The field-induced geometric deformation shifts turning points and circular-orbit domains and can eliminate a finite effective-potential well together with its bound orbits. Even without direct dipole coupling, it can also offset the Kerr periapsis advance and produce a finite-radius zero-precession orbit. Direct dipole coupling breaks the symmetry under magnetic-field reversal and shifts the radius, energy, and angular momentum of the innermost stable circular orbit in an orientation-dependent manner. The formal ultrarelativistic endpoints of these orbit branches, however, remain fixed by the background geometry and approach circular null orbits. In the Bañados--Silk--West mechanism, both magnetic effects modify finite-radius potential barriers and hence the ability of a critical particle to reach the near-horizon collision region. In the representative nonzero-field cases examined here, an exactly critical particle released from infinity is blocked before reaching an extremal horizon, although a locally admissible collision between critical and usual particles can still produce unbounded center-of-mass energy. Finite dipole coupling shifts the barriers but does not change the leading near-horizon divergence. Near a nonextremal horizon, an exactly critical particle is excluded and the collision energy remains finite.