Publication date: Feb 2010
Abstract:
We discuss non-geodesic corrections to orbital and epicyclic frequencies
of charged test particles orbiting a non-rotating neutron star with a
dipole magnetic field. Using a fully relativistic approach we consider
the influence of both the magnetic attraction and repulsion on the
orbital and epicyclic motion. The magnetic repulsion introduces a rather
complex and unusual behaviour of the circular orbital motion that is
well defined down to the radius where the vertical epicyclic frequency
loses its meaning. We demonstrate that for the intensity of the magnetic
interaction appropriately restricted, the stable circular orbits extend
down to the magnetic innermost stable circular orbit (MISCO) that is
located well under the geodetic innermost stable circular orbit (GISCO)
and even can reach the region under the photon circular orbit. The
lowest stable circular orbit at rMISCOmin =
2.73M, associated with the highest possible orbital frequency nu
_{K}^{max} = 3284, Hz (1.5 , {it M}_{odot}/it M) , corresponds to
the critical value of the particle-specific charge and the neutron star
magnetic dipole moment product (tilde{q} mu )_crit = 1.87 M^2 . For
the magnetic attraction acting above the GISCO, the situation is much
more simple and we demonstrate that the most significant correction
arises for the radial epicyclic frequency and consequently for the
location of the MISCO when the strong magnetic attraction pushes its
location far behind the location of GISCO. We show that the Lorentz
force also naturally violates the equality of the orbital and vertical
epicyclic frequencies implied by the spherical symmetry of the
background Schwarzschild geometry giving rise to the new effect of nodal
precession of the orbital motion plane. Finally, we apply the magnetic
attraction corrections on the relativistic precession model of the
twin-peak high-frequency quasiperiodic oscillations observed in the
galactic low mass x-ray binaries, showing possible high relevance of the
modified radial epicyclic frequency.
Authors:
Bakala, Pavel; Šrámková, Eva; Stuchlík, Zdeněk; Török, Gabriel;