Publication date: n/a 2008
Abstract:
We discuss the non-geodesic corrections to the formulae for orbital and
epicyclic frequencies given by the presence of a neutron star magnetic
field. In this paper we focus on the „Lorentzian“ corrections arising if
specific charge of a test particle is considered. This corrections are
valid for a slightly charged accreting matter. We consider a magnetic
field generated by an intrinsic static dipole magnetic moment of a
slowly rotating neutron star on the background of the Schwarzschild
geometry.We calculate relevant orbital and epicyclic frequencies in a
fully general relativistic form using the equations governing
perturbations of the circular motion. The nongeodesic corrections are
rather high in the vicinity of the central compact object. The most
significant correction arises for the radial epicyclic frequency. The
zero point of the corrected radial epicyclic frequency defines radius of
the effective innermost stable circular orbit (EISCO). This correction
implies an influence of a test particle charge on the effective position
of the innermost marginally stable circular orbit (EISCO) and
constraints a restriction to the specific charge under an evidence for
the orbital motion close to the central compact object. A dipole
magnetic field influences the radial epicyclic frequency and the
position of EISCO. It also cancels the equality of orbital and vertical
epicyclic frequencies present in spherically symmetric Schwarzschild
geometry. However, these corrections become substantial only for a
specific charge values which implying shift of the innermost stable
circular orbit inconsistent with the present astrophysical view of
LMXBs. Hence, in the lowest approximation realitic for observed
neutron-star binary system with QPOs, the influence of the eventual
specific charge of the accreted matter should enter the orbital QPO
models in the form of a slightly lowered radial epicyclic frequency and
slightly shifted ISCO.
Authors:
Bakala, Pavel; Sramkova, Eva; Stuchlik, Zdenek; Török, Gabriel;