Publication date: Jun 2019
Abstract:
The external Hartle─Thorne geometry, which describes the spacetime
outside a slowly rotating compact star, is characterized by the
gravitational mass M, angular momentum J, and quadrupole moment Q of the
star and gives a convenient description, which, for the rotation
frequencies of more than 95% of known pulsars, is sufficiently accurate
for most purposes. We focus here on the motion of particles in these
spacetimes, presenting a detailed systematic analysis of the frequency
properties of radial and vertical epicyclic motion and of orbital
motion. Our investigation is motivated by X-ray observations of binary
systems containing a rotating neutron star that is accreting matter from
its binary companion. In these systems, twin high-frequency quasi-
periodic oscillations (QPOs) are sometimes observed with a frequency
ratio approaching 3:2 or 5:4, and these may be explained by models
involving the orbital and epicyclic frequencies of quasi-circular
geodesic motion. In our analysis, we use realistic equations of state
for the stellar matter and proceed in a self-consistent way, following
the Hartle─Thorne approach in calculating both the corresponding values
of Q, M, and J for the stellar model and the properties of the
surrounding spacetime. Our results are then applied to a range of
geodetical models for QPOs. A key feature of our study is that it
implements the recently discovered universal relations among neutron-
star parameters so that the results can be directly used for models with
different masses M, radii R, and rotational frequencies f
rot.
Authors:
Urbancová, Gabriela; Urbanec, Martin; Török, Gabriel; Stuchlík, Zdeněk; Blaschke, Martin; Miller, John C.;