Epicyclic Oscillations in the HartleThorne External Geometry
Abstract
The external HartleThorne 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 Xray observations of binary systems containing a rotating neutron star that is accreting matter from its binary companion. In these systems, twin highfrequency quasiperiodic 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 quasicircular geodesic motion. In our analysis, we use realistic equations of state for the stellar matter and proceed in a selfconsistent way, following the HartleThorne 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 neutronstar parameters so that the results can be directly used for models with different masses M, radii R, and rotational frequencies f _{rot}.
 Publication:

The Astrophysical Journal
 Pub Date:
 June 2019
 DOI:
 10.3847/15384357/ab1b4c
 arXiv:
 arXiv:1905.00730
 Bibcode:
 2019ApJ...877...66U
 Keywords:

 stars: oscillations;
 stars: neutron;
 stars: rotation;
 Xrays: binaries;
 Astrophysics  High Energy Astrophysical Phenomena
 EPrint:
 21 pages, 15 figures, 1 table, accepted for publication in The Astrophysical Journal