Epicyclic Oscillations in the Hartle-Thorne External Geometry
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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- June 2019
- DOI:
- 10.3847/1538-4357/ab1b4c
- arXiv:
- arXiv:1905.00730
- Bibcode:
- 2019ApJ...877...66U
- Keywords:
-
- stars: oscillations;
- stars: neutron;
- stars: rotation;
- X-rays: binaries;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 21 pages, 15 figures, 1 table, accepted for publication in The Astrophysical Journal