Ocean g-Modes on Rotating Neutron Stars

Details Ocean g-Modes on Rotating Neutron Stars Ocean g-Modes on Rotating Neutron Stars

Apr 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...460..827b&link_type=abstract

Astrophysical Journal v.460, p.827

Astronomy and Astrophysics

Astronomy

58

Accretion, Accretion Disks, Stars: Neutron, Stars: Oscillations, Stars: Rotation, X-Rays: Stars

Scientific paper

We recently argued that neutron stars accreting at rates 󗓶-9 Msun yr-1 (the "Z" sources) are covered with massive oceans, and conjectured that waves in these oceans might modulate the outgoing X-ray flux at frequencies comparable to what is observed. For slowly rotating neutron stars, we showed that the low radial order (n ˜1-2) g-mode oscillations are in the 5-8 Hz range for l = 1, in rough agreement with the ubiquitous ˜6 Hz quasi-periodic oscillations (QPOs) seen in the Z sources. We now extend the thermal g-mode calculations to the case of rapidly rotating stars. The m = 0 modes are most relevant, since the m ≠ 0 modes will necessarily acquire a high frequency in the observer's nonrotating frame. For spin frequencies Ω ≫ ω we find that the lowest m = 0 g-mode frequency is - ω 1/4(Ωω1.0)1/2, where ω1,0 is the corresponding nonrotating l = 1 g-mode frequency. In the context of nonradial thermal g-modes, there are two ways to explain the fact that all six Z sources show ˜6 Hz QPOs: (1) the neutron stars are all spinning at frequencies ≲6 Hz and a low-order (i.e., only a few radial nodes) g-mode is responsible for the oscillations, or (2) the g-mode is of higher radial order (n ˜ 10-50) and is brought to ˜6 Hz by fast rotation, thus requiring that all six neutron stars have rotation frequencies within a factor of 2-3 of each other. Which radial order, if any, the star chooses to excite is not known at present.

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