Sonic-Point and Spin-Resonance Model of the Kilohertz QPO Pairs. Lamb, F. K. & Miller, M. C. arXiv Astrophysics e-prints, August, 2003.
Sonic-Point and Spin-Resonance Model of the Kilohertz QPO Pairs [link]Paper  abstract   bibtex   
KHz QPOs have now been detected in more than twenty accreting neutron stars in low-mass binary systems. Two kHz QPOs are usually detected in each star. Burst oscillations and two kHz QPOs have recently been detected in the 401 Hz accretion-powered X-ray pulsar SAX J1808.4-3658. In this star the frequency of the burst oscillation is approximately equal to the star's spin frequency nu_spin whereas the frequency separation of the two kHz QPOs is approximately nu_spin/2. If as expected the frequency of the burst oscillations in other stars is also approximately nu_spin, the frequency separation is approximately nu_spin in some stars but approximately nu_spin/2 in others. A frequency separation approximately equal to nu_spin/2 is unexplained in all existing models of the kHz QPOs. Here we propose a modified version of the sonic-point beat-frequency model that can explain within a single framework why the frequency separation is close to nu_spin in some stars but close to nu_spin/2 in others. As in the original sonic-point model, the frequency nu_QPO2 of the upper kHz QPO is close to the orbital frequency nu_orb at the radius r_\sp\ of the sonic point in the disk flow. We show that magnetic and radiation fields rotating with the star will preferentially excite vertical motions in the disk at the "spin-resonance'' radius r_\sr\ where nu_orb - nu_spin is equal to the vertical epicyclic frequency, producing vertical motions in the disk that modulate the X-ray flux at approximately nu_QPO2 - nu_spin or approximately nu_QPO2 - nu_spin/2, depending on whether the disk flow at r_\sr\ is smooth or clumped. This sonic-point and spin-resonance model can also explain quantitatively the decrease of the kHz QPO frequency separation with increasing accretion rate that is observed in many sources.
@article{lambSonicPointSpinResonanceModel2003,
	title = {Sonic-{Point} and {Spin}-{Resonance} {Model} of the {Kilohertz} {QPO} {Pairs}},
	url = {http://adsabs.harvard.edu/abs/2003astro.ph..8179L},
	abstract = {KHz QPOs have now been detected in more than twenty accreting neutron stars in low-mass binary systems. Two kHz QPOs are usually detected in each star. Burst oscillations and two kHz QPOs have recently been detected in the 401 Hz accretion-powered X-ray pulsar SAX J1808.4-3658. In this star the frequency of the burst oscillation is approximately equal to the star's spin frequency nu\_spin whereas the frequency separation of the two kHz QPOs is approximately nu\_spin/2. If as expected the frequency of the burst oscillations in other stars is also approximately nu\_spin, the frequency separation is approximately nu\_spin in some stars but approximately nu\_spin/2 in others. A frequency separation approximately equal to nu\_spin/2 is unexplained in all existing models of the kHz QPOs. Here we propose a modified version of the sonic-point beat-frequency model that can explain within a single framework why the frequency separation is close to nu\_spin in some stars but close to nu\_spin/2 in others. As in the original sonic-point model, the frequency nu\_QPO2 of the upper kHz QPO is close to the orbital frequency nu\_orb at the radius r\_\{sp\} of the sonic point in the disk flow. We show that magnetic and radiation fields rotating with the star will preferentially excite vertical motions in the disk at the
"spin-resonance'' radius r\_\{sr\} where nu\_orb - nu\_spin is equal to the vertical epicyclic frequency, producing vertical motions in the disk that modulate the X-ray flux at approximately nu\_QPO2 - nu\_spin or approximately nu\_QPO2 - nu\_spin/2, depending on whether the disk flow at r\_\{sr\} is smooth or clumped. This sonic-point and spin-resonance model can also explain quantitatively the decrease of the kHz QPO frequency separation with increasing accretion rate that is observed in many sources.},
	urldate = {2020-01-24},
	journal = {arXiv Astrophysics e-prints},
	author = {Lamb, Frederick K. and Miller, M. Coleman},
	month = aug,
	year = {2003},
	keywords = {Astrophysics},
	pages = {arXiv:astro--ph/0308179},
}

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