Sonic-Point and Spin-Resonance Model of the Kilohertz QPO Pairs

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dc.creatorLamb, Frederick K.
dc.creatorMiller, M. Coleman
dc.date2003-08-11
dc.date.accessioned2026-07-07T02:09:12Z
dc.date.available2026-07-07T02:09:12Z
dc.descriptionKHz 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.
dc.description10 pages, 2 figures, LaTeX, uses AASTeX; submitted to ApJ Letters
dc.identifierhttps://arxiv.org/abs/astro-ph/0308179
dc.identifierhttp://arxiv.org/abs/astro-ph/0308179
dc.identifier.urihttp://salesiana.dossiersoluciones.com/handle/123456789/6346
dc.subjectAstrophysics
dc.titleSonic-Point and Spin-Resonance Model of the Kilohertz QPO Pairs
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