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F. K. Lamb (U. Illinois), M. C. Miller (U. Maryland)
Kilohertz QPOs have now been detected in more than twenty accreting neutron stars in low-mass binary systems. Two such QPOs are usually detected in each star. The two detected in the 401~Hz accretion-powered X-ray pulsar SAX~J1808.4-3658 have a frequency separation consistent with half its 401~Hz stellar spin frequency \nu\rm spin. Two kilohertz QPOs have also been detected in the 191~Hz accretion-powered X-ray pulsar XTE~J1807-294 with a frequency separation consistent with its 191~Hz stellar spin frequency. Thus the frequency separation is apparently \approx \nu\rm spin in some stars but \approx \nu\rm spin/2 in others. A frequency separation approximately equal to \nu\rm spin/2 is unexplained in all existing models of the kilohertz 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\rm spin in some stars but close to \nu\rm spin/2 in others. As in the original sonic-point model, the frequency \nu\rm QPO2 of the upper kilohertz QPO is close to the orbital frequency \nu\rm orb at the radius rsp 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 rsr where \nu\rm orb - \nu\rm spin is equal to the vertical epicyclic frequency. If the flow at rsr is relatively smooth, the vertical motions excited at rsr modulate the X-ray flux at \nu\rm QPO1 \approx \nu\rm QPO2 - \nu\rm spin. If instead the gas at rsr is highly clumped, the vertical motions excited at rsr modulate the X-ray flux at \nu'\rm QPO1 \approx \nu\rm QPO2 - \nu\rm spin/2. This \textit {sonic-point and spin-resonance} beat-frequency model can also explain quantitatively the decrease of the kilohertz QPO frequency separation with increasing accretion rate that is observed in many sources. This research was supported in part by NASA grant NAG~5-12030, NSF grant AST~0098399, and funds of the Fortner Endowed Chair at Illinois, and by NSF grant AST~0098436 at Maryland.
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Bulletin of the American Astronomical Society, 36 #3
© 2004. The American Astronomical Soceity.