Other
Scientific paper
May 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000aas...195.0711l&link_type=abstract
American Astronomical Society, 195th AAS Meeting, #07.11; Bulletin of the American Astronomical Society, Vol. 32, p.874
Other
Scientific paper
We show that in the sonic-point beat-frequency model of the kilohertz QPOs, the frequency separation is close to but not exactly equal to the stellar spin frequency and generally decreases slightly with increasing mass accretion rate. We explore a simple physical model of this variation in which density fluctuations (``clumps'') in the inner disk are optically thick and interact only weakly with each other and the interclump gas but couple to the magnetic field of the star. Such clumps are expected to remain in pressure equilibrium with the stellar magnetic field and to experience a magnetic torque proportional to their surface area and the local magnetic stress. In this model there are three parameters: the mass and spin frequency of the neutron star and the coefficient of the magnetic couple, which is assumed to be constant. We compare the kilohertz QPO frequency behavior predicted by this model with Rossi X-ray Timing Explorer data on 4U 1728-34 and Sco X-1. For 4U 1728-34, the best fit of the model gives a gravitational mass of 1.74 Msun and a total χ2 of 10.5 for 6 degrees of freedom, assuming that the spin frequency is the 364 Hz frequency of the burst brightness oscillations in this source. For Sco X-1, the best fit of the model gives a gravitational mass of 1.69 Msun, a spin frequency of 348 Hz, and a total χ2 of 34.6 for 33 degrees of freedom. This research was supported in part by the NSF and NASA.
Lamb Frederick K.
Miller Michael Coleman
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