Further Evidence that Radiation Forces Affect the Frequencies Of the Kilohertz QPOs

Details Further Evidence that Radiation Forces Affect the Frequencies Of the Kilohertz QPOs Further Evidence that Radiation Forces Affect the Frequencies Of the Kilohertz QPOs

Sep 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006head....9.0146y&link_type=abstract

American Astronomical Society, HEAD meeting #9, #1.46; Bulletin of the American Astronomical Society, Vol. 38, p.336

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In the sonic-point and spin-resonance beat-frequency (SPSRBF) model of the kHz QPOs, the frequency of the upper kHz QPO is close to the azimuthal frequency of the innermost nearly circular streamline (INCS). The INCS is created by the combined effects of the radiation and magnetic forces acting on the gas in the disk and the general relativistic 'pit' in the effective gravitational potential near the star. Variations in the luminosity of the star can be produced by variations in the accretion rate through the inner disk or other mechanisms, such as variations in the nuclear burning rate. In the SPSRBF model, the time-varying radiation force caused by any luminosity variations that are independent of the accretion rate through the inner disk causes variations in the frequency of the upper kHz QPO that are out of phase with those luminosity variations. Such variations have been seen in Sco X-1 and 4U 1608-52 at the frequencies of the 6 Hz normal branch oscillation and mHz QPOs. The highest frequency of the upper kHz QPO observed at a given X-ray flux (or count rate) of 4U 1820-30 and 4U 1636-53 is anti-correlated with the flux (or count rate). If we suppose that the highest frequency kHz QPOs reflect the azimuthal frequency of the gas very close to the general relativistic innermost stable circular orbit, so that the radius of the gas is very nearly constant, then the observed anti-correlation of the maximum frequency of the upper kHz QPO and the X-ray flux is consistent with the idea that the gas circulating there is partially supported by the radial component of the radiation force.
This research was supported in part by NASA grants NAG 5-12030 and NNG05GL60G, NSF grant AST 0098399, and funds of the Fortner Endowed Chair at the University of Illinois.

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