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Mark H. Finger (USRA/MSFC)
Rossi X-ray Timing Explorer (RXTE) observations of Cen X-3 and 4U 1626-67 have shown that when a low frequency (\nu\rm qpo~50 mHz) Quasi-Periodic Oscillation (QPO) is present in the power spectrum of these sources, smaller QPOs also appear in the upper and lower sidebands of the spin frequency \nu\rm s, and in the sidebands of spin frequency harmonics. This is expected if the QPO is a modulation in the accretion rate. However, the QPO in the lower sideband of the spin frequency (\nu=\nu\rm s-\nu\rm qpo) is stronger than the QPO in the in the upper sideband (\nu = \nu\rm s+\nu\rm qpo), which is not consistent with pure accretion rate modulation.
I will present a method for measuring the phase coherence between the main QPO and any of the sideband QPOs, and apply this to RXTE observations. Preliminary results for Cen X-3 show that the QPO in the lower sideband of the spin frequency is highly coherent with the main QPO, while the QPO in the upper sideband is not. Implications for possible QPO mechanisms will be discussed.
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