Coherence between QPOs in X-Ray Pulsars

Rossi X-ray Timing Explorer (RXTE) observations of Cen X-3 and 4U 1626 have shown that when a low frequency (upsilon(sub qpo) approx. 50 mHz) Quazi-Periodic Oscillation (QPO) is presented in the power spectrum of these sources, smaller QPOs also appear in the upper and lower sidebands of the spin frequency upsilon(sub s), and in the sidebands of the 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 (upsilon = upsilon(sub s) - upsilon(sub qpo)) is stronger than the QPO in the upper sideband (upsilon = upsilon(sub s) + upsilon(sub qpo)), which is not consistent with pure acretion rate modulation. I will present a method for measuring the phase coherent 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.