The complex phase-lag behavior of the 3-12 Hz quasi-periodic oscillations during the very high state of XTE J1550-564

We present a study of the complex phase-lag behavior of the low-frequency ( <20 Hz) quasi-periodic oscillations (QPOs) in the X-ray transient and black hole candidate XTE J1550-564 during its very high state. We distinguish tw o different types of low-frequency QPOs, based on their coherence and harmo nic content. The first type is characterized by a 6 Hz QPO with a Q (the QP O frequency divided by the QPO width) of less than 3 and with a harmonic at 12 Hz. The second type of QPO is characterized by a 6 Hz QPO with a Q-valu e of greater than 6 and with harmonics at 3, 12, 18, and possibly at 9 Hz. Not only are the e-values and the harmonic content of the two types differe nt, but their phase-lag behavior also differs. For the first type of QPO, t he low-energy photons (<5 keV) of both the 6 Hz QPO and its harmonic at 12 Hz lag the hard energy photons (>5 keV) by as much as 1.3 rad. The phase la gs of the second type of QPO are more complex. The soft photons (<5 keV) of the 3 and 12 Hz QPOs lag the hard photons (>5 keV) by as much as 1.0 rad. However, the soft photons of the 6 Hz QPO precede the hard ones by as much as 0.6 rad. This means that different harmonics of this type of QPO have di fferent signs for their phase lags. This unusual behavior is hard to explai n when the lags are due to light-travel time differences between the photon s at different energies, e.g., in a Comptonizing region surrounding the are a in which the QPOs are formed.