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We present the evolution of the phase lags associated with the type-C QPO inGX 339--4 during the rising phase of the 2006/2007 outburst. We find that thephase lags at the QPO frequency are always positive (hard), and show verydifferent behavior between QPOs with frequencies below and above $\sim1.7$ Hz:when the QPO frequency is below $\sim1.7$ Hz, the phase lags increase both withQPO frequency and energy, while when the QPO frequency is above $\sim1.7$ Hz,the phase lags remain more or less constant. When the QPO frequency is higherthan $\sim1.7$ Hz, a broad feature is always present in the lag-energy spectraat around 6.5 keV, suggesting that the reflection component may have asignificant contribution to the phase lags. Below $\sim1.7$ Hz, the QPO rmsfirst decreases with energy and then turns to almost flat, while above$\sim1.7$ Hz, the QPO rms increases with energy. During the transition from thelow-hard state to the hard-intermediate state, the second harmonic andsubharmonic of this QPO appear in the power density spectra. Thesecond-harmonic and subharmonic phase lags show very similar evolution withtheir centroid frequencies. However, the energy dependence of thesecond-harmonic and subharmonic phase lags are quite different. Our resultssuggest that, at different phases of the outburst, different mechanisms may beresponsible for the phase lags of the QPO. We briefly discuss the possiblescenarios for producing the lags.

The Evolution of the Phase Lags Associated with the Type-C Quasi-periodic Oscillation in GX 339--4 during the 2006/2007 Outburst