Using all the observations from Rossi X-ray Timing Explorer for the accreting neutron star XTE J1701-462, we carry out a systematic study on the cross-correlation function between its soft and hard light curves. During the entire outburst, XTE J1701-462 evolves from super-Eddington luminosities to quiescence, and shows both Z and atoll behaviours. Following the previous work, we divide the outburst into five intervals, one Cyg-like interval, three Sco-like intervals, and one atoll interval, according to their different behaviors in the corresponding color-color diagrams (CCDs). With cross-correlation analyses, the anti-correlation, positive and ambiguous correlations are found in the different intervals in this source. Both the anti-correlated soft and hard time lags are detected, where the hard lags mean that the hard photons lag behind the soft ones and the soft lags are reverse. In the Cyg-like interval, the anti-correlations are presented in the hard vertex and upper normal branch (NB), and the positive correlations dominate the horizontal branch (HB) and lower NB. In the first two Sco-like intervals, the anti-correlations are firstly detected and most of them show up the HB and/or upper NB, and the positive correlations are mostly detected in the lower NB and flaring branch (FB). While in the following interval, i.e., the third Sco-like interval, the anticorrelations occur on the upper FB, and the positive correlations mainly distribute in the lower FB. The different intervals are corresponding to the various luminosities, therefore, the position of anti-correlations in the CCD might depend on the luminosity. It is noted that, in the Cyg-like interval, the positive correlations dominate the HB, which is not consistent with that of the Cyg-like Z sources GX 5-1 and Cyg X-2 whose HBs host ambiguous correlations and anti-correlations. Hence, for comparing with GX 5-1, we analyze the spectra of the HB and the hard vertex of the Cyg-like interval. The fitting results show that, different from GX 5-1, the ratio of the hard emission to the soft emission basically keeps unvaried from the HB to the hard vertex, which might result in the positive correlation. Additionally, we compare the spectra of the third Sco-like interval with atoll source 4U 1735-44, and find their spectral evolution along the tracks in the CCD is similar, indicating that in this interval, XTE J1701-462 could approach an atoll source. The truncated disk model might be responsible for the detected time lags in XTE J1701-462, and the possible origins of the anti-correlated hard time lags and soft time lags are also discussed.

Using all the observations from Rossi X-ray Timing Explorer for Z source GX 349+2, we systematically carry out cross-correlation analysis between its soft and hard X-ray light curves. During the observations from January 9 to January 29, 1998, GX 349+2 traced out the most extensive Z track on its hardness-intensity diagram, making a comprehensive study of cross-correlation on the track. The positive correlations and positively correlated time lags are detected throughout the Z track. Outside the Z track, anti-correlations and anti-correlated time lags are found, but the anti-correlated time lags are much longer than the positively correlated time lags, which might indicate different mechanisms for producing the two types of time lags. We argue that neither the short-term time lag models nor the truncated accretion disk model can account for the long-term time lags in neutron star low mass X-ray binaries (NS-LMXBs). We suggest that the extended accretion disk corona model could be an alternative model to explain the long-term time lags detected in NS-LMXBs.