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We investigate the nature of the inner accretion disk in the neutron starsource GX 5-1 by making a detailed study of time lags between X-rays ofdifferent energies. Using the cross-correlation analysis, we foundanti-correlated hard and soft time lags of the order of a few tens to a fewhundred seconds and the corresponding intensity states were mostly thehorizontal branch (HB) and upper normal branch (NB). The model independent anddependent spectral analysis showed that during these time lags the structure ofaccretion disk significantly varied. Both eastern and western approaches wereused to unfold the X-ray continuum and systematic changes were observed in softand hard spectral components. These changes along with a systematic shift inthe frequency of quasi-periodic oscillations (QPOs) made it substantiallyevident that the geometry of the accretion disk is truncated. Simultaneousenergy spectral and power density spectral study shows that the production ofthe horizontal branch oscillations (HBOs) are closely related to theComptonizing region rather than the disk component in the accretion disk. Wefound that as the HBO frequency decreases from the hard apex to upper HB, thedisk temperature increases along with an increase in the coronal temperaturewhich is in sharp contrast with the changes found in black hole binaries wherethe decrease in QPO frequency is accompanied by a decrease in the disktemperature and a simultaneous increase in the coronal temperature. We discussthe results in the context of re-condensation of coronal material in the innerregion of the disk.

Anti-correlated time lags in the Z source GX 5-1: Possible evidence for a truncated accretion disk