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

TL;DR

This study analyzes X-ray time lags in the neutron star source GX 5-1, revealing evidence for a truncated accretion disk and linking spectral changes to disk geometry and QPO behavior.

Contribution

It provides the first detailed analysis of anti-correlated X-ray time lags and their relation to accretion disk truncation in GX 5-1, with insights into disk-corona interactions.

Findings

Anti-correlated X-ray time lags of tens to hundreds of seconds observed.

Spectral analysis indicates significant variation in accretion disk structure during lags.

QPO frequency shifts are linked to changes in disk and coronal temperatures.

Abstract

We investigate the nature of the inner accretion disk in the neutron star source GX 5-1 by making a detailed study of time lags between X-rays of different energies. Using the cross-correlation analysis, we found anti-correlated hard and soft time lags of the order of a few tens to a few hundred seconds and the corresponding intensity states were mostly the horizontal branch (HB) and upper normal branch (NB). The model independent and dependent spectral analysis showed that during these time lags the structure of accretion disk significantly varied. Both eastern and western approaches were used to unfold the X-ray continuum and systematic changes were observed in soft and hard spectral components. These changes along with a systematic shift in the frequency of quasi-periodic oscillations (QPOs) made it substantially evident that the geometry of the accretion disk is truncated. Simultaneous energy spectral and power density spectral study shows that the production of the horizontal branch oscillations (HBOs) are closely related to the Comptonizing region rather than the disk component in the accretion disk. We found that as the HBO frequency decreases from the hard apex to upper HB, the disk temperature increases along with an increase in the coronal temperature which is in sharp contrast with the changes found in black hole binaries where the decrease in QPO frequency is accompanied by a decrease in the disk temperature and a simultaneous increase in the coronal temperature. We discuss the results in the context of re-condensation of coronal material in the inner region of the disk.