Anti-correlated Soft Lags in the Intermediate State of Black Hole Source GX 339-4

TL;DR

This paper reports the detection of anti-correlated soft lags in GX 339-4 during intermediate states, linking spectral and temporal variability to changes in accretion disk geometry and corona condensation.

Contribution

It provides the first observation of anti-correlated soft lags in GX 339-4 and explains these phenomena within the truncated disk accretion framework.

Findings

Anti-correlated soft lags observed in GX 339-4 during intermediate states.

Spectral and temporal properties change during lag timescales.

Evidence supports a truncated accretion disk geometry with corona condensation.

Abstract

We report the few hundred second anti-correlated soft lags between soft and hard energy bands in the source GX 339-4 using RXTE observations. In one observation, anti-correlated soft lags were observed using the ISGRI/INTEGRAL hard energy band and the PCA/RXTE soft energy band light curves. The lags were observed when the source was in hard and soft intermediate states, i.e., in a steep power-law state.We found that the temporal and spectral properties were changed during the lag timescale. The anti-correlated soft lags are associated with spectral variability during which the geometry of the accretion disk is changed. The observed temporal and spectral variations are explained using the framework of truncated disk geometry. We found that during the lag timescale, the centroid frequency of quasi-periodic oscillation is decreased, the soft flux is decreased along with an increase in the hard flux, and the power-law index steepens together with a decrease in the disk normalization parameter. We argue that these changes could be explained if we assume that the hot corona condenses and forms a disk in the inner region of the accretion disk. The overall spectral and temporal changes support the truncated geometry of the accretion disk in the steep power-law state or in the intermediate state.