The evolution of the X-ray phase lags during the outbursts of the black hole candidate GX 339-4

TL;DR

This study systematically analyzes the evolution of X-ray phase lags in GX 339-4 during outbursts, revealing correlations with spectral states, physical changes in the corona/jet, and similarities with Cygnus X-1, enhancing understanding of black hole accretion processes.

Contribution

First comprehensive analysis of X-ray lag evolution in GX 339-4 across outburst states, linking lag behavior to spectral and physical changes in the system.

Findings

X-ray lags always lag behind soft photons, consistent with previous studies.

Lag maxima coincide with drops in optical/NIR flux and spectral state transitions.

Lag evolution in GX 339-4 resembles that of Cygnus X-1, indicating similar underlying phenomena.

Abstract

Owing to the frequency and reproducibility of its outbursts, the black-hole candidate GX 339-4 has become the standard against which the outbursts of other black-hole candidate are matched up. Here we present the first systematic study of the evolution of the X-ray lags of the broad-band variability component (0.008-5 Hz) in GX 339-4 as a function of the position of the source in the hardness-intensity diagram. The hard photons always lag the soft ones, consistent with previous results. In the low-hard state the lags correlate with X-ray intensity, and as the source starts the transition to the intermediate/soft states, the lags first increase faster, and then appear to reach a maximum, although the exact evolution depends on the outburst and the energy band used to calculate the lags. The time of the maximum of the lags appears to coincide with a sudden drop of the Optical/NIR flux, the fractional RMS amplitude of the broadband component in the power spectrum, and the appearance of a thermal component in the X-ray spectra, strongly suggesting that the lags can be very useful to understand the physical changes that GX 339-4 undergoes during an outburst. We find strong evidence for a connection between the evolution of the cut-off energy of the hard component in the energy spectrum and the phase lags, suggesting that the average magnitude of the lags is correlated with the properties of the corona/jet rather than those of the disc. Finally, we show that the lags in GX 339-4 evolve in a similar manner to those of the black-hole candidate Cygnus X-1, suggesting similar phenomena could be observable in other black-hole systems.