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

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/near-infrared flux, the fractional rms amplitude of the broad-band 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.

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ORCID for Diego Altamirano: orcid.org/0000-0002-3422-0074

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