Correlated X-ray spectral and fast-timing behaviour of 4U 1636-53.

Abstract

We present a study of the persistent X-ray emission from the low-mass X-ray binary 4U 1636-53 based on the entire archival EXOSAT ME data set. We performed a homogeneous analysis of all five EXOSAT observations in terms of the correlated rapid X-ray variability and X-ray spectral properties by means of power spectra, and colour-colour and hardness-intensity diagrams, respectively. Over the years we find similar patterns in the colour-colour diagram and the hardness-intensity diagram, but small shifts in their positions occur. We find one case of an island-banana transition. On all but one occasion the differences in colours are smaller than 5%. Between the two "island state" data sets we find a shift of 8% in the soft colour, which must be at least partly intrinsic. Clear correlations are present between the X-ray colours (which are governed by the mass accretion rate) and the power spectrum. With increasing mass accretion rate the fractional rms amplitude of the power-law shaped power spectral component that dominates the power spectra below ~1Hz rises, while the band-limited noise component at higher frequencies drops. The cut-off frequency of the band-limited component increases when its amplitude decreases. This is reminiscent of the behaviour of black-hole candidates in the low state.