Astron. Astrophys. 319, 498-506 (1997)

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

S. Prins and M. van der Klis

Astronomical Institute "Anton Pannekoek", University of Amsterdam, and Center for High Energy Astrophysics, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands

Received 22 July 1996 / Accepted 31 July 1996

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 spectra l component that dominates the power spectra below 1 Hz 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 candidate s in the low state.

Key words: accretion, accretion disks stars: individual: 4U 1636-53 stars: neutron X-rays: stars X-rays: bursts

Send offprint requests to: S. Prins (saskia@astro.uva.nl)

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998

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