Detection of a Strong Excess Noise Component in the Low State of 4U 1705-44

Abstract

The timing analysis of four Exosat observations of the bright Galactic burst source 4U 1705-44 is presented. A correlation is found between the aperiodic noise components and the spectral variability. 4U 1705-44 shows in its low-intensity state a strong variability of the X-ray flux in the frequency range 1-64 Hz, corresponding to an rms modulation of 20 percent. During this low-intensity state, where the spectrum is hardest and type I bursts are most frequent, the hardness of the energy spectrum shows a negative correlation with intensity. In the high-intensity state, the shape of the power spectrum is best described by a power law. In this state, no significant signal at frequencies greater than 1 Hz is measured and the spectral hardness shows no clear correlation with intensity.