Spectral and QPO analysis of GX5-1 at very high intensity

Abstract

From a short EXOSAT observation of the bright low mass X-ray binary GX 5-1 we have derived information about the spectral and the QPO behaviour of this source during a very high intensity state. GX 5-1 was found to be on the `horizontal branch' in the hardness ratio/intensity diagram. The X-ray luminosity (1-20 keV) varied from 3.8 ^*10³⁸ to 4.5^*10³⁸ erg/s (for 10 kpc) on a time scale of hour.

The spectral analysis showed that at all intensities two spectral components are required to fit the data. One component always is a black body spectrum with a temperature around kT=1.1 keV. Intensity variation are mainly due to this constant temperature BB component. For a distance of 10 kpc we derive a BB radius of 35 km. For the second spectral component we cannot single out a particular model because the data are fitted equally well different spectral functions.

We found the QPO phenomenon to be present at a weak level, with a rms value of 2.4% for the QPO peak and 2.8% for low frequency noise. The centroid frequency of the QPO peak was found at 41.5 Hz. These values are in good agreement with QPO parameters extrapolated from lower intensity levels.