The properties of outbursts and long-term activity of the soft X-ray transient 4U 1608–52 (QX Nor)

Abstract

An analysis of the long-term activity of 4U 1608–52, based on the data, is presented. Although the individual outbursts in 4U 1608–52 differ in both their duration and peak intensity, their rising and final decay branches display remarkable similarities. The variations of the soft X-ray color between 1.5–4 keV are found to follow the luminosity during the decay of outburst much better than the variations between 4–12 keV. The recurrence time T C of outbursts was analyzed by the method of the residuals. The mean value of T C is 330 days between the years 1996–2003 but a strong long-term trend of a decreasing TC can be resolved. A series of four or five consecutive outbursts displays a clear correlation between the decreasing TC and increasing peak intensity, but no correlation with the outburst duration. The observed behaviour is discussed in terms of the thermal instability model. We argue that the disk viscosity achieves similar values for the individual outbursts in 4U 1608–52. Only during the brightest outbursts does the luminosity appear to be high enough for the disk to be ionized out to its outer rim by the X-rays from the neutron star, according to the model by King & Ritter ([CITE]). We also argue that the disk is never considerably depleted in outburst. A comparison with the similar system, Aql X-1, helps us to find the common links between the disk behaviour in the individual neutron-star soft X-ray transients.