Long-term activity of the soft X-ray transient 4U 1608-52

Abstract

Abstract An analysis of the long-term activity of the neutron-star (NS) soft X-ray transient 4U 1608–52, based on the ASM/RXTE data, is presented. It is found that although the individual X-ray outbursts differ in both their duration and the peak intensity, their final decay branches display remarkable similarities. The prevailing value of the recurrence time T C of outbursts was found to be 330 days between the years 1996–2003 but a long-term trend of a decreasing T C can be resolved. A series of four or five consecutive outbursts displays a correlation between the decreasing T C and increasing peak intensity of outburst, but no correlation with the outburst duration. The observed behaviour is discussed in terms of the thermal instability model. We argue that just the inner disk is kept in the hot state by the irradiation by the neutron star during most events and the viscosity near the outer edge of the radius kept in the hot state by irradiation achieves quite similar values for the individual events.