Long-term X-ray activity of the ultra-compact binary 4U 1820-30

An analysis of the long-term X-ray activity of the ultra-compact binary 4U 1820-30 within the years 1996-2002, using the ASM/RXTE data, is presented. The X-ray light curve displays complicated large-amplitude variations, even on short time scales of a few days. Episodes of brief low states (BLS), lasting for several days, are superposed on the general course of the cycle, the length of which we determined to be Tc=172.780 days by the method of the O-C residuals. We found no significant secular trend in Tc; this is consistent with the stable period found over longer time intervals by Chou & Grindlay (2001) and is thus consistent with the triple model. The statistical distribution of 1.5-12 keV intensity, Isum, for all data is not far from Gaussian although some deviations exist. The Isum curve, folded with the 172 day cycle and smoothed, shows a rapid rise, double-peaked maximum, and a slow decline. The course of the smoothed residuals of this curve, the so called (simga)sum curve, is more complicated than the Isum curve, with several minima and maxima. Two sharp maxima of (sigma)sum coincide with the rising branch and the dip after the maximum of the Isum curve. We have shown that the scatter of the folded Isum curve is dominated by the BLS. The activity is discussed in terms of several models. Thermal instability of the disk can be excluded. The increases of the mass transfer from the donor star are likely to explain the 172 day cycle of activity. However, we argue that the highly asymmetric X-ray light curve of this cycle, with the long and complicated profile of the decay branch, cannot be explained purely by the influence of a third body via the Mazeh & Shaham (1979) mechanism. Instead, a hybrid model in which the irradiation-driven instability of the donor star governs the complicated profile of the decay with the superposed BLS appears to be more viable.