The complex long-term X-ray activity of the compact binary 4U 1915–05

An analysis of the long-term X-ray activity of the compact binary 4U 1915–05 from 1996–2003, using the ASM/RXTE data, shows that the behaviour of the long-term X-ray light curve inside this interval was considerably different from that reported previously. We find that the X-ray light curve is dominated by complicated variations in the form of outbursts with rather sharp peaks, each lasting for about 20–30 days. The period search by weighted wavelet transform (WWZ) reveals that the cycle-length of the activity is highly variable and the periods can persist only through several epochs. We find that a cycle of about 230–250 days (similar to the ~199 day cycle reported by Priedhorsky & Terrell 1984, ApJ, 280, 661) occurred only in a limited time interval – this cycle manifests in the variations of the recurrence time and amplitude of the outbursts. Generally, the system spends more time in the low state than in the high state (outbursts), and the standard deviation and skewness display variations on the time scale of the order of hundreds of days. We show that the decays of the individual outbursts in 4U 1915–05 display a relatively broad range of slopes. The slow average decay rate $\tau_{\rm D}\approx 0.2$ ct/s/day can be related to the viscous time of the disk. We discuss the models and offer an interpretation in terms of the thermal instability of the disk, modified by irradiation by the neutron star, following the model by King & Ritter (1998, MNRAS, 293, L42). We also compare the activity of 4U 1915–05 with that of a similar system, 4U 1820–30, and interpret the largely divergent kinds of activity in terms of the different mean mass transfer rates. This rate is lower in 4U 1915–05 and can allow thermal instability of the disk, unlike the case of 4U 1820–30. We argue that the disk in 4U 1915–05 can oscillate between thermally stable and unstable state