Long-term activity of the neutron star soft X-ray transients

The properties of the outburst behaviour of six soft X-ray transients (SXTs) with a neutron star (NS) accretor are analyzed using the ASM/RXTE observations, with an impact on the observations with INTEGRAL and on the observing strategy. It is found that the individual outbursts in each NS SXT are dependent on each other because they display well-defined trends in the O - C diagrams for the outburst recurrence time T-C. In most cases, there is no relation (or at most a weak) between the peak outburst intensity I-max and TC in a given SXT. Nevertheless, the NS SXTs with shorter TC tend to possess more uniform for the individual events. The mean TC in the NS SXTs analyzed here is 60350 days, which means that up to 19 outbursts have been observed in some of them by ASM/RXTE and hence it can be expected that at least a similar number of events may be observed by INTEGRAL. Generally, the hardness (X-ray color) increases with the increasing intensity, if exceeds some threshold, and the variations of the X-ray color between 1.5-4 keV tend to follow better than the color changes between 4-12 keV. It is also shown that hard X-ray spectral states in the early rise and late decline of outbursts can be particularly suitable for INTEGRAL. Aql X-1 is brighter than 17 mag(V) during outburst and hence it cold be detected by OMC in co-added frames.