Long-term activity of the Rapid Burster

The recurrence time T_C of outbursts of the remarkable and unique system, the Rapid Burster (MXB 1730-335), observed by ASM/RXTE, is analyzed by the method of the O-C residuals. The variations of T_C are large and occur all the time, but generally they are not chaotic; the mean value of T_C is 160 days between the years 1996-2005 but a large shortening of T_C, accompanied by a large decrease of the maximum intensity I_max and the relative energy RE (energy output) of most outbursts, occurred in this interval. The outbursts are found to display a correlation between RE and I_max, but no correlation with the outburst duration. The observed behaviour is discussed in terms of the thermal instability of the accretion disk. A comparison of this prototype with other neutron star soft X-ray transients, like Aql X-1 and 4U 1608-52, helps us find the common links in the disk behaviour in such systems.