Hard X-rays from Type II bursts of the Rapid Burster and its transition toward quiescence

We report on 4 BeppoSAX Target Of Opportunity observations of MXB 1730-335, the Rapid Burster (RB), made during the 1998 February-March outburst.. In the first observation, approximately 20 days after the outburst peak, the X -ray light curve showed Type II bursts at a rate of 43 hr(-1). Nine days la ter, during the second BeppoSAX pointing, only 5 Type II bursts were detect ed at the beginning of the observation. During the third pointing no X-ray bursts were detected and in the fourth and final observation the RE was not detected at all. Persistent emission from the RE was detected up to 10 keV during the first three pointings. The spectra of the persistent and bursti ng emissions below 10keV were best fit with a model consisting of two black bodies. An additional component (a power law) was needed to describe the 1- 100 keV bursting spectrum when the persistent emission was subtracted. To o ur knowledge, this is the first detection of the RE beyond 20 keV. We discu ss the evolution of the spectral parameters for the bursting and persistent emission during the outburst decay. The light curve, after the second Bepp oSAX pointing, showed a steepening of the previous decay trend, and a sharp er decay rate leading to quiescence was observed with BeppoSAX in the two s ubsequent observations. We interpret this behaviour as caused by the onset of the propeller effect. Finally, we infer a neutron star magnetic field B similar to 4 x 10(8) Gauss.