2000A&A...363..188M

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 later, during the second BeppoSAX pointing, only 5 Type II bursts were detected at the beginning of the observation. During the third pointing no X-ray bursts were detected and in the fourth and final observation the RB was not detected at all. Persistent emission from the RB was detected up to 10keV during the first three pointings. The spectra of the persistent and bursting emissions below 10keV were best fit with a model consisting of two blackbodies. An additional component (a power law) was needed to describe the 1-100keV bursting spectrum when the persistent emission was subtracted. To our knowledge, this is the first detection of the RB beyond 20keV. We discuss the evolution of the spectral parameters for the bursting and persistent emission during the outburst decay. The light curve, after the second BeppoSAX pointing, showed a steepening of the previous decay trend, and a sharper decay rate leading to quiescence was observed with BeppoSAX in the two subsequent observations. We interpret this behaviour as caused by the onset of the propeller effect. Finally, we infer a neutron star magnetic field B∼4x10⁸Gauss.