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 10 keV during the first three pointings .
The spectra of the persistent and bursting emissions below 10 keV were best fit with a model consisting of two blackbodies .
An additional component ( a power law ) was needed to describe the 1–100 keV bursting spectrum when the persistent emission was subtracted .
To our knowledge , this is the first detection of the RB beyond 20 keV .
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 \sim 4 \times 10 ^ { 8 } Gauss .