A giant outburst from SGR 1900+14 observed with the BeppoSAX Gamma-Ray Burst Monitor

We report the detection by the Gamma-Ray Burst Monitor on board BeppoSAX of the strongest and longest outburst ever detected from SGR 1900+14. Oscilla tions are detectable with a period of similar to 5.16 s for the entire dura tion of the event (similar to 300 s). The temporal analysis reveals also a remarkable periodic substructure: after about 35 a from the event onset, ea ch 5.16 s pulse shows a pattern of four subpulses and a dip, each separated by similar to 1 s. Significant spectral variation is detected during the e vent and for each individual oscillation. The first and most intense part o f the outburst is quite hard and similar to what was previously detected fr om the "March 5 event." A hard nonthermal spectral component persists for s imilar to 200 s. SGR 1900+14 was proposed to be a strongly magnetized neutr on star (B greater than or equal to 10(14) G) undergoing violent instabilit ies by internal magnetic/crustal stresses. However, the onset of an apparen t 1 s periodicity within the 5.16 s pulsations and the observed spectral pr operties show a complex behavior that is not satisfactorily modeled yet.