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. Oscillations are detectable with a period of similar to 5.16 s for the entire duration of the event (similar to 300 s). The temporal analysis reveals also a remarkable periodic substructure: after about 35 a from the event onset, each 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 event and for each individual oscillation. The first and most intense part of the outburst is quite hard and similar to what was previously detected from the "March 5 event." A hard nonthermal spectral component persists for similar to 200 s. SGR 1900+14 was proposed to be a strongly magnetized neutron star (B greater than or equal to 10(14) G) undergoing violent instabilities by internal magnetic/crustal stresses. However, the onset of an apparent 1 s periodicity within the 5.16 s pulsations and the observed spectral properties show a complex behavior that is not satisfactorily modeled yet.