Abstract
We report the detection by the Gamma Ray Burst Monitor onboard BeppoSAX of the strongest and longest ever detected outburst from SGR 1900+14. Oscillations are detectable with a period of ~5.16 s for the entire duration of the event ~300 s. The temporal analysis reveals also a remarkable periodic substructure: after about 35 s from the event onset each 5.16-s pulse shows a pattern of four subpulses and a dip, each separated by ~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 previously detected from the `March 5th event'. A hard non-thermal spectral component persists for ~200 s. SGR 1900+14 was proposed to be a strongly magnetized neutron star (B>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 behaviour that is not satisfactorily modelled yet.
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