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M. Feroci (IAS-CNR, Rome), F. Frontera (ITESRE-CNR, Bologna and Univ. of Ferrara), M. Tavani (IFC-CNR, Milano and Columbia University, NY), L. Amati, E. Costa (IAS-CNR, Rome)
We report the detection by the Gamma Ray Burst Monitor on board of BeppoSAX of the strongest and longest ever detected outburst from SGR 1900+14. This event, first reported by Cline et al. (1998), is characterized by a sharp rise (~1s) to a peak emission lasting ~2s, and by a fast quasi-exponential decay, followed by a slower exponential longer decay phase. Oscillations are detectable with a period of 5.16-s for the entire duration of the event. A fine temporal analysis reveals a remarkable periodic substructure: after about 35s from the event onset each 5.16-s pulse showed a pattern of four subpulses of period ~1s. No apparent relative phase variations or subpulse damping were observed in the oscillations for about 100s. Strong spectral evolution is detected across 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 strong outburst of the `March 5th event', SGR0525-66. The measurement of the 5.16-s period (Hurley et al. 1998) and of its period derivative (Kouveliotou et al. 1998) support the interpretation of SGR1900+14 as a strongly magnetized neutron star (magnetar) with B >1014 G undergoing a violent instability by either internal or crustal magnetic stresses. However, the coexistence of two apparent periodicities at about 5 and 1s and the observed spectral evolution, challenge current SGR models.
The author(s) of this abstract have provided an email address for comments about the abstract: feroci@ias.rm.cnr.it