An Unusual Burst from Soft Gamma Repeater SGR 1900+14: Comparisons with Giant Flares and Implications for the Magnetar Model

The soft gamma-ray repeater SGR 1900+14 entered a remarkable phase of activity during the summer of 1998. This activity peaked on 1998 August 27, when a giant periodic γ-ray flare resembling the famous 1979 March 5 event from SGR 0526-66 was recorded. Two days later (August 29), a strong, bright burst was detected simultaneously with the Rossi X-Ray Timing Explorer (RXTE) and the Burst and Transient Source Experiment (BATSE). This event reveals several similarities to the giant flares of August 27 and March 5 and shows a number of unique features not previously seen in SGR bursts. Unlike typically short SGR bursts (duration ~0.1 s), this event exhibits a 3.5 s burst peak that was preceded by an extended (~1 s) complex precursor, and followed by a long (~10³ s) pulsating tail modulated at the 5.16 s stellar rotation period. Spectral analysis shows a striking distinction between the spectral behavior of the precursor, main peak, and long tail. While the spectrum is uniform during the peak, a significant hard-to-soft spectral evolution is detected in both the precursor and tail emissions. Temporal behavior shows a sharp rise (~10 ms) at the precursor onset, a rapid cutoff (~17 ms) at the end of the burst peak, and a gradual decay (~17 minutes) of the pulsating tail. The tail pulsations show a simple pulse profile that did not evolve with time. The contrasted spectral and temporal signatures of the event suggest that the precursor, main peak, and extended tail are produced by different physical mechanisms, and that the observed tail represents a new emission component from SGRs. We discuss these features and their implications in the context of the magnetar model. The bright 3.5 s component is consistent with a very hot (kT ~ 1 MeV) trapped fireball, and the precursor with magnetospheric emission in which the radiating particles are heated more continuously. Less than 1% of the fireball energy will be conducted into the exposed surface of the neutron star, thereby dissociating heavy elements and even helium, and inducing rapid transformations between neutrons and protons. The extended "afterglow" tail of the August 29 burst is consistent with a cooling hot spot of small area (~13 km²), and indicates that the energy release in an SGR burst is strongly localized.