The Soft Gamma-ray Repeater SGR 1900+14 entered a remarkable phase of activity during the summer of 1998 .
This activity peaked on August 27 , 1998 when a giant periodic \gamma -ray flare resembling the famous March 5 , 1979 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 \sim 0.1 s ) , this event features a 3.5 s burst peak that was preceded by an extended ( \sim 1 s ) complex precursor , and followed by a long ( \sim 10 ^ { 3 } s ) periodic tail modulated at the 5.16 s stellar rotation period .
The tail also shows several short recurrent bursts .
Spectral analysis shows a striking distinction between the spectral behavior of the precursor , main peak and long tail .
While the spectrum during the peak is uniform , a significant hard-to-soft spectral evolution is detected in both the precursor and tail emissions .
Temporal behavior shows a sharp rise ( \sim 9.8 ms ) at the event onset and a rapid cutoff ( \sim 17 ms ) at the end of the burst peak .
The tail pulsations show a simple pulse profile consisting of one 5.16 s peak that did not evolve with time .
The spectral and temporal signatures of this event imply that the precursor , main peak , and extended tail are produced by different physical mechanisms .
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 ( T \sim 1 MeV ) trapped fireball , and the precursor with magnetospheric emission in which the radiating particles are heated more continuously .
Less than a percent 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 hotspot of a very small area ( \sim 1.3 km ^ { 2 } ) , and indicates that the energy release in an SGR burst is strongly localized .
The energetics of the August 29 event , and its close proximity to the August 27 flare , suggest that it is an ‘ aftershock ’ of the preceding giant flare .