DETECTION OF SPECTRAL EVOLUTION IN THE BURSTS EMITTED DURING THE 2008-2009 ACTIVE EPISODE OF SGR J1550–5418

Abstract

In early 2008 October, the soft gamma repeater SGR J1550−5418 (1E 1547.0−5408, AX J155052−5418,
\nPSR J1550−5418) became active, emitting a series of bursts which triggered the Fermi Gamma-ray Burst Monitor
\n(GBM) after which a second especially intense activity period commenced in 2009 January and a third, less active
\nperiod was detected in 2009 March–April. Here, we analyze the GBM data for all the bursts from the first and
\nlast active episodes. We performed temporal and spectral analysis for all events and found that their temporal
\ncharacteristics are very similar to the ones of other SGR bursts, as well the ones reported for the bursts of the main
\nepisode (average burst durations ∼170 ms). In addition, we used our sample of bursts to quantify the systematic
\nuncertainties of the GBM location algorithm for soft gamma-ray transients to 8◦. Our spectral analysis indicates
\nsignificant spectral evolution between the first and last set of events. Although the 2008 October events are best
\nfitted with a single blackbody function, for the 2009 bursts an optically thin thermal bremsstrahlung is clearly
\npreferred.We attribute this evolution to changes in the magnetic field topology of the source, possibly due to effects
\nfollowing the very energetic main bursting episode.