Detection of spectral evolution in the bursts emitted during the 2008-2009 active episode of SGR J1550 - 5418

TL;DR

This study analyzes spectral and temporal properties of bursts from SGR J1550-5418 during its 2008-2009 active episodes, revealing spectral evolution likely linked to magnetic field topology changes.

Contribution

It provides the first detailed spectral evolution analysis of bursts from SGR J1550-5418 across different active episodes, highlighting changes in spectral models over time.

Findings

Spectral evolution observed between 2008 and 2009 bursts.

2008 bursts fit single blackbody model, 2009 bursts fit OTTB model.

Temporal characteristics consistent with other SGR bursts.

Abstract

In early October 2008, the Soft Gamma Repeater SGRJ1550 - 5418 (1E 1547.0 - 5408, AXJ155052 - 5418, PSR J1550 - 5418) became active, emitting a series of bursts which triggered the Fermi Gamma-ray Burst Monitor (GBM) after which a second especially intense activity period commenced in 2009 January and a third, less active period was detected in 2009 March-April. Here we analyze the GBM data all the bursts from the first and last active episodes. We performed temporal and spectral analysis for all events and found that their temporal characteristics are very similar to the ones of other SGR bursts, as well the ones reported for the bursts of the main episode (average burst durations \sim 170 ms). In addition, we used our sample of bursts to quantify the systematic uncertainties of the GBM location algorithm for soft gamma-ray transients to < 8 deg. Our spectral analysis indicates significant spectral evolution between the first and last set of events. Although the 2008 October events are best fit with a single blackbody function, for the 2009 bursts an Optically Thin Thermal Bremsstrahlung (OTTB) is clearly preferred. We attribute this evolution to changes in the magnetic field topology of the source, possibly due to effects following the very energetic main bursting episode.