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We present the results of our broadband spectral analysis of 42 SGRJ1550-5418 bursts simultaneously detected with the Swift/X-ray Telescope (XRT)and the Fermi/Gamma-ray Burst Monitor (GBM), during the 2009 January activeepisode of the source. The unique spectral and temporal capabilities of the XRTWindowed Timing mode have allowed us to extend the GBM spectral coverage forthese events down to the X-ray domain (0.5-10 keV). Our earlier analysis of theGBM data found that the SGR J1550-5418 burst spectra were described equallywell with a Comptonized model or with two blackbody functions; the two modelswere statistically indistinguishable. Our new broadband (0.5 - 200 keV)spectral fits show that, on average, the burst spectra are better describedwith two blackbody functions than with the Comptonized model. Thus, our jointXRT/GBM analysis clearly shows for the first time that the SGR J1550-5418 burstspectra might naturally be expected to exhibit a more truly thermalizedcharacter, such as a two-blackbody or even a multi-blackbody signal. Using theSwift and RXTE timing ephemeris for SGR J1550-5418 we construct thedistribution of the XRT burst counts with spin phase and find that it is notcorrelated with the persistent X-ray emission pulse phase from SGR J1550-5418.These results indicate that the burst emitting sites on the neutron star neednot be co-located with hot spots emitting the bulk of the persistent X-rayemission. Finally, we show that there is a significant pulse phase dependenceof the XRT burst counts, likely demonstrating that the surface magnetic fieldof SGR J1550-5418 is not uniform over the emission zone, since it isanticipated that regions with stronger surface magnetic field could triggerbursts more efficiently.

Broadband Spectral Investigations of SGR J1550-5418 Bursts