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T. E. Strohmayer (NASA/GSFC), A. I. Ibrahim (GWU)
We present evidence indicating the presence of a 6.4 keV emission line during a burst from the soft gamma-ray repeater SGR 1900+14. The Rossi X-ray Timing Explorer (RXTE) monitored this source extensively during its outburst in the summer of 1998. A strong burst observed on August 29, 1998 revealed a number of unique properties. The burst exhibits a precursor and is followed by a long (~103 s) tail modulated at the 5.16 s stellar rotation period. The precursor has a duration of ~0.85 s and shows both significant spectral evolution as well as an emission feature centered near 6.4 keV during the first 0.3 s of the event, when the X-ray spectrum was hardest. The continuum during the burst is well fit with an optically thin thermal bremsstrahlung (OTTB) spectrum with the temperature ranging from as high as ~40 to as low as 10 keV. The line is strong, with an equivalent width of ~400 eV, and the centroid is consistent with Fe K-\alpha fluorescence from relatively cool material. If the rest-frame energy is indeed 6.4 keV, then the lack of an observed redshift indicates that the source of the line is at least ~80 km above the surface of the neutron star. We briefly discuss the implications of the line detection in the context of models for SGRs.