Discovery of a 6.4 keV Emission Line in a Burst from SGR 1900+14

We present evidence of a 6.4 key emission line during a burst from the soft gamma 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 1998 August 29 revealed a number of unique properties. The burst exhibits a precursor and is followed by a long (approx. 10(exp 3) s) tail modulated at the 5.16 s stellar rotation period. The precursor has a duration of approx. equals 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 spectrum with the temperature ranging from approx. equals 40 to 10 keV. The line is strong, with an equivalent width of approx. 400 eV, and is consistent with Fe K(alpha) fluorescence from relatively coot material. If the rest-frame energy is indeed 6.4 keV, then the lack of an observed redshift indicates that the source is at least approx. 80 km above the neutron star surface. We discuss the implications of the line detection in the context of models for SGRs.