We present evidence of a 6.3 keV emission line during a burst from the soft
gamma repeater SGR 1900+14. The Rossi X-Ray Timing Explorer (RXTE) monitor
ed this source extensively during its outburst in the summer of 1998. A str
ong burst observed on 1998 August 29 revealed a number of unique properties
. The burst exhibits a precursor and is followed by a long (similar to 10(3
) a) tail modulated at the 5.16 s stellar rotation period. The precursor ha
s a duration of approximate to 0.85 s and shows both significant spectral e
volution as well as an emission feature centered near 6.4 keV during the fi
rst 0.3 a 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 approximate to 40 to 10 keV. The
line is strong, with an equivalent width of similar to 400 eV, and is cons
istent with Fe K alpha fluorescence from relatively cool material. If the r
est-frame energy is indeed 6.4 keV, then the lack of an observed redshift i
ndicates that the source is at least similar to 80 km above the neutron sta
r surface. We discuss the implications of the line detection in the context
of models for SGRs.