Discovery of a 6.4 [CLC]ke[/CLC]V Emission Line in a Burst from SGR 1900+14

Abstract

We present evidence of a 6.4 keV 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 (~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 spectrum with the temperature ranging from ~40 to 10 keV. The line is strong, with an equivalent width of ~400 eV, and is consistent with Fe Kα 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 is at least ~80 km above the neutron star surface. We discuss the implications of the line detection in the context of models for SGRs.