Discovery of an iron K absorption line in the low-mass X-ray binary GX 13+1

We report the discovery of an iron K absorption-line feature with ASCA in the spectra of the low-mass X-ray binary GX 13 + 1. The absorption line is detected at 7.0 keV with an equivalent width of 35 +/- 8 eV, which is attributed to resonant scattering of the K alpha line from Fe XXVI ions in the line of sight whose column density is larger than similar to 10(18) cm(-2). The detection of such a spectral feature from a neutron star binary rules out the hypothesis that it is unique to the Galactic superluminal jet sources. From the similarity of the line parameters to those of the superluminal sources, we infer that we view the X-ray emission through a highly ionized plasma in outer parts of the accretion disk with a relatively high inclination angle. This picture is consistent with the universal presence of such plasmas in X-ray binaries accreting through disks.