2003ApJ...591L..29B

The gravitational redshift measured by Cottam, Paerels, and Mendez for the neutron star (NS) in the low-mass X-ray binary EXO 0748 -676 depends on the identification of an absorption line during a type I burst as the Hα line from hydrogenic Fe. We show that Fe is present above the photosphere as long as M{dot}>4x10^–13 M_☉yr^–1 during the burst. In this limit, the total Fe column is N_Fe~3x10¹⁹.cm^–2 for incident material of solar abundances and depends only on the nuclear physics of the proton spallation. The Fe destruction creates many heavy elements with Z<26 that may imprint photoionization edges on the NS spectra during a radius expansion event or in a burst cooling tail. Detecting these features in concert with those from Fe would confirm a redshift measurement. We also begin to address the radiative transfer problem and find that a concentrated Fe layer with kT=1.2-1.4 keV and column N_Fe=(7-20)x10²⁰/cm² (depending on the line depth) above the hotter continuum photosphere is required to create the Hα line of the observed strength. This estimate must be refined by considerations of non-LTE effects as well as resonant line transport. Until these are carried out, we cannot say whether the Fe column from accretion and spallation is in conflict with the observations. We also show that hydrogenic Fe might remain in the photosphere due to radiative levitation from the high burst flux.