Atomic Spectral Features During Thermonuclear Flashes on Neutron Stars

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 H$\alpha$ line from hydrogenic Fe. We show that a concentrated Fe layer with $kT=1.2-1.8\ {\rm keV}$ and column depth $N_{\rm Fe}=4-50\times 10^{19}\ {\rm cm}^{-2}$ (depending on the line depth) above the hotter continuum photosphere would make a line close to the observed strength. For these Fe columns and reasonable atmospheric temperatures and pressures, we show that the Fe Ly$\alpha$ absorption feature equivalent width never exceeds 30 eV and that the Fe photoedge optical depth remains $4\times 10^{-13}M_\odot {\rm yr^{-1}}$, we show that the Fe column is independent of $\dot M$ and only depends on the nuclear physics of the proton spallation, resulting in $N_{\rm Fe}\approx 3\times 10^{19}{\rm cm^{-2}}$ for incident material of solar abundances. The Fe destruction creates many heavy elements with $Z<26$ which may imprint photo-ionization 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 show that hydrogenic Fe might remain in the photosphere due to radiative levitation from the high burst flux.