Formation of Resonant Atomic Lines during Thermonuclear Flashes on Neutron Stars

Motivated by the measurement of redshifted Fe Hα lines during type I X-ray bursts on EXO 0748-676 (Cottam, Paerels, & Mendez), we study the formation of atomic Fe lines above the photosphere of a bursting neutron star (k_BT_eff ≈ 1-2 keV). We discuss the effects of Stark broadening, resonant scattering, and NLTE (level population) on the formation of hydrogenic Fe Hα, Lyα, and Paα lines. From the observed equivalent width (EW) of the Fe Hα line, we find an implied Fe column of × 10²⁰ cm^-2, which is 3-10 times larger than the Fe column calculated from the accretion/spallation model of Bildsten, Chang, & Paerels. We also estimate that the implied Fe column is about a factor of 2-3 larger than a uniform solar metallicity atmosphere. We discuss the effects of rotational broadening and find that the rotation rate of EXO 0748-676 must be slow, as confirmed by the recent measurement of a 45 Hz burst oscillation by Villareal & Strohmayer. We also show that the Fe Lyα EW ≈ 15-20 eV (redshifted 11-15 eV) and the Paα EW ≈ 4-7 eV (redshifted 3-5 eV) when the Hα EW is 10 eV (redshifted 8 eV). The Lyα line is rotationally broadened to a depth of ≈10%, making it difficult to observe with Chandra. We also show that radiative levitation can likely support the Fe column needed to explain the line.