Formation of resonant atomic lines during thermonuclear flashes on neutron stars

Motivated by the measurement of redshifted Fe H alpha 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(B)T(eff) approximate to 1-2 keV). We discuss the effects of Stark broadening, resonant scattering, and NLTE ( level population) on the formation of hydrogenic Fe H alpha, Ly alpha, and Pa alpha lines. From the observed equivalent width ( EW) of the Fe H alpha line, we find an implied Fe column of 13 x 10(20) cm(-2), which is 3-10 times larger than the Fe column calculated fromthe 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 Ly alpha EW approximate to 15-20 eV ( redshifted 11-15 eV) and the Pa alpha EW approximate to 4-7 eV( redshifted 3-5 eV) when theH alpha EW is 10 eV ( redshifted 8 eV). The Ly alpha line is rotationally broadened to a depth of approximate to 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.