Absorption features in the spectra of X-ray bursting neutron stars

Context: The discovery of photospheric absorption lines in XMM-Newton spectra of the X-ray bursting neutron star in EXO 0748-676 by Cottam and collaborators allows us to constrain the neutron star mass-radius ratio from the measured gravitational redshift. A radius of R=9{-}12 km for a plausible mass range of M=1.4{-}1.8 M_⊙ was derived by these authors.
Aims: It has been claimed that the absorption features stem from gravitationally redshifted (z=0.35) n=2{-}3 lines of H- and He-like iron. We investigate this identification and search for alternatives.
Methods: We compute LTE and non-LTE neutron-star model atmospheres and detailed synthetic spectra for a wide range of effective temperatures (T_eff = 1{-}20 MK) and different chemical compositions.
Results: We are unable to confirm the identification of the absorption features in the X-ray spectrum of EXO 0748-676 as n=2{-}3 lines of H- and He-like iron (Fe XXVI and Fe XXV). These are subordinate lines that are predicted by our models to be too weak at any T_eff. It is more likely that the strongest feature is from the n=2{-}3 resonance transition in Fe XXIV with a redshift of z=0.24. Adopting this value yields a larger neutron star radius, namely R=12{-}15 km for the mass range M=1.4{-}1.8 M_⊙, favoring a stiff equation-of-state and excluding mass-radius relations based on exotic matter. Combined with an estimate of the stellar radius R > 12.5 km from the work of Özel and collaborators, the z=0.24 value provides a minimum neutron-star mass of M > 1.48 M_⊙, instead of M > 1.9 M_⊙, when assuming z=0.35.
Conclusions: The current state of line identifications in the neutron star of EXO 0748-676 must be regarded as highly uncertain. Our model atmospheres show that lines other than those previously thought must be associated with the observed absorption features.