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 Hand 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 (Teff =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 Hand He-like iron (Fe xxvi and Fe xxv). These are subordinate lines that are predicted by our models to be too weak at any Teff . 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 > 13.8 km from the work of Özel and collaborators, the z = 0.24 value provides a minimum neutron-star mass of M > 1.63 M⊙, instead of M > 2.10 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.