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

¹ Institute for Astronomy and Astrophysics, Kepler Center for Astro and Particle Physics, Eberhard Karls University, Sand 1, 72076 Tübingen, Germany e-mail: rauch@astro.uni-tuebingen.de
² Kazan State University, Kremlevskaja Str. 18, Kazan 420008, Russia

Received: 5 May 2008
Accepted: 22 August 2008

Abstract

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 km for a plausible mass range of was derived by these authors.

Aims. It has been claimed that the absorption features stem from gravitationally redshifted () 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 () 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 lines of H- and He-like iron ( and ). 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 resonance transition in with a redshift of . Adopting this value yields a larger neutron star radius, namely km for the mass range , favoring a stiff equation-of-state and excluding mass-radius relations based on exotic matter. Combined with an estimate of the stellar radius km from the work of Özel and collaborators, the value provides a minimum neutron-star mass of , instead of , when assuming .

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.