XMM-Newton RGS spectrum of RX J0720.4−3125: An absorption feature at 0.57 keV

Aims. By measuring the gravitational redshift of spectral features in the spectrum of thermal radiation emitted from neutron stars, useful constraints for the equation of state of superdense matter can be obtained via an estimate of the mass-to-radius ratio. We searched for spectral line features in the high-resolution X-ray spectrum of the isolated neutron star RX J0720.4−3125. Methods. Our target was observed by XMM-Newton on many occasions. We used the XMM-Newton SAS task rgscombine in order to create a co-added RGS spectrum of RX J0720.4−3125. We modeled the resulting spectrum with absorbed blackbody radiation with a Gaussian absorption features using the XSPEC package. Results. We found a narrow absorption feature at 0.57 keV in the co-added RGS spectrum of the isolated neutron star RX J0720.4−3125 with an equivalent width of 1.35 ± 0.3 eV and FWHM ∼ 6.0 eV. The feature was identified with an absorption line of highly ionized oxygen O VII, most probably originating in the ambient medium of RX J0720.4−3125. An extensive investigation with the photo-ionization code CLOUDY indicates the possibility that the optical flux excess observed in the spectrum of RX J0720.4−3125 at least partially originates in a relatively dense (e.g. nH ∼ 108cm−3) slab, located in the vicinity of the neutron star (e.g. ∼ 10cm).