Context :

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 \pm 0.3 eV and FWHM \sim 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 . \mathrm { n } _ { H } \sim 10 ^ { 8 } \mathrm { cm } ^ { -3 } ) slab , located in the vicinity of the neutron star ( e.g . \sim \mathrm { 10 } ^ { 10 } \mathrm { cm } ) .

Conclusions :