Constraints of the compactness of the isolated neutron stars via X-ray phase-resolved spectroscopy

A model with a condensed iron surface and partially ionized hydrogen-thin atmosphere allows us to fit simultaneously the observed general spectral shape and the broad absorption feature (observed at 0.3 keV) in different spin phases of the isolated neutron star RBS 1223. We constrain some physical properties of the X-ray emitting areas, i.e. the temperatures (T _pole1 ~ 105 eV, T _pole2 ~ 99 eV), magnetic field strengths (B _pole1 ≈ B _pole2 ~ 8.6 × 10¹³ G) at the poles, and their distribution parameters (a1 ~ 0.61, a2 ~ 0.29, indicating an absence of strong toroidal magnetic field component). In addition, we are able to place some constraints on the geometry of the emerging X-ray emission and the gravitational redshift (z ~ 0.16^0.03 _-0.01) of the isolated neutron star RBS 1223.