Observational constraints of the compactness of isolated neutron stars

We report on our observational attempt to constrain the compactness of the isolated neutron stars via X-ray spin phase-resolved spectroscopy. There are seven thermally emitting neutron stars known from X-ray and optical observations, which are young (up to few Myrs), nearby (hundreds of pc), and radio-quiet with blackbody-like X-ray spectra. 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. We constrain a number of physical properties of the X-ray emitting areas, including their temperatures, magnetic field strengths at the poles, and their distribution parameters. In addition, we place some constraints on the geometry of the emerging X-ray emission and the gravitational redshift of three isolated neutron stars.