Constrains on the mass and radius of pulsars from X-ray observations of their polar caps

The properties of X-ray radiation from the polar caps predicted by the radio pulsar models depend on the surface chemical composition, magnetic field and star's mass and radius as well as on the cap temperature, size and position. Fitting the radiation spectra and light curves with the neutron star atmosphere models enables one to infer these parameters. We present here results obtained from the analysis of the soft X-ray radiation of PSR J0437-4715. In particular, with the aid of radio polarization data, we put constraints on the pulsar mass-to-radius ratio.