Neutron stars

Almost 50 years after confirmation of the existence of the neutron star, the equation of state of the matter that comprises these stars is still under discussion.  In neutron stars, the density in the center of the star exceeds a few times the nuclear density. Many theoretical models of the equation of state (EOS) of superdense matter have been proposed. Astronomical observations are the only way to verify the EOS of neutron stars, because in Earth laboratories we are unable to reproduce conditions similar to neutron star interiors. A very important property of theoretical models is the existence of a maximum mass for the neutron star and a unique mass-radius relation for each assumed EOS. There exist various methods to constrain the EOS with astronomical observations.
There exist methods that allow for simultaneous mass and radius determination, and consequently, to determine the EOS. One such method is fitting observed spectra with model atmospheres.

Our group develop an unique model atmosphere of hot neutron star and accretion disks (ATM24 code). This code calculates the radiative transfer equation in a plane-parallel geometry. It takes into account the effect of Compton scattering on free, relativistic electrons, where initial photon energies can approach the electron rest mass. We assume  the equation of state of ideal gas being in local thermodynamical equilibrium (LTE). We solve the model atmosphere together with hydrostatic and radiative equilibrium equations. The influences of the magnetic field and accretion onto the neutron star are not included. Our code takes into account energy-dependent opacities from hydrogen, helium and heavy element ions in LTE. The ionization equilibrium is fully solved. We neglect the effect of electron degeneracy, which is unimportant in the hot atmospheres.

Our theoretical spectra could be fitted to the observed spectra of hot neutron stars in different type of objects like X-ray bursters, isolated neutron stars or X-ray transient in the period when the neutron star is not accreting matter.

               Theoretical spectra of hot neutron star atmosphere with the effective temperature
              T_eff=2.20x10^7 K and different logarithm of surface gravities form log(g)=14.20
               to 14.60. Strong iron lines are clearly visible.