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In this paper we show that X-ray spectral observations of the ATHENA mission,which is planned to launch in 2031, can constrain the equation of state ofsuperdense matter. We use our well-constrained continuum fitting method formass and radius determination of the neutron star. Model spectra of theemission from a neutron star were calculated using the atmosphere code ATM24.In the next step, those models were fitted to a simulated spectra of theneutron star calculated for ATHENA's WFI detector, using the satellitecalibration files. To simulate the spectra we assumed three different values ofeffective temperatures, surface gravities and gravitational redshifts. Therecases are related to the three different neutron star masses and radii. Thisanalysis allows us to demonstrate the precision of our method and demonstratethe need for a fast detector onboard of ATHENA. A large grid of theoreticalspectra was calculated with various parameters and a hydrogen-helium-ironcomposition of solar proportion. These spectra were fitted to the simulatedspectrum to estimate the precision of mass and radius determination. In eachcase, we obtained very precise mass and radius values with errors in the range3--10% for mass and in the range 2--8% for radius within the 1-sigma confidenceerror. We show here that with the ATHENA WFI detector, such a determinationcould be used to constrain the equation of state of superdense neutron starmatter.

Precision of mass and radius determination for neutron star using the ATHENA mission