Precision of Mass and Radius Determination for Neutron Stars from the ATHENA Mission

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 (EOS) of superdense matter. We use our well-constrained continuum-fitting method to determine the mass and radius of a neutron star. Model spectra of the emission from a neutron star were calculated using the atmosphere code ATM24. In the next step, those models were fitted to simulated spectra of the neutron star calculated for ATHENA's Wide Field Imager (WFI) detector, using satellite calibration files. To simulate the spectra we assumed three different values of effective temperature, surface gravity, and gravitational redshift. These cases relate to three different neutron star masses and radii. This analysis allows us to demonstrate both the precision of our method and the need for a fast detector onboard ATHENA. A large grid of theoretical spectra was calculated with various parameters and a hydrogen-helium-iron composition of solar proportions. These spectra were fitted to the simulated spectrum to estimate the precision of mass and radius determination. In each case, we obtained very precise values with errors in the range 3%-10% for mass and 2%-8% for radius within 1σ confidence. We show here that, with the ATHENA WFI detector, such a determination could be used to constrain the EOS of superdense neutron star matter.