Observational Constraints on the Surface Gravitational Redshift of a Neutron Star

Abstract

A neutron star (NS) is a natural laboratory to probe the nature of the ultra-dense matter. Its observational parameters, e.g., mass and radius, may constrain the equation of state of the supranuclear matter. Here, we analyzed the thermonuclear X-ray burst from GRS 1747−312 observed with Suzaku to constrain the mass-radius relation of a NS. The burst had a long duration with the photospheric radius expansion. We detected for the first time a broad absorption edge like feature above 6 keV in the energy spectrum during the decay phase of the burst. The feature was most plausibly explained by the absorption edges of hydrogen-like Fe and Zn ions, which were smeared by the rapid NS spin. Assuming those ions are responsible for the feature, the surface gravitational redshift is estimated as 1 + z = 1.56 ± 0.03. This value may be feasible if the NS is as heavy as 2M ⊙ .