2026ApJ...998...70F

Thermonuclear (Type I) X-ray bursts from accreting neutron stars (NSs) offer a means to determine NS mass (M) and radius (R) and thereby probe the properties of matter at supranuclear density. A subset of these events, photospheric radius-expansion bursts, provide a particularly powerful tool to constrain the NS M and R. Here, we apply the direct cooling-tail method to 2S 0918-549, using a rare superexpansion burst observed by Rossi X-ray Timing Explorer. We fit only the posttouchdown data within F/F_td ∈ [0.6, 0.95], employing modern atmosphere models (pure-He and metal-enriched). The pure-He atmosphere yields a good description of the cooling tail (χ²/ν = 18.12/14), whereas metal-rich models fail; information-criterion tests (Akaike information criterion/Bayesian information criterion) disfavor adding a free absorption edge in every time bin, indicating that heavy-element ashes are unnecessary. The joint fit gives a distance d = 4.1-5.3 kpc and mass-radius constraints M = 1-2 M_⊙ and R = 9.7-11.9 km (99% confidence). These results suggest that representative families of both gravity-bound and self-bound equations of state remain viable at the 1σ confidence level.