Mass-Radius Constraints for 2S 0918-549 from an RXTE Superexpansion Burst: A Direct Cooling-Tail Analysis

TL;DR

This study uses a rare superexpansion burst observed by RXTE to constrain the mass and radius of neutron star 2S 0918-549 through a direct cooling-tail analysis, informing dense matter physics.

Contribution

It applies the direct cooling-tail method with modern atmosphere models to a superexpansion burst, providing new neutron star mass-radius constraints.

Findings

Pure helium atmosphere models fit the cooling tail well.

Metal-rich models are disfavored by statistical tests.

The neutron star's mass is constrained between 1 and 2 solar masses, and radius between 9.7 and 11.9 km.

Abstract

Thermonuclear (Type I ) X-ray bursts from accreting neutron stars offer a means to determine neutron-star (NS) mass ($M$) and radius ($R$) and thereby probe the properties of matter at supranuclear density. A subset of these events, photospheric radius-expansion (PRE) bursts, provide a particularly powerful tool to constrain the neutron-star $M$ and $R$. Here, we apply the direct cooling-tail method to 2S~0918$-$549, using a rare superexpansion burst observed by \emph{RXTE}. We fit only the post-touchdown data within \(F/F_{\rm td}\in[0.6,0.95]\), employing modern atmosphere models (pure He and metal-enriched). The pure-He atmosphere yields a good description of the cooling tail (\(\chi^{2}/\nu=18.12/14\)), whereas metal-rich models fail; information-criterion tests (AIC/BIC) 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 \(M=1-2\,M_\odot\) 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 $1\sigma$ confidence level.