2S 0918-549的质量和半径约束：来自RXTE超膨胀爆发的直接冷却尾分析
Mass-Radius Constraints for 2S 0918-549 from an RXTE Superexpansion Burst: A Direct Cooling-Tail Analysis
AI总结：
利用RXTE观测到的2S 0918-549超膨胀爆发，通过直接冷却尾分析确定中子星质量和半径约束，表明重力束缚和自束缚方程状态在1σ置信度下仍有效。
摘要：
中子星吸积过程中的热核（类型I）X射线爆发为确定中子星质量（M）和半径（R）提供了手段，从而探测超核密度下的物质性质。其中一部分事件，即光球半径膨胀（PRE）爆发，为约束中子星的M和R提供了特别有力的工具。本文应用直接冷却尾方法分析2S 0918-549，利用RXTE观测到的罕见超膨胀爆发。仅拟合$F/F_{\rm td}$在[0.6,0.95]范围内的post-touchdown数据，采用现代大气模型（纯氦和金属富集）。纯氦大气能良好描述冷却尾（$χ^{2}/ν=18.12/14$），而金属丰富模型失败；信息准则测试（AIC/BIC）不支持在每个时间-bin中加入自由吸收边，表明重元素残渣不需要。联合拟合给出距离$d=4.1-5.3$kpc和质量-半径约束$M=1-2\,M_\odot$和$R=9.7-11.9$km（99%置信度）。这些结果表明，代表性的重力束缚和自束缚方程状态族在$1σ$置信度水平上仍可行。
英文摘要：
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 (\(χ^{2}/ν=18.12/14\)), 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 $1σ$ confidence level.
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