We present a recent Chandra observation of the quiescent low-mass X-ray binary containing a neutron star (NS), located in the globular cluster M30. We fit the thermal emission from the NS to extract its mass and radius. We find no evidence of flux variability between the two observations taken in 2001 and 2017, nor between individual 2017 observations, so we analyse them together to increase the signal-to-noise ratio. We perform simultaneous spectral fits using standard light-element composition atmosphere models (hydrogen or helium), including absorption by the interstellar medium, correction for pile-up of X-ray photons on the detector, and a power law for count excesses at high photon energy. Using a Markov chain Monte Carlo approach, we extract mass and radius credible intervals for both chemical compositions of the atmosphere: |$\mbox{$R_{\rm NS}$}=7.94\mbox{{$\scriptstyle ^{ + 0.76}_{- 1.21}$}}$| km and |$\mbox{$M_{\rm NS}$}\lt 1.19$| M_⊙ assuming pure hydrogen, and |$\mbox{$R_{\rm NS}$}=10.50\mbox{{$\scriptstyle ^{ + 2.88}_{- 2.03}$}}$| km and |$\mbox{$M_{\rm NS}$}\lt 1.78$| M_⊙ for helium, where the uncertainties represent the 90 per cent credible regions. For H, the small radius is difficult to reconcile with most current nuclear physics models (especially for nucleonic equations of state) and with other measurements of NS radii, with recent preferred values generally in the 11–14 km range. Whereas for He, the measured radius is consistent with this range. We discuss possible sources of systematic uncertainty that may result in an underestimation of the radius, identifying the presence of surface temperature inhomogeneities as the most relevant bias. According to this, we conclude that either the atmosphere is composed of He or it is anH atmosphere with a significant contribution of hotspots to the observed radiation.

中文翻译：

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球状星团 M30 中静止低质量 X 射线双星的光谱分析

我们展示了最近对包含中子星 (NS) 的静止低质量 X 射线双星的钱德拉观测，该双星位于球状星团 M30 中。我们拟合来自 NS 的热辐射以提取其质量和半径。我们没有发现 2001 年和 2017 年两次观测之间存在通量变化的证据，也没有发现 2017 年个别观测之间的通量变化，因此我们将它们一起分析以提高信噪比。我们使用标准轻元素组成大气模型（氢或氦）进行同步光谱拟合，包括星际介质的吸收、对探测器上 X 射线光子堆积的校正，以及用于高光子计数过剩的幂律活力。使用马尔可夫链蒙特卡罗方法，我们提取了大气化学成分的质量和半径可信区间：|$\mbox{$R_{\rm NS}$}=7.94\mbox{{$\scriptstyle ^{ + 0.76}_{- 1.21}$}}$| km 和 |$\mbox{$M_{\rm NS}$}\lt 1.19$| M_⊙ 假设纯​​氢，|$\mbox{$R_{\rm NS}$}=10.50\mbox{{$\scriptstyle ^{ + 2.88}_{- 2.03}$}}$| km 和 |$\mbox{$M_{\rm NS}$}\lt 1.78$| M_⊙ 氦气，其中的不确定性代表 90% 的可信区域。对于 H，小半径很难与大多数当前的核物理模型（特别是对于核子状态方程）和其他 NS 半径的测量相协调，最近的首选值通常在 11-14 公里范围内。而对于 He，测量的半径与此范围一致。我们讨论了可能导致半径低估的系统不确定性的可能来源，将表面温度不均匀性的存在确定为最相关的偏差。