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arXiv · 2605.14264

The compact neutron star in 4U 1746-37 revisited: Reassessing the mass and radius

Kwang Hyun Sung·Myungkuk Kim·Young-Min Kim·Kyujin Kwak·Chang-Hwan Lee
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Abstract

A recent analysis of photospheric radius expansion X-ray bursts from the low-mass X-ray binary 4U 1746-37 reported unusually small mass and radius estimates for the neutron star, suggesting it could be a quark star or quark-cluster star. Here, we propose an alternative interpretation: the star's mass and radius could be underestimated from significant blocking of the X-ray flux. Significant blocking factors ($\mathcal{B} \gtrsim 0.8$, reducing the observed flux to $\sim17\%$ of the intrinsic emission) permit neutron star parameters consistent with the canonical values: $M = 1.59 \pm 0.69 M_{\odot}$, $R = 13.0 \pm 5.45\,\mathrm{km}$, or $M = 2.12 \pm 1.08 M_{\odot}$, $R = 9.80 \pm 4.13\,\mathrm{km}$. The blocking factor, which varies with the photospheric radius, provides a natural explanation for the anomalously large peak-to-touchdown flux ratio ($\sim2.0$) and highlights the importance of accounting for geometric system configuration in neutron star mass--radius estimates.

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Kwang Hyun Sung, Myungkuk Kim, Young-Min Kim, Kyujin Kwak, Chang-Hwan Lee. 2026-05-14. The compact neutron star in 4U 1746-37 revisited: Reassessing the mass and radius. https://arxiv.org/abs/2605.14264

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