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

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 $\sim 17\%$ of the intrinsic emission) permit neutron star parameters consistent with the canonical values: $M=1.59\pm 0.69M_{\odot }$, $R=13.0\pm 5.45\mathrm{km}$, or $M=2.12\pm 1.08M_{\odot }$, $R=9.80\pm 4.13\mathrm{km}$. The blocking factor, which varies with the photospheric radius, provides a natural explanation for the anomalously large peak-to-touchdown flux ratio ($\sim 2.0$) and highlights the importance of accounting for geometric system configuration in neutron star mass--radius estimates.