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

TL;DR

This paper suggests that the previously estimated small mass and radius of the neutron star in 4U 1746-37 may be underestimated due to flux blocking, allowing for more typical neutron star parameters.

Contribution

It introduces the idea that flux blocking significantly affects neutron star parameter estimates, offering an alternative to exotic star models.

Findings

Significant flux blocking (over 80%) can reconcile observations with canonical neutron star parameters.

Blocking factors vary with photospheric radius, influencing flux measurements.

Accounting for geometric effects is crucial in neutron star mass-radius estimations.

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.