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

¹ Department of Physics, Ulsan National Institute of Science and Technology, Ulsan 44919, Korea
² Center for Exotic Nuclear Studies, Institute for Basic Science, Daejeon 34126, Korea
³ Korea Astronomy and Space Science Institute, Daejeon 34055, Korea
⁴ Department of Astronomy and Space Science, University of Science and Technology, Daejeon 34113, Republic of Korea
⁵ Department of Physics, Pusan National University, Busan 46241, Korea

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Received: 29 September 2025
Accepted: 16 April 2026

Abstract

Context. 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.

Aims. By introducing a blocking factor to account for the systematic reduction of observed flux relative to the intrinsic emission from the neutron star’s photosphere, we investigated whether the reduction in observed flux can reconcile anomalous mass–radius estimates with canonical neutron star properties.

Methods. We defined the blocking factor as the fraction of the neutron star photosphere obscured from view, which scales both the observed touchdown flux and the effective emitting area. We solved the modified photospheric radius expansion equations analytically, which yields two distinct mathematical branches of mass–radius solutions, and employed Monte Carlo simulations for high-blocking scenarios.

Results. Significant blocking factors (ℬ ≳ 0.8, reducing the observed flux to ∼17% of the intrinsic emission) permit neutron star parameters consistent with the canonical values: M = 1.59 ± 0.69 M_⊙, R = 13.0 ± 5.45 km, or M = 2.12 ± 1.08 M_⊙, R = 9.80 ± 4.13 km. The blocking factor, which varies with the photospheric radius, provides a natural explanation for the anomalously large peak-to-touchdown flux ratio (∼2.0) and highlights the importance of accounting for geometric system configuration in neutron star mass–radius estimates.