X-ray Eclipse Mapping Constrains the Binary Inclination and Mass Ratio of Swift J1858.6-0814

TL;DR

This study models X-ray eclipses in the binary Swift J1858.6-0814 to determine the system's inclination, mass ratio, and companion star properties, revealing irradiation effects and providing key parameters for understanding the system.

Contribution

It introduces detailed eclipse profile modeling of Swift J1858.6-0814, constraining inclination, mass ratio, and companion star characteristics, with evidence of irradiation-driven ablation.

Findings

Inclination ~81 degrees

Mass ratio ~0.14

Companion mass 0.183-0.372 M_sun

Abstract

X-ray eclipse mapping is a promising modelling technique, capable of constraining the mass and/or radius of neutron stars (NSs) or black holes (BHs) in eclipsing binaries and probing any structure surrounding the companion star. In eclipsing systems, the binary inclination, $i$, and mass ratio, $q$ relate via the duration of totality, $t_{e}$. The degeneracy between $i$ and $q$ can then be broken through detailed modelling of the eclipse profile. Here we model the eclipses of the NS low-mass X-ray binary Swift J1858.6$-$0814 utilising archival NICER observations taken while the source was in outburst. Analogous to EXO 0748$-$676, we find evidence for irradiation driven ablation of the companion's surface by requiring a layer of stellar material to surround the companion star in our modelling. This material layer extends $\sim 7000 - 14000$ km from the companion's surface and is likely the cause of the extended, energy-dependent and asymmetric ingress and egress that we observe. Our fits return an inclination of $i \sim 81^{\circ}$ and a mass ratio $q \sim 0.14$. Using Kepler's law to relate the mass and radius of the companion star via the orbital period ($\sim$ 21.3 hrs), we subsequently determine the companion to have a low mass in the range $0.183 M_{\odot} \leq M_{cs} \leq 0.372 M_{\odot}$ and a large radius in the range $1.02 R_{\odot} \leq R_{cs} \leq 1.29 R_{\odot}$. Our results, combined with future radial velocity amplitudes measured from stellar absorption/emission lines, can place precise constraints on the component masses in this system.