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X-ray eclipse mapping is a promising modelling technique, capable ofconstraining 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, $q$ relatevia the duration of totality, $t_{e}$. The degeneracy between $i$ and $q$ canthen be broken through detailed modelling of the eclipse profile. Here we modelthe eclipses of the NS low-mass X-ray binary Swift J1858.6$-$0814 utilisingarchival NICER observations taken while the source was in outburst. Analogousto EXO 0748$-$676, we find evidence for irradiation driven ablation of thecompanion's surface by requiring a layer of stellar material to surround thecompanion 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 fitsreturn 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 theorbital period ($\sim$ 21.3 hrs), we subsequently determine the companion tohave 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 $1.02 R_{\odot} \leq R_{cs} \leq 1.29R_{\odot}$. Our results, combined with future radial velocity amplitudesmeasured from stellar absorption/emission lines, can place precise constraintson the component masses in this system.

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