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

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 $0.183M_{\odot }\le M_{cs}\le 0.372M_{\odot }$ and a large radius in the range $1.02R_{\odot }\le R_{cs}\le 1.29R_{\odot }$. 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.

12 pages, 8 figures, accepted for publication in MNRAS

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Cited by in corpus (4)

• A shared accretion instability for black holes and neutron stars
• The $\text{False Widow}$ Link Between Neutron Star X-ray Binaries and Spider Pulsars
• A rapid optical and X-ray timing study of the neutron star X-ray binary Swift J1858.6-0814
• X-ray Iron absorption line in Swift J1858.6-0814