Shedding far-ultraviolet light on the donor star and evolutionary state of the neutron-star LMXB Swift J1858.6−0814

The evolution of accreting X-ray binary systems is closely coupled to the properties of their donor stars. Consequently, we can constrain the evolutionary track a system is by establishing the nature of its donor. Here, we present far -ultra violet (far -UV) spectroscopy of the transient neutron-star low-mass X-ray binary J1858 in different accretion states (low-hard, high-hard, and soft). All of these spectra exhibit anomalous N V , C IV , Si IV , and He II lines, suggesting that its donor star has undergone CNO processing. We also determine the donor's ef fecti ve temperature, T d ≃ 5700 K, and radius, R d ≃ 1.7 R ≃, based on photometric observations obtained during quiescence. Lastly, we leverage the transient nature of the system to set an upper limit of M acc × 10 -8 . 5 M ⊙yr -1 on the present-day mass-transfer rate. Combining these with the orbital period of the system, P orb = 21.3 h, we search for viable evolution paths. The initial donor masses in the allowed solutions span the range 1 M ⊙ ≲ M d , i ≲ 3.5 M ⊙. All but the lowest masses in this range are consistent with the strong CNO-processing signature in the UV line ratios. The present-day donor mass in the permitted tracks are 0.5 M ⊙ ≲ M d ,obs ≲ 1.3 M ⊙, higher than suggested by eclipse modelling. Since P orb is close to the so-called bifurcation period, both converging and diverging binary tracks are permitted. If the former is confirmed, J1858 will end its life as an ultracompact system with a substellar donor.