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-ultraviolet (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 effective temperature, T-d similar or equal to 5700 K, and radius, R-d similar or equal to 1.7 R-circle dot, based on photometric observations obtained during quiescence. Lastly, we leverage the transient nature of the system to set an upper limit of (M) over dot(acc) less than or similar to 10(-8.5) M-circle dot 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-circle dot <= M-d,M-i less than or similar to 3.5 M-circle dot. 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-circle dot less than or similar to M-d,M-obs less than or similar to 1.3 M-circle dot, 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.