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 ≃ 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 $\dot{M}_{\rm acc} \lesssim 10^{-8.5}~{\rm M}_{\odot }~\mathrm{ yr}^{-1}$ 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.