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

Castro Segura, N.; Knigge, C.; Matthews, J. H.; Vincentelli, F. M.; Charles, P.; Long, K. S.; Altamirano, D.; Buckley, D. A. H.; Modiano, D.; Torres, M. A. P.; Buisson, D. J. K.; Fijma, S.; Alabarta, K.; Degenaar, N.; Georganti, M.; Baglio, M. C.
Referencia bibliográfica

Monthly Notices of the Royal Astronomical Society

Fecha de publicación:
1
2024
Número de autores
16
Número de autores del IAC
2
Número de citas
0
Número de citas referidas
0
Descripción
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, Td ≃ 5700 K, and radius, Rd ≃ 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, Porb = 21.3 h, we search for viable evolution paths. The initial donor masses in the allowed solutions span the range 1 M⊙ ≲ Md,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⊙ ≲ Md,obs ≲ 1.3 M⊙, higher than suggested by eclipse modelling. Since Porb 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.