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

Journal Article · 12 October 2023 · Monthly Notices of the Royal Astronomical Society

DOI: https://doi.org/10.1093/mnras/stad3109 · OSTI ID:2208853

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.

ABSTRACT 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.

• Sponsoring Organization: USDOE
• OSTI ID: 2208853
• Journal Information: Monthly Notices of the Royal Astronomical Society, Journal Name: Monthly Notices of the Royal Astronomical Society Journal Issue: 2 Vol. 527; ISSN 0035-8711
• Publisher: Oxford University PressCopyright Statement
• Country of Publication: United Kingdom
• Language: English