[HP99] 159 – Properties of the first supersoft X-ray source with a helium star donor

Journal article

[HP99] 159 is remarkable as the first supersoft X-ray source (SSS) identified with an evolved helium star donor. With a likely orbital period of 1.164 d or 2.327 d, the origin of the SSS component is controversial, with the two current models being either steady He-burning on the white dwarf surface, or that it is a helium nova in the decaying phase. To help resolve this issue we present extensive new long-term spectroscopy (with SALT) and photometry (at SAAO and with OGLE) of [HP99] 159 which (a) supports 2.327 d as the orbital period, and (b) finds only a small He ii radial velocity modulation. The latter is surprising as it implies a very low inclination system, whereas our light curve modelling suggests , and hence that the He ii must be produced in outflowing material further above, or beyond, the disc. We find that the decaying nova model cannot fit our OGLE light-curve and the observed SSS flux level. [HP99] 159 has been essentially constant as an SSS over several decades, implying a sustained high level of mass-transfer from its He star donor, making it the only confirmed single-degenerate scenario SN Ia progenitor. We have updated the known SSS binary parameters and find a clear 1.5 mag difference in their when compared to the properties of LMXBs, likely due to the larger irradiated areas and more luminous donors.

Authors: Szegedi, H; Charles, PA; Buckley, DAH; Meintjes, PJ; Mróz, P

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Oxford University Press
• Journal: Monthly Notices of the Royal Astronomical Society
• Volume: 546
• Issue: 3
• Article number: stag189
• Publication date: 2026-01-27
• Acceptance date: 2026-01-23
• DOI: 10.1093/mnras/stag189
• EISSN: 1365-2966
• ISSN: 0035-8711
• Language: English
• Keywords: binaries: close; white dwarfs; supernovae: general; X-rays: individual([HP99]159)