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

ABSTRACT 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 i 50^, 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 M ₕ when compared to the M ₕ \!-\! properties of LMXBs, likely due to the larger irradiated areas and more luminous donors.