[HP99] 159 -- Properties of the first Supersoft X-ray Source with a Helium star donor

TL;DR

This study presents comprehensive multi-epoch spectroscopy (SALT/HRS and SpUpNIC) and long-term photometry (SAAO SHOC and OGLE-IV) of [HP99] 159, clarifying its nature as the first supersoft X-ray source with a helium-star donor. The authors confirm a 2.327 d orbital period and argue for a relatively high inclination around i ≈ 50°, with radial-velocity amplitudes suggesting disc- or outflow–related dilution, thereby challenging the decaying helium nova scenario. The system remains a sustained SSS over decades, consistent with a high, steady mass transfer from a He-star donor and aligning with single-degenerate SN Ia progenitor pathways; the analysis also highlights a roughly 1.5 mag offset in M_V relative to LMXB SSSs due to enhanced irradiation and donor luminosity. Overall, [HP99] 159 provides key empirical support for He-star–donor SSSs and informs SN Ia progenitor evolutionary channels and disc-outflow physics.

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 $i{\sim}50^\circ$, 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 $\sim$1.5 mag difference in their $M_{\rm V}$ when compared to the $M_{\rm V} - Σ$ properties of LMXBs, likely due to the larger irradiated areas and more luminous donors.

Figures (8)

• Figure 1: OGLE-IV $I$-band light curve of [HP99] 159 obtained Mar 2010 -- Jan 2024, with a linear fit (dashed line) giving a mean of $I$=16, and demonstrating no detectable decline over the last 14 yrs. The blue and purple lines represent the 28 yr and 80 yr decaying nova models, respectively, of Kato2023. The red region indicates the time interval of the XMM-Newton and eROSITA observations.
• Figure 2: PDM periodogram of the OGLE-IV light curve (Fig. \ref{['fig:ogle']}) in the frequency range of $0.1 < f < 2.5 {\rm ~cycles~day^{-1}}$. The two strongest signals are marked, and are the same as those seen in our and G23's Lomb-Scargle periodograms.
• Figure 3: The phase-folded light curve on the proposed 2.33 d orbital period obtained with the SAAO 1.0-m/SHOC (3 -- 29 Jan 2024). Two cycles are shown for clarity.
• Figure 4: SAAO 1.9-m/SpUpNIC spectrum of [HP99] 159 on 3 Oct 2024. All the emission lines are attributed to He i and He ii, confirming the absence of hydrogen.
• Figure 5: He ii and He i line profiles in SALT/HRS spectra, obtained between Dec 2022 and Feb 2024. The plots are offset vertically by an arbitrary constant.