WX Cen is X‑ray quiet: Does this signify a high‑mass or a low‑mass binary?

Abstract

We present results from analysis of the XMM–Newton observation (2021 February) of the enigmatic object WX Cen: a presumed massive Wolf–Rayet (WR) binary, (WN3 + WC4), or a low‑mass binary. WX Cen was not detected in X‑rays. Its UV emission shows periodic variability with a period (0.405–0.434 day) similar to that found in the optical for this object (0.417 day). The X‑ray nondetection of WX Cen is inconsistent with the fact that WR binaries are X‑ray sources. Also, the X‑ray emission from its early WN‑type component would have been clearly detected in the XMM–Newton exposure. Thus, the X‑ray nondetection does not support the physical picture of WX Cen as a massive WR binary. Alternatively, WX Cen might be a low‑mass binary, presumably a supersoft X‑ray source (a V Sagittae star), and the X‑ray nondetection of WX Cen could be consistent with such a physical picture.