Optical identification of the transient supersoft X-ray source RX J0527.8-6954, in the LMC

TL;DR

This paper reports the optical identification of the transient supersoft X-ray source RX J0527.8-6954 in the LMC, linking it to a B5e V star with extended Halpha emission, and discusses implications for its orbital period.

Contribution

The study provides the first optical identification of RX J0527.8-6954 and characterizes its stellar environment, offering insights into its nature as a supersoft X-ray source.

Findings

Identified the X-ray source with a B5e V star with extended Halpha emission.

Suggested the orbital period exceeds 21 hours, not 9.4 hours.

Provided observational evidence supporting the white dwarf accretor model.

Abstract

Context. Close binary supersoft X-ray sources (CBSS) are binary systems that contain a white dwarf with stable nuclear burning on its surface. These sources, first discovered in the Magellanic Clouds, have high accretion rates and near-Eddington luminosities (10^37 - 10^38 erg/s) with high temperatures (T = 2 - 7 x 10^5 K). Aims. The total number of known objects in the MC is still small and, in our galaxy, even smaller. We observed the field of the unidentified transient supersoft X-ray source RX J0527.8-6954 in order to identify its optical counterpart. Methods. The observation was made with the IFU-GMOS on the Gemini South telescope with the purpose of identifying stars with possible He II or Balmer emission or else of observing nebular extended jets or ionization cones, features that may be expected in CBSS. Results. The X-ray source is identified with a B5e V star that is associated with subarcsecond extended Halpha emission, possibly bipolar. Conclusions. If the primary star is a white dwarf, as suggested by the supersoft X-ray spectrum, the expected orbital period exceeds 21 h; therefore, we believe that the 9.4 h period found so far is not associated to this system.