Discovery of four super-soft X-ray sources in XMM-Newton observations of the Large Magellanic Cloud

TL;DR

This study identified four new super-soft X-ray sources in the Large Magellanic Cloud using XMM-Newton data, revealing their diverse natures and contributing to the understanding of such sources in our galaxy and beyond.

Contribution

The paper reports the discovery and detailed analysis of four new super-soft X-ray sources, including their classification and potential origins, expanding the known population in the Magellanic Clouds and our galaxy.

Findings

XMMU J050452.0-683909 is a planetary nebula central star.

XMMU J051854.8-695601 and XMMU J050815.1-691832 are likely new soft intermediate polars.

XMMU J044626.6-692011 shows coherent pulsations indicating a magnetic cataclysmic variable.

Abstract

Super-soft X-ray sources were established as a heterogeneous class of objects from observations of the Large Magellanic Cloud (LMC). We have searched for new sources of this class in the X-ray images obtained from the XMM-Newton survey of the LMC and additional archival observations. We first selected candidates by visual inspection of the image, and screened out artefacts which can mimic super-soft X-ray sources as well as bright foreground stars which create optical loading on the CCD image. We finally obtained 4 new super-soft X-ray sources for which we performed detailed X-ray timing and spectral analysis and searched for possible optical counterparts to identify their nature. XMMU J050452.0-683909 is identified as the central star of the planetary nebula SMP LMC21 in the LMC. We suggest XMMU J051854.8-695601 and XMMU J050815.1-691832 as new soft intermediate polars from the nature of their X-ray spectrum. Their estimated absorption-corrected luminosities and the blackbody radii indicate that they are located in our Galaxy, rather than the LMC. We discovered coherent pulsations of 497 s from XMMU J044626.6-692011 which indicates a magnetic cataclysmic variable nature of the source. The location of XMMU J044626.6-692011 in the LMC or our Galaxy is less clear. It could either be a white dwarf in the LMC with nuclear burning on its surface near the Eddington limit, or another soft intermediate polar in our Galaxy. The discovery of new super-soft X-ray sources makes a significant contribution to the known population in our own Galaxy. An observed higher density of sources in the direction of the Magellanic Clouds can likely be explained by the relatively low Galactic column density in their direction as well as a large number of existing observations sensitive at low X-ray energies.