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

Abstract

Context. Super-soft X-ray sources were established as a heterogeneous class of objects from observations of the Large Magellanic Cloud (LMC). Aims. 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. Methods. We first selected candidates by visual inspection of the image and screened out the artefacts that can mimic super-soft X-ray sources as well as the bright foreground stars that create optical loading in the detectors. We obtained four new super-soft X-ray sources for which we performed detailed X-ray timing and spectral analyses and searched for possible optical counterparts to identify their nature. We also looked at archival ROSAT and Swift observations to investigate the long-term behaviour of the sources. Results. XMMU J050452.0−683909 is identified as the central star of the planetary nebula SMP LMC 21 in the LMC. We suggest XMMU J051854.8−695601 and XMMU J050815.1−691832 as new soft intermediate polars based on 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. Conclusions. The discovery of new super-soft X-ray sources makes a significant contribution to the known population in our 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.