2006A&A...458..285K

We report the discovery of the super-soft X-ray source XMMU J052016.0-692505 in the LMC with XMM-Newton. We analyse the EPIC spectra of XMMU J052016.0-692505 and study the likely optical counterpart LMCV2135. Using an absorbed blackbody spectrum we derive a bolometric luminosity of >10³⁴erg/s for the X-ray source at LMC distance. Assuming that the bolometric luminosity does not exceed the Eddington luminosity of a ∼1.0M_☉ star we derive a blackbody temperature of (25-70)eV and an LMC absorbing column density of <6x10²¹cm^–2. The likely optical counterpart of XMMU J052016.0-692505 is the LMC variable star LMCV2135 which is a MACHO and OGLE variable. The infrared and optical colors and magnitudes of this star are consistent with a hot star of likely spectral type B. The long-term MACHO light curve shows variability with a timescale of ∼500 and ∼1000 days. The optical spectra obtained at the 1.9-meter telescope of the South African Astronomical Observatory show strong H_α and H_β emission lines (with EW_Hα∼34Å) which indicate a B0-3e star. The radial velocities of the H_α and H_β emission lines show a variation from ∼400-450km/s to ∼5-20km/s which is consistent with the systemic velocity of the LMC and an intrinsic variation most likely due to the rotation of the Be disk. We discuss LMCV2135/XMMU J052016.0-692505 as a Be/white dwarf binary system in the LMC. The super-soft X-ray spectrum of the source could be due to a stable nuclear burning white dwarf with a mass of ∼0.9-1.0M_☉.