The super-soft source XMMU J052016.0-692505 in the LMC - a likely white dwarf Be/X-ray binary

Abstract

Aims: We report the discovery of the super-soft X-ray source XMMU J052016.0-692505 in the LMC with XMM-Newton.

Methods: We analyse the EPIC spectra of XMMU J052016.0-692505 and study the likely optical counterpart LMCV2135.

Results: Using an absorbed blackbody spectrum we derive a bolometric luminosity of ⪆ 1034 erg s-1 for the X-ray source at LMC distance. Assuming that the bolometric luminosity does not exceed the Eddington luminosity of a ~ 1.0 Mȯ star we derive a blackbody temperature of (25-70) eV and an LMC absorbing column density of < 6× 1021 cm-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 Hα and Hβ emission lines (with EWH_α˜ 34 Å) which indicate a B0-3e star. The radial velocities of the Hα and Hβ emission lines show a variation from ˜ 400-450 km s-1 to ˜ 5-20 km s-1 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 ˜ 0.9-1.0 Mȯ.