The super-soft source XMMU J052016.0-692505 in the LMC

A likely white dwarf Be/X-ray binary

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85741 Garching, Germany e-mail: [pkahabka;fwh]@mpe.mpg.de
² Centre for Astronomy, James Cook University, Townsville, Queensland 4811, Australia
³ University of Western Sydney, Locked Bag 1797, Penrith South DC, NSW 1797, Australia

Received: 25 April 2006
Accepted: 28 July 2006

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 for the X-ray source at LMC distance. Assuming that the bolometric luminosity does not exceed the Eddington luminosity of a ~ star we derive a blackbody temperature of and an LMC absorbing column density of <. 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 emission lines (with  Å) which indicate a B0–3e star. The radial velocities of the H_α and H emission lines show a variation from ∼ to ∼ 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 ∼.