A new super-soft X-ray source in the Small Magellanic Cloud: Discovery of the first Be/white dwarf system in the SMC?

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
e-mail: rsturm@mpe.mpg.de
² School of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK
³ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, via E. Bassini 15, 20133 Milano, Italy
⁴ Laboratoire AIM, CEA/DSM – CNRS – Université Paris Diderot, IRFU/SAp, CEA-Saclay, 91191 Gif-sur-Yvette, France
⁵ Warsaw University Observatory, Aleje Ujazdowskie 4, 00-478 Warsaw, Poland

Received: 29 July 2011
Accepted: 25 November 2011

Abstract

Context. The Small Magellanic Cloud (SMC) hosts a large number of Be/X-ray binaries, however no Be/white dwarf system is known so far, although population synthesis calculations predict that they might be more frequent than Be/neutron star systems.

Aims. XMMU J010147.5-715550 was found as a new faint super-soft X-ray source (SSS) with a likely Be star optical counterpart. We investigate the nature of this system and search for further high-absorbed candidates in the SMC.

Methods. We analysed the XMM-Newton X-ray spectrum and light curve, optical photometry, and the I-band OGLE III light curve.

Results. The X-ray spectrum is well represented by black-body and white dwarf atmosphere models with highly model-dependent temperature between 20 and 100 eV. The likely optical counterpart AzV 281 showed low near infrared emission during X-ray activity, followed by a brightening in the I-band afterwards. We find further candidates for high-absorbed SSSs with a blue star as counterpart.

Conclusions. We discuss XMMU J010147.5-715550 as the first candidate for a Be/white dwarf binary system in the SMC.