Supersoft sources in XMM-Newton Small Magellanic Cloud fields

A symbiotic and a close binary or cooling neutron star

Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85741 Garching, Germany e-mail: [pkahabka;fwh]@mpe.mpg.de

Received: 15 September 2005
Accepted: 10 February 2006

Abstract

We report the detection and study of two faint ROSAT supersoft X-ray sources in the SMC field with XMM-Newton, RX J0059.1-7505  and RX J0059.4-7118. Due to the larger effective area of XMM-Newton we can constrain the X-ray spectra of both sources. RX J0059.1-7505 is optically identified with the symbiotic LIN 358 in the SMC. A ~ blackbody component dominates the observed spectrum. The soft blackbody component is consistent with steady nuclear burning in a shell although the spectrum is more complex than a simple blackbody continuum. RX J0059.4-7118 is not optically identified and we derive with the Optical Monitor (OM) a V magnitude 19.3 assuming an M0 spectral type. The X-ray spectrum is fitted with a blackbody component with a temperature of ~ and an additional spectrally hard component which can be reproduced with a powerlaw. The luminosity of RX J0059.4-7118 would be ~4  at the distance of the SMC. This is too large for a Cataclysmic Variable (CV). The spectral appearance is not in agreement with a supersoft source in the SMC. Thus we suggest that RX J0059.4-7118 is a Galactic source. As the optical magnitude derived from the OM data may be too faint for a normal Galactic CV we examined the possibility that RX J0059.4-7118 is a polar CV in the Galaxy, an isolated cooling neutron star (INS) at distance ~ kpc, a pulsar with a brown dwarf companion, or a Galactic quiescent low-mass X-ray binary (qLMXB). We favor the hypothesis of a Galactic CV because of variability in the EPIC-pn data with a timescale of ~1 h. A third supersoft ROSAT source, RX J0050.5-7455, is not detected with XMM-Newton.