Supersoft sources inXMM-NewtonSmall Magellanic Cloud fields

Abstract

We report the detection and study of two faint ROSAT supersoft X-ray sources in the SMC field withXMM-Newton, RX J0059.1−7505 and RX J0059.4−7118. Due to the larger eff ective area ofXMM-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 ∼20 eV 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 ∼90 eV and an additional spectrally hard component which can be reproduced with a powerlaw. The luminosity of RX J0059.4−7118 would be ∼4 × 10 34 erg s −1 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 ∼(1−2) 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.