1994A&A...288..538K

Five super-soft X-ray sources (SSS) have been observed in the field of the Small Magellanic Cloud with ROSAT during the all-sky Survey and during deep pointed observations. They show absorbed blackbody spectra with temperatures of 10 to 60 eV and luminosities of at least 3x10³⁶erg/s to well above the Eddington limit for accretion onto a 1M_☉ compact object. One of the new super-soft X-ray sources detected by ROSAT, RX J0048.4-7332, is identified with the symbiotic nova SMC 3 in the SMC, two further SSS discovered by ROSAT , RX J0058.6-7146 and RX J0112.9-7521, are not yet confirmed as SMC members. The two SSS in the SMC detected by the Einstein satellite, 1E0056.8-7154, correlating with the planetary nebula N67, and 1E0035.4-7230 were observed by ROSAT as well. Several models are considered to explain the nature of these sources: Near-Eddington accretion onto neutron stars (NSs), hydrogen (or helium) burning on the central stars of planetary nebulae (PNe), or on accreting white dwarfs (WDs), hot and luminous extended accretion disks surrounding compact WDs, accretion on subdwarfs. The high luminosities deduced from the X-ray observations by ROSAT favor WDs accreting at high rates (≥10^–7M_☉/yr) and stable burning hydrogen or helium. If accretion occurs over system evolutionary timescales then NSs may be formed by accretion induced collapse. The detection of an X-ray eclipse by ROSAT in Cal 87, a SSS in the LMC, supports the model of a hot region occulted by the optical companion star. The observation of an X-ray turn-on from RX J0058.6-7146 with a duration of about one day is an important signature for short-time variability of SSS. The different behavior of the SSS argues against the description of a unique new class of objects.