The discovery and properties of the ultra-soft X-ray transient EXO 1846-031

We report the discovery of a previously uncataloged ultra-soft X-ray transient EXO 1846-031 which was in outburst during 1985 April to September. The X-ray spectrum consists of an ultra-soft component and a high-energy power-law tail that extends to at least 25 keV. The ultra-soft component may be modeled by either a cutoff power-law or a multicolor blackbody disk model. The latter model allows the evolution in spectrum and intensity observed during the outburst to be accounted for by the change in a single parameter - the temperature at the innermost disk radius. We demonstrate that at least one other accretion disk model is able to account for these changes by the variation of a single parameter. During one of the three EXOSAT observations, EXO 1846-031 exhibited significant intensity variability which probably originates from the power-law component. We derive a position for this unidentified source and present the results of a search for the optical counterpart.