Accreting white dwarf models for CAL 83, CAL 87 and other ultrasoft X-ray sources in the LMC

Abstract

The study demonstrates that the ultrasoft X-ray emission observed in the three strong LMC X-ray sources CAL 83, CAL 87, and RXJ 0527.8-6954 can be explained by steady nuclear burning of hydrogen accreted onto white dwarfs with masses in the range of 0.7 to 1.2 solar mass. The observed optical and X-ray characteristics of the binary systems CAL 83 and CAL 87 are shown to be consistent with such a model. In both systems the companions are main-sequence stars with masses in the range of 1.5 to 2 solar masses. They are transferring mass unstably on a thermal time scale by Roche-lobe overflow, at rates between 1.0 and 4.0 x 10 exp -7 solar mass/yr. It is argued that the stellar wind emanating from the heated star interacts with that from the disk to generate the He II 4686 emission line with a radial velocity amplitude much lower than the actual radial velocity amplitude of the white dwarf, thus yielding an apparently excessively large mass estimate of this compact star. It is suggested that CAL 83 and CAL 87 are the white dwarf analogs of X-ray binaries like Her X-1.