IONIZATION NEBULAE SURROUNDING SUPERSOFT X-RAY SOURCES

In this work we carry out a theoretical investigation of a new type of astrophysical gaseous nebula, viz., ionized regions surrounding super soft X-ray sources. Supersoft X-ray sources, many of which have charac teristic luminosities of approximately 10(37)-10(38) ergs s-1 and effe ctive temperatures of approximately 4 x 10(5) K, were first discovered with the Einstein Observatory. These sources have now been shown to c onstitute a distinct class of X-ray source and are being found in subs tantial numbers with ROSAT. We predict that these sources should be su rrounded by regions of ionized hydrogen and helium with properties tha t are distinct from other astrophysical gaseous nebulae. We present ca lculations of the ionization and temperature structure of these ioniza tion nebulae, as well as the expected optical line fluxes. The ionizat ion profiles for both hydrogen and helium exhibit substantially more g radual transitions from the ionized to the unionized state than is the case for conventional H II regions. The calculated optical line inten sities are presented as absolute fluxes from sources in the Large Mage llanic Cloud and as fractions of the central source luminosity. We fin d, in particular, that [O III] lambda5008 and He II lambda4686 are esp ecially prominent in these ionization nebulae as compared to other ast rophysical nebulae. We propose that searches for supersoft X-rays via their characteristic optical lines may reveal sources in regions where the soft X-rays are nearly completely absorbed by the interstellar me dium.