LUMINOUS SUPERSOFT X-RAY SOURCES

Luminous supersoft X-ray sources were discovered with the Einstein obs ervatory and have been established as an important new class of X-ray binaries on the basis of observations with the Roentgen Satellite (ROS AT). They have extremely soft spectra (equivalent blackbody temperatur es of similar to 15-80 eV) and are highly luminous (bolometric luminos ities of 10(36)-10(38) erg s(-1)). Correcting for the heavy extinction of soft X rays by interstellar neutral hydrogen, their numbers in the disks of ordinary spiral galaxies like our own and M31 are estimated to be of the order of 10(3). Their observed characteristics are consis tent with those of white dwarfs, which are steadily or cyclically burn ing hydrogen-rich matter accreted onto the surface at a rate of order 10(-7) M-. year(-1). The required high accretion rates can be supplied by mass transfer on a thermal time scale (10(6)-10(7) years) from clo se companion stars that are more massive than the white dwarf accretor , typically 1.3-2.5M(.). Steady burning can also occur in a post-nova stage, but for shorter time scales, and it has been observed in a few classical novae and symbiotic novae. A few supersoft sources have been found to be recurrent transients. They are possibly connected with ve ry massive white dwarfs accreting at high rates. Luminous supersoft so urces may make a considerable contribution to the Type Ia supernova ra te in spiral and irregular galaxies.