DELAYED RECOMBINATION AS A MAJOR SOURCE OF THE SOFT-X-RAY BACKGROUND

THE origin of the soft X-ray background radiation has remained masteri ous(1) since its discovery(2), although it is clear from the lack of a bsorption of the tow-energy X-rays that there must be a strong local c ontribution(3). Recent results(4,5) demonstrate, however, that there a re significant more distant contributions, whose origins are also uncl ear. Here we propose an explanation for both the local and more distan t contributions to the soft X-ray background-they seem to arise from t he rapid adiabatic expansion of hot gas, driven by the explosions of m assive stars. This hot gas cools quickly, 'freezing in' highly ionized atomic states. The X-ray emission arises from the delayed recombinati on of ions and electrons at relatively low temperatures, and is theref ore distinct from the more usual line emission excited by collisions w ith electrons. The X-ray flux is thus relatively insensitive to the lo cal gas kinetic temperature, as the gas is far from ionization equilib rium.