Delayed recombination as a major source of the soft X-ray background

Abstract

THE origin of the soft X-ray background radiation has remained mysterious¹ since its discovery², although it is clear from the lack of absorption of the low-energy X-rays that there must be a strong local contribution³. Recent results^4,5 demonstrate, however, that there are significant more distant contributions, whose origins are also unclear. Here we propose an explanation for both the local and more distant contributions to the soft X-ray background—they seem to arise from the rapid adiabatic expansion of hot gas, driven by the explosions of massive stars. This hot gas cools quickly, 'freezing in' highly ionized atomic states. The X-ray emission arises from the delayed recombination of ions and electrons at relatively low temperatures, and is therefore distinct from the more usual line emission excited by collisions with electrons. The X-ray flux is thus relatively insensitive to the local gas kinetic temperature, as the gas is far from ionization equilibrium.