Trapped line radiation in X-ray sources of Population II

Abstract

We suggest that an X-ray source of Population II is produced by gas accreting onto a collapsed star from an extended quasi-transparent envelope formed in the distant past in a collision with a normal star. Most of the envelope is only weakly ionized; energy balance and hydrostatic equilibrium are ensured mainly by secondary line radiation, which is trapped there for very long times before it escapes or is reabsorbed. However, in the fully ionized innermost regions of the envelope the atomic particles that can absorb and re-emit line radiation are not present and matter is steadily accreting onto the black hole, producing the primary X-rays.

On the basis of the mechanism that controls the escape of line photons, we distinguish between lines of the ‘first’ and ‘second’ kind. The first should fall in the visible or the infrared and appear in emission; the second, resulting exclusively from allowed transitions, should fall in the ultraviolet and appear as very broad absorption lines. Heat deposited by partial absorption of X-rays is withdrawn by radiation of the first kind; hydrostatic equilibrium is ensured primarily by that of the second.

The collapsed star is probably a moderately massive (≲10M _⊙) black hole.