Electron-positron Annihilation and the Cosmic X-ray Background

The possibility that the processes responsible for the cosmic X-ray background (CXB) would also produce an e(-)-e(+) annihilation feature is examined. Under the assumption that these processes are thermal, the absence of a strong e(-)-e(=) annihilation feature places constraints on the compactness (L/R ratio) of these sources. Observations favor souces of small compactness ratio.