Possible Detection of a Dense Intergalactic Plasma

Observations in the 1.5-8 keV range indicate that the isotropic X-ray background spectrum breaks at low energies, implying that a flux observed at 0.27 keY has an independent origin. This soft X-ray flux is attributed to free-free emission from a dense (105_106 particle cm-3), hot (3-8 X 1O~ ° K) inter- galactic gas. A very soft X-ray source has been found. It is shown that the integral of such sources in other galaxies probably could not provide the observed high-latitude soft flux