THE 6.4-KEV FLUORESCENT IRON LINE FROM CLUSTER COOLING FLOWS

The fate of the cooling gas in the central regions of rich clusters of galaxies is not well understood. In one plausible scenario clouds of atomic or molecular gas are formed. However the mass of the cold gas, inferred from measurements of low-energy X-ray absorption, is hardly c onsistent with the absence of powerful CO or 21-cm emission lines from the cooling flow region. Among the factors which may affect the detec tability of the cold clouds are their optical depth, shape and coverin g fraction. Thus, alternative methods to determine the mass in cold cl ouds, which are less sensitive to these parameters, are important. For the inner region of the cooling flow (e.g. within a radius of similar to 50-100 kpc) the Thomson optical depth of the hot gas in a massive cooling flow can be as large as similar to 0.01. Assuming that the coo ling time in the inner region is few times shorter than the lifetime o f the cluster, the Thomson depth of the accumulated cold gas can be ac cordingly higher (if most of the gas remains in the form of clouds). T he illumination of the cold clouds by the X-ray emission of the hot ga s should lead to the appearance of a 6.4-keV iron fluorescent line, wi th an equivalent width proportional to tau(T). The equivalent width on ly weakly depends on the detailed properties of the clouds, e.g. on th e column density of individual clouds, as long as the column density i s less than a few 10(23) cm(-2). Another effect also associated exclus ively with the cold gas is a flux in the Compton shoulder of bright X- ray emission lines. It also scales linearly with the Thomson optical d epth of the cold gas. With the new generation of X-ray telescopes, com bining large effective area and high spectral resolution, the mass of the cold gas in cooling flows (and its distribution) can be measured.