HEAD Division Meeting 1999, April 1999
Session 13. Clusters and Galaxies
Oral, Tuesday, April 13, 1999, 8:30am-10:10am, Colonial Room

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[13.02] Luminous Soft Emission from Clusters of Galaxies - Spatial and Spectral Diagnostics

Richard Lieu (UAH)

The recent missions of EUVE, ROSAT, BeppoSAX, and RXTE, through their wide passband coverage, revealed that the intracluster medium is considerably more complicated than that given by the hitherto accepted model of virial temperature gas with a cooling flow. An important aspect of these discoveries is the luminous `soft excess', measured spatially by EUVE and SAX to exhibit emission contours with the same anisotropic shape which do not follow the X-ray distribution. For the Coma cluster there is suggestive evidence of non-thermal origin with the soft excess being part of the same power-law component as that which accounts for the hard X-ray `tail' - the other significant new phenomenon found by SAX and RXTE. For those regular clusters which show no signatures of recent merging activities, however, the 0.1 - 0.2 keV excess is > 10 times above expectation, with no accompanying soft X-ray excess. This can only be explained by a line-type spectrum, from either warm under-ionized gas of, e.g., a mixing layer (with consequent mass implications) or a totally surprising reaction process which, though unlikely, is not yet completely excluded.

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