Properties of the double β model for intracluster gas

We present an extensive study of the double beta model for the X-ray surface brightness profiles of clusters, and derive analytically the gas density and total masses of clusters under the hydrostatic equilibrium hypothesis. It is shown that the employment of the double beta model instead of the conventional single beta model can significantly improve the goodness-of-fit to the observed X-ray surface brightness profiles of clusters, which will in turn lead to a better determination of the gas and total mass distributions in clusters. In particular, the observationally fitted beta parameter for the extended component in a double beta model may become larger. This opens a new possibility of resolving the long-standing beta discrepancy for clusters. Using an ensemble of 33 ROSAT PSPC observed clusters drawn from the Mohr, Mathiesen & Evrard sample, we find that the asymptotic value of beta (fit) is 0.83 +/-0.33 at large radii, consistent with both the average spectroscopic parameter beta (spec)=0.78 +/-0.37 and the result given by numerical simulations.