2016MNRAS.460.2625L

Traditionally the thermodynamic profiles (gas density, temperature, etc.) of galaxy clusters are obtained by assuming spherical symmetry and modelling projected X-ray spectra in each annulus. The outer annuli contribute to the inner ones and their contribution needs to be subtracted to obtain the temperature and density of spherical shells. The usual deprojection methods lead to propagation of errors from outside to in and typically do not model the covariance of parameters in different radial shells. In this paper we describe a method based on a free-form model of clusters with cluster parameters (density, temperature) given in spherical shells, which we jointly forward fit to the X-ray data by constructing a Bayesian posterior probability distribution that we sample using the MCMC technique. By systematically marginalizing over the nuisance outer shells, we estimate the inner entropy profiles of clusters and fit them to various models for a sample of Chandra X-ray observations of 17 clusters. We show that the entropy profiles in almost all of our clusters are best described as cored power laws. A small subsample is found to be either consistent with a power law, or alternatively their cores are not fully resolved (smaller than, or about few kpc). We find marginal evidence for bimodality in the central values of entropy (and cooling time) corresponding to cool-core and non cool-core clusters. The minimum value of the ratio of the cooling time and the free-fall time (min[t_cool/tff]; correlation is much weaker with core entropy) is anti-correlated with H α and radio luminosity. H α emitting cold gas is absent in our clusters with min(t_cool/tff) >= 10. Our lowest core entropies are systematically and substantially lower than the values quoted by the ACCEPT sample.