On the Unusually High Temperature of the Cluster of Galaxies 1E 0657-56

Abstract

A recent X-ray observation of the cluster 1E0657-56 (z=0.296) with ASCA implied an unusually high temperature of ~17 keV. Such a high temperature would make it the hottest known cluster and severely constrain cosmological since, in a Universe with critical density Omega=1 the probability of observing such a cluster is only 4e-5. Here we test the robustness of this observational result since it has such important implications. We analysed the data using a variety of different data analysis methods and spectral analysis assumptions and find a temperature of ~11-12 keV in all cases, except for one class of spectral fits. These are fits in which the absorbing column density is fixed at the Galactic value. We show that a high temperature of ~17 keV is artificially obtained if the true spectrum has a stronger low- energy cut-off than that for Galactic absorption only. The extra absorption may be astrophysical in origin, or it may be a problem with the low-energy CCD efficiency. Although significantly lower than previous measurements, this temperature of kT ~11-12 keV is still relatively high since only a few clusters have been found to have temperatures higher than 10 keV and the data therefore still present some difficulty for an Omega=1 Universe. Our results will also be useful to anyone who wants to estimate the systematic errors involved in different methods of background subtraction of ASCA data for sources with similar S/N to that of the 1E0657-56 data reported here.