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

A recent X-ray observation of the cluster 1E0657-56 (z=0.296) with ASCAimplied an unusually high temperature of ~17 keV. Such a high temperature wouldmake it the hottest known cluster and severely constrain cosmological since, ina Universe with critical density Omega=1 the probability of observing such acluster is only 4e-5. Here we test the robustness of this observational resultsince it has such important implications. We analysed the data using a varietyof different data analysis methods and spectral analysis assumptions and find atemperature 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 Galacticvalue. We show that a high temperature of ~17 keV is artificially obtained ifthe true spectrum has a stronger low- energy cut-off than that for Galacticabsorption only. The extra absorption may be astrophysical in origin, or it maybe a problem with the low-energy CCD efficiency. Although significantly lowerthan previous measurements, this temperature of kT ~11-12 keV is stillrelatively high since only a few clusters have been found to have temperatureshigher than 10 keV and the data therefore still present some difficulty for anOmega=1 Universe. Our results will also be useful to anyone who wants toestimate the systematic errors involved in different methods of backgroundsubtraction of ASCA data for sources with similar S/N to that of the 1E0657-56data reported here.Comment: 14 pages plus 2 figures. Latex with separate postscript figure files. AASTEX 4.0 macro. Accepted for the Astrophysical Journal Letter