NONISOTHERMALITY OF THE HOTTEST CLUSTER ABELL-2163

Preliminary results of the ASCA observation of A2163 are presented. K alpha lines of He-like and H-like iron have been observed in the SIS spectrum at a redshift of 0.200 (0.190-0.206; hereafter the quoted errors are 90%). The flux ratio in these lines corresponds to T-e= 16 keV or a 90% lower limit of 12 keV, unambiguously confirming the presence of very hot gas in this cluster. The GIS spectrum in the 2.5-12 keV band shows a temperature of T-e = 12.7 +/- 2.0 keV, and an iron abundance of 0.38 +/- 0.13, assuming isothermality. Comparison of the deconvolved GIS cluster images in the energy bands 2.5-7 keV and 7-12 keV (rest frame) shows considerable gas temperature variations in the central 3-4 core radii region. A similar kind of nonisothermality is independently indicated, although marginally, by SIS images in the narrow energy intervals which include the 6.67 keV and 6.97 keV iron line blends. The overall picture suggests that neither the galaxies (Soucail et al. 1994) nor even the gas in A2163 are in equilibrium, and we may be currently observing a merger. If the cluster gas is not isothermal, the somewhat too high temperature derived from the iron line ratio may require a higher iron abundance in the hotter regions of the gas.