1995A&A...293..337E

We obtained a deep exposure of the exceptionally hot cluster of galaxies A2163 with the PSPC on board the ROSAT satellite. These data, combined with X-ray spectroscopic data previously obtained with the GINGA satellite clearly confirm the very high temperature and mass of this cluster. The low background of the PSPC enabled us to map the gas distribution very far from the cluster center and we found that the gas extends to at least 4.6Mpc or 15 core radii. The PSPC image revealed that the cluster is probably in a non fully relaxed dynamical state. A joint analysis of ROSAT and GINGA spectroscopic data is used to better constrain the overall temperature and iron abundance and to constrain non isothermal models. The spectroscopic data are well fitted with an isothermal model with L_X[2-10 keV]=6.0x10⁴⁵ergs/s, kT=14.6_–0.8^+0.9keV and an heavy element abundance equal to 0.40_–0.14^+0.15 solar value. There is no strong evidence for any departure from isothermality. Finally these data enabled us to significantly improve our determination of the gas and binding masses (assuming simple analytical models). A2163 is found to be 2.6 times more massive than Coma with a similar gas mass over binding mass ratio of about 0.3: M_gas(r≤4.6Mpc)=1.43 ±0.05x10¹⁵M_☉ and M_binding(r≤4.6Mpc)=4.6_–1.5^+0.4x10¹⁵M_☉. Assuming that the baryon mass fraction in clusters reflects the corresponding fraction in the Universe (i.e. no hot dark matter) the high baryonic mass fraction {OMEGA}_b/{OMEGA}>0.28 observed in A2163, combined with the baryon density obtained by nucleosynthesis models consistent with observations, implies a low density value for the Universe, far from the critical closure density: {OMEGA}<0.2.