Long RXTE observations of A2163

A2163 was observed by the RXTE satellite for similar to 530 ks during a 6 month period, starting in 2004 August. The cluster primary emission is from very hot intracluster gas with kT similar to 15 keV, but this component does not by itself provide the best-fitting model. A secondary emission component is quite clearly needed, and while this could also be thermal at a temperature significantly lower than kT similar to 15 keV, the best fit (to the combined PCA and HEXTE data sets) is obtained with a power-law secondary spectral component. The deduced parameters of the nonthermal (NT) emission imply a significant fractional flux amounting to similar to 25% of the integrated 3-50 keV emission. NT emission is expected, given the intense level of radio emission, most prominently from a large extended (halo'') central region of the cluster. Interpreting the deduced NT emission as Compton scattering of the radio-emitting relativistic electrons by the CMB, we estimate the volume-averaged value of the magnetic field in the extended radio region to be B similar to 0.4 +/- 0.2 mu G.