Long RXTE Observations of A2163
  A2163 was observed by the RXTE satellite for 530 ks during a 6 month period starting in August 2004. The cluster primary emission is from very hot intracluster gas with kT~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~15 keV, the best fit (to the combined PCA and HEXTE datasets) is obtained with a power law secondary spectral component. The deduced parameters of the non-thermal (NT) emission imply a significant fractional flux amounting 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=0.4+/-0.2 microG. 
Yoel Rephaeli, Duane Gruber, and Yinon Arieli