2006ApJ...649..673R

A2163 was observed by the RXTE satellite for ∼530 ks during a 6 month period, starting in 2004 August. 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 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 ∼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 µG.