Spectral Analysis of RXTE Observations of A3667
  X-ray emission from the cluster of galaxies A3667 was measured by the PCA and HEXTE experiments aboard the RXTE satellite during the period December 2001 - July 2002. Analysis of the ~141 ks RXTE observation and lower energy ASCA/GIS data, yields only marginalevidence for a secondary power-law emission component in the spectrum. The 90% confidence upper limit on nonthermal emission in the 15-35 keV band is determined to be 2.6x10^{-12} erg/(cm^{2}s). When combined with the measured radio flux and spectral index of the dominant region of extended radio emission, this upper limit implies a lower limit of ~0.4 microgauss on the mean, volume-averaged intracluster magnetic field in A3667. 
Yoel Rephaeli and Duane Gruber