X-ray Constraints on Cluster Magnetic Fields
  Lower limits to the intracluster magnetic field are derived from modeling broad band X-ray spectra of 6 galaxy clusters with radio halos (A401, A754, A1367, Coma (A1656), A2256, and A2319). The 0.7 - 10 keV spectra for all clusters were obtained with the Advanced Satellite for Cosmology and Astrophysics (ASCA) Gas Imaging Spectrometer (GIS). For A401, A754, A1367, and A1656, data from the HEAO1-A2 High Energy Detector (HED) (2-60 keV) was included in a joint fit to further constrain the allowed non-thermal component. Upperlimits to the non-thermal X-ray emission are a factor of approximately 10 improvement over previous values. Lower limits to the average magnetic field derived from these upperlimits and the radio spectral parameters range from 0.08 - 0.30 microGauss. A seed intracluster field amplified by turbulence from galaxy motion through the intracluster medium should have an average value in the range of 0.1-0.2 (Goldman & Rephaeli, 1991; De Young, 1992), thus the lack of any detections makes it unlikely that field amplification is due to galaxy wakes. In addition, all of these clusters have an asymmetric surface brightness distribution while several also have X-ray spectroscopic evidence of a merger. We suggest that cluster mergers rather than galaxy wakes are the source of the magnetic field amplification. 
Mark Henriksen (University of North Dakota)