1999ApJ...511..666H

An 8.3 hr observation of the Abell 2256 galaxy cluster using the Rossi X-Ray Timing Explorer proportional counter array produced a high-quality spectrum in the 2-30 keV range. Joint fitting with the 0.7-11 keV spectrum obtained with the ASCA gas imaging spectrometer gives an upper limit of ∼2.3x10^–7 photons.cm^–2.s^–1.keV^–1 for nonthermal emission at 30 keV. This yields a lower limit to the mean magnetic field of 0.36 µG and an upper limit of 1.8x10^–13 ergs.cm^–3 for the cosmic-ray electron energy density. The resulting lower limit to the central magnetic field is ∼1-3 µG. While a magnetic field of ∼0.1-0.2 µG can be created by galaxy wakes, a magnetic field of several microgauss is usually associated with a cooling flow or, as in the case of the Coma Cluster, a subcluster merger. However, for A2256 the evidence for a merger is weak and the main cluster shows no evidence of a cooling flow. Thus there is presently no satisfactory hypothesis for the origin of an average cluster magnetic field as high as greater than 0.36 µG in the A2256 cluster.