RXTE and ASCA Constraints on Non-thermal Emission from the A2256 Galaxy Cluster

An 8.3 hour observation of the Abell 2256 galaxy cluster using the Rossi X-ray Timing Explorer (RXTE) proportional counter array (PCA) produced a high quality spectrum in the 2 30 keV range. Joint fitting with the 0.7 11 keV spectrum obtained with the Advanced Satellite for Astrophysics and Cosmology (ASCA) Gas Imaging spectrometer (GIS) gives an upperlimit of ∼2.3×10−7 photons cm−2 sec−1 keV−1 for non-thermal emission at 30 keV. This yields a lower limit to the mean magnetic field of 0.36μG and an upperlimit of 1.8×10−13 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 μG 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 > 0.36 μG in the A2256 cluster. Subject headings: galaxies: clusters: individual (A2256) – galaxies: intergalactic medium – galaxies: magnetic fields – X-rays: galaxies