RXTE Observations of A2256

The cluster of galaxies A2256 was observed by the PCA and HEXTE experiments aboard the RXTE satellite during the period July 2001 - January 2002, for a total of 343 ks and 88 ks, respectively. Most of the emission is thermal, but the data analysis yields evidence for two components in the spectrum. Based on statistical likelihood alone, the secondary component can be either thermal or power-law. Inclusion in the analysis of data from ASCA measurements leads to a more definite need for a second component.Joint analysis of the combined RXTE-ASCA data sets yields $k{T}_1=7.9_{-0.2}^{+0.5}$ and $kT_2 = 1.5^{+1.0}{-0.4}$,whenthesecondcomponentisalsothermal,and$kT = 7.7^{+0.3}{-0.4}$and$\alpha = 2.2^{+0.9}_{-0.3}$, if the second component is fit by a power-law with (photon) index $\alpha$; all errors are at 90% confidence. Given the observed extended regions of radio emission in A2256, it is reasonable to interpret the deduced power-law secondary emission as due to Compton scattering of the radio producing relativistic electrons by the cosmic microwave background radiation. If so, then the {\it effective, mean volume-averaged} value of the magnetic field in the central 1${}^o$ region of the cluster – which contains both the halo' and relic’ radio sources – is $B\sim 0.2_{-0.1}^{+1.0}$ $\mu G$.