The diffuse, relic radio source in Abell 85: estimation of cluster scale magnetic field from inverse Compton X-rays

We report the first detection of an inverse Compton X-ray emission, spatially correlated with a very steep spectrum radio source (VSSRS), 0038-096, without any detected optical counterpart, in cluster Abell 85. The ROSAT PSPC data and its multiscale wavelet analysis reveal a large scale (linear diameter of the order of 500 h^{-1}_{50} kpc), diffuse X-ray component, in excess to the thermal bremsstrahlung, overlapping an equally large scale VSSRS. The primeval 3 K background photons, scattering off the relativistic electrons can produce the X-rays at the detected level. The inverse Compton flux is estimated to be (6.5\pm 0.5) 10^{-13} erg s^{-1}cm^{-2} in the 0.5-2.4 keV X-ray band. A new 327 MHz radio map is presented for the cluster field. The synchrotron emission flux is estimated to be (6.6\pm 0.90) \times 10^{-14} erg s^{-1} cm^{-2} in the 10-100 MHz radio band. The positive detection of both radio and X-ray emission from a common ensemble of relativistic electrons leads to an estimate of (0.95\pm 0.10) 10^{-6} G for the cluster-scale magnetic field strength. The estimated field is free of the `equipartition' conjecture, the distance, and the emission volume. Further, the radiative fluxes and the estimated magnetic field imply the presence of `relic' (radiative lifetime > 10^{9} yr) relativistic electrons with Lorentz factors \gamma \approx 700-1700, that would be a significant source of radio emission in the hitherto unexplored frequency range \nu \approx 2-10 MHz.