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

Abstract

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 addition 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+/-0.5)x10^-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+/-0.90)x10^-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+/-0.10)x10^-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~700-1700 this would be a significant source of radio emission in the hitherto unexplored frequency range nu~2-10 MHz.