Accepted???. Received????; in original form???? 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±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 ± 0.90) × 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 ±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> ∼ 109 yr) relativistic electrons with Lorentz factors γ ≈ 700–1700, that would be a significant source of radio emission in the hitherto unexplored frequency range ν ≈ 2–10 MHz.
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Bagchi et al. (1998) studied this question.