2006ApJ...645..256I

An XMM-Newton observation of the east radio lobe of the nearby radio galaxy Fornax A is reported. The diffuse hard X-ray emission associated with the east lobe, which was initially discovered by ASCA and ROSAT, is confirmed with significant signal statistics, after strictly removing 59 sources detected within the MOS field of view. Its X-ray spectrum is described by a single power-law model, which is absorbed by a medium with a column density consistent with that toward the object. The best-fit X-ray photon index, Γ_X=1.62^+0.24_–0.15, agrees with the synchrotron radio index, Γ_R=1.68±0.1, determined from the radio spectrum between 29.9 MHz and 5 GHz. Hence, the inverse Compton interpretation for the diffuse X-rays is justified. The X-ray flux density in the east lobe is measured to be 90±21 nJy at 1 keV (including both statistical and systematic errors) with the index fixed at the radio value. This gives electron and magnetic energy densities of 3.0^+1.5_–1.0x10^–13 and 6.1^+5.7_–3.5x10^–14 ergs/cm3, respectively. The latter corresponds to a magnetic field strength of 1.24^+0.50_–0.40 µG, which is smaller than the field estimated under the minimum energy condition, 1.55 µG, although with a slightly large error. Reevaluation is also made of the ASCA result on the west lobe, to show that both lobes share a similar physical condition in terms of energetics.