Hard X-Ray Spectrum from West Lobe of Radio Galaxy Fornax A Observed with Suzaku

Abstract

An observation of the West lobe of radio galaxy Fornax A (NGC 1316) with Suzaku is reported. Since Feigelson et al. (1995, ApJ, 449, L149) and Kaneda et al. (1995, ApJ, 453, L13) discovered the cosmic microwave background boosted inverse-Comptonized (IC) X-rays from the radio lobe, the magnetic field and electron energy density in the lobes have been estimated under the assumption that a single component of the relativistic electrons generates both the IC X-rays and the synchrotron radio emission. However, electrons generating the observed IC X-rays in the 1–10 keV band do not possess sufficient energy to radiate the observed synchrotron radio emission under the estimated magnetic field of a few $\mu$G. On the basis of observations made with Suzaku, we show in the present paper that a 0.7–20 keV spectrum is well described by a single power-law model with an energy index of 0.68 and a flux density of 0.12$\pm$0.01 nJy at 1 keV from the West lobe. The derived multiwavelength spectrum strongly suggests that a single electron energy distribution over a Lorentz factor $\gamma$$=$ 300–90000 is responsible for generating both the X-ray and radio emissions. The derived physical quantities are not only consistent with those reported for the West lobe, but are also in very good agreement with those reported for the East lobe.