Measurements of the Field and Particle Energy Distributions in the West Lobe of the Radio Galaxy NGC 1316 ( Fornax A )

A follow-up X-ray study was made of the west lobe of the radio galaxy Fornax A, (NGC 1316) based on new ASCA observations made in 1997 for 98 ks, and incorporating the previous observation in 1994 for 39 ks. The 0.7–10 keV spectrum of the emission can be described by a power-law of energy index 0.74 ± 0.10, which agrees with the synchrotron radio index of 0.9 ± 0.2. Therefore, the X-rays are reconfirmed to arise via inverse-Compton scattering of the cosmic microwave photons, as Kaneda et al. (1995) and Feigelson et al. (1995) concluded. The surface brightness of the inverse-Compton X-rays exhibits a relatively flat distribution over the west lobe, indicative of an approximately spherical emissivity distribution with a radius of ∼ 11 ′ (75 kpc). In contrast, the 1.4 GHz radio image by Ekers et al. (1983) exhibits a rim-brightened surface brightness, consistent with a shell-like emissivity distribution whose inner and outer boundaries are 4 ′ and 11 ′ , respectively. These morphological differences between radio and X-rays suggest that the relativistic electrons are distributed homogeneously over the lobe volume, whereas the magnetic field is amplified toward the lobe rim region.