X-Ray 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 FornaxA, (NGC 1316) based on new ASCA observations made in 1997 for 98 ks, andincorporating the previous observation in 1994 for 39 ks. The 0.7--10 keVspectrum 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 scatteringof the cosmic microwave photons, as Kaneda et al. (1995) and Feigelson et al.(1995) concluded. The surface brightness of the inverse-Compton X-rays exhibitsa relatively flat distribution over the west lobe, indicative of anapproximately spherical emissivity distribution with a radius of ~11' (75 kpc).n contrast, the 1.4 GHz radio image by Ekers et al. (1983) exhibits a rim-brightened surface brightness, consistentwith a shell-like emissivitydistribution whose inner and outer boundaries are 4' and 11', respectively.These morphological differences between radio and X-rays suggest that therelativistic electrons are distributed homogeneously over the lobe volume,whereas the magnetic field is amplified toward the lobe rim region.