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 \pm 0.10 , which agrees with the synchrotron radio index of 0.9 \pm 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 \sim 11 ^ { \prime } ( 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 ^ { \prime } and 11 ^ { \prime } , 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 .