The spectral index variations shown in Figure 9 indicate that there are regions
2.4' back towards C from the brightest
region in each lobe that have unusually flat spectra
(
-0.3), flatter even than the hot spots.
The only extended synchrotron sources known with spectra this flat are
a few Galactic supernova remnants ([Berkhuijsen 1986]).
The spectral index structure in NVSS 2146+82 is unlike the systematic steepening of the spectrum away from the hot spots that is usually interpreted as an effect of spectral aging in extended lobes. In such interpretations, electrons are presumed to be injected into a high field region in or around the hot spots, and their energy spectrum steepens with distance as they diffuse into lower field regions of the extended lobes. Clearly no such interpretation can be made here.
These flatter spectrum regions occur in the transition zone from the featureless parts of the lobes (closer to the core) to the parts near the regions of enhanced emission that contain significant filamentary structure. The anomalous regions are near the midline of the lobes; the southern region is centered on the path of the jet and the northern region is at one end of a prominent filament (the path of the jet is uncertain). The relative symmetry of the flatter spectrum regions of the lobes suggests that they might be produced by an intrinsic property of the source, such as a variable spectral index in the injection spectrum of the relativistic electrons from the jet, rather than local environmental effects.
If the magnetic field has values near those estimated by the equipartition calculations given above, then the energy loss of the radiating electrons is dominated by inverse Compton scattering against the Cosmic Microwave Background. In the low density, low magnetic fields in these lobes, the aging effects will be slow and the history of a variable electron spectrum could be maintained along the length of the lobe.