Canonical Particle Acceleration in FR I Radio Galaxies

Matched resolution multi-frequency VLA observations of four radio galaxiesare used to derive the asymptotic low energy slope of the relativistic electrondistribution. Where available, low energy slopes are also determined for othersources in the literature. They provide information on the acceleration physicsindependent of radiative and other losses, which confuse measurements of thesynchrotron spectra in most radio, optical and X-ray studies. We find a narrowrange of inferred low energy electron energy slopes, n(E)=const*E^-2.1 for thecurrently small sample of lower luminosity sources classified as FRI (notclassical doubles). This distribution is close to, but apparently inconsistentwith, the test particle limit of n(E)=const*E^-2.0 expected from strongdiffusive shock acceleration in the non-relativistic limit. Relativistic shocksor those modified by the back-pressure of efficiently accelerated cosmic raysare two alternatives to produce somewhat steeper spectra. We note for furtherstudy the possiblity of acceleration through shocks, turbulence or shear in theflaring/brightening regions in FRI jets as they move away from the nucleus.Jets on pc scales and the collimated jets and hot spots of FRII (classicaldouble) sources would be governed by different acceleration sites andmechanisms; they appear to show a much wider range of spectra than for FRIsources.Comment: 16 figures, including 5 color. Accepted to Astrophysical Journa