Canonical particle acceleration in FR I radio galaxies

Matched-resolution multifrequency VLA observations of four radio galaxies are used to derive the asymptotic low-energy slope of the relativistic electron distribution. When available, low-energy slopes are also determined for other sources in the literature. They provide information on the acceleration physics independent of radiative and other losses, which confuse measurements of the synchrotron spectra in most radio, optical, and X-ray studies. We find a narrow range of inferred low-energy electron energy slopes n(E) ∝ E^-2.1 for the currently small sample of lower luminosity sources classified as FR I (not classical doubles). This distribution is close to, but apparently inconsistent with, the test particle limit of n(E) ∝ E^-2.0 expected from strong diffusive shock acceleration in the nonrelativistic limit. Relativistic shocks or those modified by the back-pressure of efficiently accelerated cosmic rays are two alternatives to produce somewhat steeper spectra. We note for further study the possibility of acceleration through shocks, turbulence, or shear in the flaring and brightening regions in FR I jets as they move away from the nucleus. Jets on parsec scales and the collimated jets and hot spots of FR II (classical double) sources would be governed by different acceleration sites and mechanisms; they appear to show a much wider range of spectra than those for FR I sources.