The spectra of jet bases in FR I radio galaxies: Implications for particle acceleration

Abstract

We present accurate, spatially resolved imaging of radio spectra at the bases of jets in eleven low-luminosity (Fanaroff-Riley I) radio galaxies, derived from Very Large Array (VLA) observations.We pay careful attention to calibration and to the effects of random and systematic errors, and we base the flux-density scale on recent measurements of VLA primary amplitude calibrators by Perley and Butler. We show images and profiles of spectral index over the frequency range 1.4-8.5 GHz, together with values integrated over fiducial regions defined by our relativistic models of the jets.We find that the spectral index α, defined in the sense Ivα v-α, decreases with distance from the nucleus in all of the jets. The mean spectral indices are 0.66 ± 0.01 where the jets first brighten abruptly and 0.59± 0.01 after they recollimate. The mean change in spectral index between these locations, which is independent of calibration and flux-density scale errors and is therefore more accurately and robustly measured, is -0.067plusmn; 0.006. Our jet models associate the decrease in spectral index with a bulk deceleration of the flow from ≈0.8c to ≲0.5c. We suggest that the decrease is the result of a change in the characteristics of ongoing particle acceleration. One possible acceleration mechanism is the first-order Fermi process in mildly relativistic shocks: in the Bohm limit, the index of the electron energy spectrum, p = 2α + 1, is slightly larger than 2 and decreases with velocity upstream of the shock. This possibility is consistent with our measurements, but requires shocks throughout the jet volume rather than at a few discrete locations. A second possibility is that two acceleration mechanisms operate in these jets: one (with p = 2.32) dominant close to the galactic nucleus and associated with high flow speeds, another (with p = 2.18) taking over at larger distances and slower flow speeds. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.