Spectral Ages of Giant Radio Sources

Multifrequency observations with the GMRT and the VLA are used to determine the spectral breaks in consecutive strips along the lobes of a sample of selected giant radio sources (GRSs) in order to estimate their spectral ages. The maximum spectral ages estimated for the detected radio emission in the lobes of our sample of ten sources has a median value of $\sim$20 Myr. The spectral ages of these GRSs are significantly older than smaller sources. In all but one source (J1313+6937) the spectral age gradually increases with distance from the hotspot regions, confirming that acceleration of the particles mainly occurs in the hotspots. Most of the GRSs do not exhibit zero spectral ages in the hotspots. This is likely to be largely due to contamination by more extended emission due to relatively modest resolutions. The injection spectral indices range from $\sim$0.55 to 0.88 with a median value of $\sim$0.6. We show that the injection spectral index appears to be correlated with luminosity and/or redshift as well as with linear size.