On spectral aging in lobes of double radio sources

Abstract

The sensitivity of the ages derived from the observed spatial gradients of spectral index along the lobes of double radio sources to the presence of small variations of magnetic field is discussed. Age variations inconsistent with the simple picture of lobe formation can be inferred if a uniform magnetic field throughout the lobe is assumed, particularly when the magnetic energy density in some regions is comparable to that of the cosmic microwave background. Reanalysis of the multifrequency radio data of Alexander for the western lobe of 3C 234, incorporating a small magnetic field gradient indicated by the equipartition estimates, provides no clear evidence for the claimed in situ acceleration.