2004A&A...427...79J

The radio spectrum of the relic radio galaxy B2 0924+30 is investigated using integrated flux densities between 0.151 and 10.6GHz, as well as images at 0.325, 0.609, 1.400 and 4.750GHz. Allowing for synchrotron and inverse Compton losses in this source, the observed spectral steepening yields a break frequency of ∼7 GHz, implying a mean particle age of about 50Myrs. The corresponding particle advance speed is of the order of 2000km/s. This relatively small particle lifetime following the exhaustion of the central source can explain the paucity of such sources and implies that such objects would easily escape detection if searched for in the cm wavelength regime.