2005A&A...433..467J

Deep 4860 and 8350 MHz observations with the VLA and 100-m Effelsberg telescopes, supplementing available radio survey maps at the frequencies of 327MHz (WENSS survey) and 1400MHz (NVSS survey), are used to study the synchrotron spectra and radiative ages of relativistic particles in opposite lobes of the giant radio galaxy J1343+3758 (Machalski & Jamrozy, 2000A&A...363L..17M). The classical spectral ageing analysis (e.g. Myers & Spangler, 1985ApJ...291...52M) with assumption of equipartition magnetic fields gives a mean separation velocity (<v_sep>) of about 0.16c and 0.12c measured with respect to the emitting plasma, and suggests a maximum particle age of about 48 and 50Myr in the NE and SW lobes, respectively. On the contrary, a mean jet-head advance speed (<v_adv>) in the above lobes, derived from ram-pressure arguments, is about 0.016c and 0.017c, respectively. This would imply a substantial backflow of the lobe material from the hotspot regions towards the radio core, v_bf, which is not supported by the available radio maps. A compromise is achieved by assuming an enhancement of 3 to 5 times the equipartition magnetic field strengths in the lobes which gives <v_sep≥∼0.06c and <v_adv≥~<v_bf≥∼0.03c, hence a dynamical age of the source of 204±40Myr. A comparison of the radiative and dynamical ages of the investigated giant radio galaxy implies that the dynamical age is about 4 times the maximum synchrotron age of the emitting particles, which supports the conclusion of Blundell & Rawlings (2000AJ....119.1111B) that the spectral and dynamical ages are comparable only when they are ≪10Myr, and suggests that for FRII-type sources the discrepancy between these ages increases with age.