Synchrotron self-absorption in the GPS radio galaxy B0108+388

Abstract

Context. The compact radio source in the nucleus of the GPS radio galaxy B0108+388 consists of two jets that have spectra that turn over at frequecies lower than ∼5 GHz. There is evidence that the spectral turnover in B0108+388 results from free-free absorption by non-uniform thermal plasma in central tens of parsecs. Aims. To reveal the real cause of the low frequency cutoff in the jet spectra, it is necessary to consider different mechanisms and, in particular, syncrotron self-absorption. Methods. We suggest that spectral turnovers at low frequencies of the jets are caused by synchrotron self-absorption. Estimations of physical parameters of these jets were obtained with a method based on a non-uniform synchrotron source model. Results. We find that magnetic fields in these jets are nearly uniform with 10 −2 < H⊥ < 10 −1 G and with relativistic electron density 10 1 < ne < 10 2 cm −3 . The relativistic electron energy is higher everywhere in the jets than the magnetic field energy, but by taking the estimated uncertainties into account it is possible that energy equipartition is in effect. We compare the physical conditions in the jets and in the nuclei of classical doubles. Conclusions. We present evidence that the spectral turnover in the B0108+388 results from synchrotron self-absorption.