Equipartition doppler factors for a sample of active galactic nuclei

The Doppler factor of the outflow from compact radio cores of active galactic nuclei (AGNs) can be estimated from single-epoch radio observations by assuming that the particles and magnetic field are in equipartition, as suggested by Readhead. This estimate of the Doppler factor is called the equipartition Doppler factor, δ_eq. To test whether δ_eq is a good estimator of the true Doppler factor, equipartition Doppler factors are computed for a sample of 105 radio sources and compared with the corresponding inverse Compton (self-Compton) Doppler factors, δ_IC, computed for this same sample by Ghisellini and coworkers, by assuming the observed X-ray flux to be of inverse Compton origin. The Ghisellini et al. sample consists of 33 BL Lacertae objects, 24 core-dominated high-polarization quasars, 29 core-dominated low-polarization quasars (including seven core-dominated quasars with no polarization data), 11 lobe-dominated quasars, and eight radio galaxies. The relevant assumptions for the computation of both the equipartition Doppler factor, δ_eq, and the inverse Compton Doppler factor, δ_IC, are discussed. A high correlation is found between these two estimates of the true Doppler factor, suggesting that they are both reliable. In fact, it appears that δ_eq/δ_IC is on the order of unity. This seems to indicate that the sources are near equipartition, and thus confirms the possibility of using δ_eq to estimate the true Doppler factor of a source from single-epoch radio data. It appears that the Doppler factors of radio galaxies and lobe-dominated quasars are lower than those of the other categories of sources. This may be related to orientation effects, and could therefore be used to constrain orientation unified models. In any case, equipartition Doppler factors are likely to play a crucial role in our understanding of the physics at work in compact radio sources.