EQUIPARTITION DOPPLER FACTORS FOR A SAMPLE OF ACTIVE GALACTIC NUCLEI

The Doppler factor of the outflow from compact radio cores of active g alactic nuclei (AGNs) can be estimated from single-epoch radio observa tions by assuming that the particles and magnetic field are in equipar tition, as suggested by Readhead. This estimate of the Doppler factor is called the equipartition Doppler factor, delta(eq). To test whether delta(eq) is a good estimator of the true Doppler factor, equipartiti on Doppler factors are computed for a sample of 105 radio sources and compared with the corresponding inverse Compton (self-Compton) Doppler factors, delta(IC), computed for this same sample by Ghisellini and c oworkers, by assuming the observed X-ray flux to be of inverse Compton origin. The Ghisellini et al. sample consists of 33 BL Lacertae objec ts, 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 galaxie s. The relevant assumptions for the computation of both the equipartit ion Doppler factor, delta(eq), and the inverse Compton Doppler factor, delta(IC), are discussed. A high correlation is found between these t wo estimates of the true Doppler factor, suggesting that they are both reliable. In fact, it appears that delta(eq)/delta(IC) is on the orde r of unity. This seems to indicate that the sources are near equiparti tion, and thus confirms the possibility of using delta(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 Doppl er factors are likely to play a crucial role in our understanding of t he physics at work in compact radio sources.