An Improved Measure of Quasar Orientation

Radio core dominance, usually measured by R, the rest frame ratio of core to lobe flux density, has been used as an indicator of Doppler boosting of quasars' radio jets and hence the inclination of the central engine's spin axis to the line of sight. The use of lobe flux density as a means of normalizing the boosted core flux density to the available intrinsic power of the central engine introduces scatter. This is because the emission from the radio lobes depends strongly on the interaction of the jet with the environment at distances beyond several Kpc from the nucleus. Total kinetic power in the extended radio emission is approximately proportional to emission line luminosity, and emission line luminosity is proportional to the luminosity of AGNs' featureless continua -- both over 4 orders of magnitude. Thus, quasars' optical luminosity may be an excellent measure of available intrinsic jet power. Therefore we define a new core dominance parameter, R_V, the ratio of radio core to optical (rest frame V band) continuum luminosity, that is not directly dependent on jet interactions with the super-Kpc-scale environment. We show that the use of R_V, rather than R, results in significantly improved inverse correlations with the beaming angle as deduced from apparent superluminal velocities and inverse Compton scattered X-ray emission, and with the FWHM of quasars' broad H\\beta emission line. We discuss some implications and applications of the new parameter.

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