Relativistic Beaming and Flux Variability in Active Galactic Nuclei

We discuss the impact of special relativistic effects on the observed light curves and variability duty cycles of radio-loud active galactic nuclei (AGNs). We model the properties of AGN light curves at radio wavelengths using a simulated shot noise process in which the occurrence of major flaring events in a relativistic jet is governed by Poisson statistics. We show that flaring sources whose radiation is highly beamed toward us are able to reach very high flux levels, but will in fact spend most of their time in relatively low flaring states. This is primarily due to relativistic Doppler contraction of flaring time scales in the observer frame. The fact that highly beamed AGNs are not observed to return to a steady-state quiescent level between flares implies that their weakly beamed counterparts should have highly stable flux densities that result from a superposition of many long-term, low-amplitude flares. The “apparent” quiescent flux levels of these weakly beamed AGNs (identified in many unified models as radio galaxies) will be significantly higher than their ”true” quiescent (i.e., non-flaring) flux levels. We have also performed Monte Carlo simulations to examine how relativistic beaming and source variability bias the selection statistics of flat-spectrum AGN samples. We find that in the case of the Caltech-Jodrell Flat-spectrum survey (CJ-F), the predicted orientation bias towards jets seen end-on is weakened if the parent population is variable, since the highly beamed sources have a stronger tendency to be found in low flaring states. This effect is small, however, due to the fact that highly beamed sources are relatively rare, and in most cases their flux densities will be boosted sufficiently above the survey limit such that they will be selected regardless of their flaring level. We find that for larger flat-spectrum AGN surveys with fainter flux density cutoffs, variability should not be an appreciable source of selection bias. Subject headings: relativity — galaxies : active — quasars : general — radio continuum : galaxies