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 (A GNs). We model the properties of AGN light curves at radio wavelengths usin g a shot-noise process in which the occurrence of major flaring events in a relativistic jet is governed by Poisson statistics. We show that flaring s ources whose radiation is highly beamed toward us are able to reach very hi gh 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 timescales in the observer frame. The fact that highly beamed AG Ns are not observed to return to a steady state quiescent level between fla res implies that their weakly beamed counterparts should have highly stable flux densities that result from a superposition of many long-term, low-amp litude flares. The "apparent" quiescent flux levels of these weakly beamed AGNs (identified in most unified models as radio galaxies) will be signific antly higher than their "true" quiescent (i.e., nonflaring) flux levels. We have also performed Monte Carlo simulations to examine how relativistic be aming 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, the predicted orientation bias toward jets seen end-on is weakened if the parent population is variable, because highly beamed sources have a stronger tendency to be found in low flaring states. This effect is small, however, because highly beamed sources are relatively rare, and in most ca ses their flux densities will be boosted sufficiently above the survey limi t that they will be selected regardless of their flaring level. We find tha t, for larger flat-spectrum AGN surveys with fainter fl density cutoffs, va riability should not be an appreciable source of selection bias.