  It has been argued that certain broad absorption line quasars are viewed
within 35 degrees of the axis of a relativistic radio jet, based on two-epoch
radio flux density variability. It is true if the surface brightness of a radio
source is observed to change by a sufficiently large amount, the inferred
brightness temperature will exceed 10^12 K and Doppler beaming in our direction
must be invoked to avoid a Compton cooling catastrophe. However, flux density
changes cannot be linked to surface brightness changes without knowledge of the
size of the source. If an optically thick source changes in projected area but
not surface brightness, its brightness temperature is constant and its flux
variability yields no constraint on its orientation. Moreover, as pointed out
by Rees, spherical expansion of an emission source at relativistic speeds
yields an apparently superluminal increase in its projected area, which can
explain short-timescale flux density variability without requiring a
relativistic jet oriented near to our line of sight. Therefore, two-epoch radio
flux density variability by itself cannot unambiguously identify sources with
jets directed towards us. Only VLBI imaging can robustly determine the fraction
of broad absorption line quasars which are polar.
