Abstract
The probability density that a randomly selected beamed source lies at angle θ to the line of sight is shown in Fig. 3. For reasonable values of the Lorentz factor, the Doppler boost exponent and the N(S) exponent, p(θ) peaks at a few degrees, well inside the angle for maximum apparent transverse velocity. Theoretical distributions of the apparent transverse velocity are sharply peaked near the maximum possible value, but the observed distribution is peaked at low velocities. Some of the difference is due to the inhomogeneous nature of the sample. BL Lacs have a lower average velocity than quasars. From a purely statistical point of view this might be due to their being the low-θ tail of the quasar distribution; this also would help to explain the rapid intensity variations and high continuum/line ratios. However this explanation is unlikely, because the redshift distributions are so different.
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Cohen, M.H. (1989). Statistics of superluminal sources. In: Maraschi, L., Maccacaro, T., Ulrich, M.H. (eds) BL Lac Objects. Lecture Notes in Physics, vol 334. Springer, Berlin, Heidelberg. https://doi.org/10.1007/BFb0031139
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DOI: https://doi.org/10.1007/BFb0031139
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