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P. J. Wiita (GSU), Gopal-Krishna (NCRA/TIFR), S. Dhurde (IUCAA)
By making allowance for the conical shape of ultrarelativistic blazar jets with opening angles of a few degrees on parsec-scales we show that their bulk Lorentz factors and viewing angles can be much larger than the values usually inferred by combining their flux variability with proper motion measurements and assuming the jets to be cylindrical. The effective values of apparent transverse velocity and flux boosting factors for the VLBI radio knots of jets are found by integrating over the angular distributions of these quantities across the widths of jets with finite opening angles. For high bulk Lorentz factors (\Gamma > 10) variations across the jet can be quite large if the opening angle on sub-parsec scales is even a few degrees. As we had shown earlier, the resulting apparent speeds are often much slower than those obtained from the usual analyses that assume cylindrical jets. Ultrarelativistic jets, with \Gamma > 30, have been inferred from high frequency VLBI measurements in some blazars. Such ultrarelativistic but conical jets can reconcile the relatively slow (usually subluminal) apparent motions of VLBI knots in TeV blazars with the extremely fast flows implied by their rapid \gamma-ray variability without assuming strong deceleration or large velocity gradients across the jet or extremely small viewing angles. We now find that this conical jet geometry also implies that de-projected jet opening angles will typically be significantly underestimated from VLBI measurements. Also, de-projected jet lengths will be considerably overestimated if high Lorentz factors and significant opening angles are not taken into account.
This work was supported in part by NSF grant AST 0507529.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.