Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006aas...209.0809w&link_type=abstract
2007 AAS/AAPT Joint Meeting, American Astronomical Society Meeting 209, #08.09; Bulletin of the American Astronomical Society, V
Other
Scientific paper
In order to better understand the impact of the jet opening angle on certain key parameters inferred from VLBI radio observations of blazar nuclear jets we present analytical modeling of different types of conical relativistic jets. We had earlier shown that for a constant velocity jet, the discrepancy between low speeds indicated by VLBI knot motions and the high ones inferred from the emission of TeV photons could be reconciled if ultrarelativistic jets had modest opening angles. The key parameters we now evaluate are the the viewing angle of the jet and the apparent speed and Doppler factor of the radio knots on parsec scales. We make quantitative comparisons of the influence of the jet opening angle on these radio knot parameters, as would be estimated for two frequently considered types of relativistic nuclear jets: those having uniform bulk speed and those in which the bulk Lorentz factor of the flow decreases with distance from the jet axis (a `spine--sheath' flow). Our analysis shows that for both types of jet velocity distributions the expectation value of the jet orientation angle at first falls dramatically with increases in the (central) jet Lorentz factor, but it levels off at a fraction of the opening angle for extremely relativistic jets. We also find that the effective values of the apparent speeds and Doppler factors of the knots always decline substantially with increasing jet opening angle, but that this effect is strongest for ultrarelativistic jets with uniform bulk speed. We suggest that the rarity of highly superluminal parsec-scale radio components in TeV blazars can be understood if their jets are both highly relativistic and intrinsically weaker, so probably less well collimated than the jets in other blazars. This work was supported in part by NSF grant 0507529.
Dhurde Samir
Gopal-Krishna ..
Sircar Pronoy
Wiita Paul J.
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