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Session 22 - Radio - Loud AGN & Extragalactic Jets.
Display session, Wednesday, January 07
Exhibit Hall,
We present first epoch VLBI observations of four X-ray selected BL Lacs (XBLs) from our ongoing program to image the parsec scale structure of a well defined sample of radio bright XBLs. In general XBLs are not as extreme as RBLs, with the former being less powerful, less variable, and with spectral energy distributions that peak at higher energies. These distinctions could be a consequence of different jet orientations, with RBL jets being oriented closer to the line of sight, or there may be intrinsic differences in the physical conditions in the jets.
Our VLBI images show XBLs to have the same core-jet morphology as RBLs, but with more varied polarization properties. The could be the consequence of significant differences in the relative importance of shocks and entrainment in XBLs and RBLs. We have also compared the radio properties of our sample of XBLs with soft X-ray data. Estimates of the distribution of Doppler factors based on SSC constraints show variations between RBLs and XBLs beyond those easily explained by orientation effects alone. However the similar correlations between the core and kpc-scale extended radio emission in both RBLs and XBLs are best understood in terms of just such an orientation difference. One possibility for these apparently contridictory results is that RBLs are intrinsically different in some way, perhaps with larger Lorentz factors, which would also bias RBL samples to have sources with jets oriented closer to the line of sight. As work on our VLBI sample continues we will be able to measure component speeds for a large number of XBLs in order to confirm or constrain such ideas.
