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Session 99 - Blazars.
Display session, Thursday, January 16
Metropolitan Ballroom,

[99.01] The Radio Properties of BL Lac Objects

M. F. Aller, H. D. Aller, P. A. Hughes (U. Michigan)

We present results on the cm-wavelength total flux density and linear polarization properties of radio-loud BL Lac objects based on a 17-year study with the University of Michigan 26-meter paraboloid of a flux-limited sample. The objects are found to exhibit large fractional changes in total flux density produced either by rapid, well-defined outbursts or by long-term declines and rises. Discrete events are characteristically nearly-simultaneous at 14.5, 8.0 and 4.8 GHz, and the spectra generally remain flat, even during active periods. This contrasts with the behavior in QSOs where frequency-dependent effects are common during outburst evolution due to self-absorption in the emitting region. The polarization also exhibits characteristic differences. We find large erratic changes in the polarization position angle in many sources consistent with the suggestion that shocks form more readily in BL Lacs than in other active galactic nuclei; when compared with VLBI/P information, the polarization data show that during stable periods the magnetic fields in the emitting regions are characteristically oriented perpendicular to the flow direction. This contrasts with the behavior in QSOs where we often find long-term stability and an alignment along the flow.

The data support a scenario in which the emission from BL Lac objects is dominated by contributions from evolving source components rather than from an underlying jet, as appears to be the case for QSOs. On the basis of comparison with relativistic hydrodynamical simulations, we suggest that class differences can be attributed to velocity-dependent differences in the parsec-scale flows in the jets.

This work was partially supported by NSF grant AST-9421979.

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