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| Paper: |
Radioband Variability Properties of BL Lac Objects |
| Volume: |
159, BL Lac Phenomenon |
| Page: |
31 |
| Authors: |
Aller, Margo F. |
| Abstract: |
Centimeter-wavelength flux and linear polarization observations have quantified the high levels of activity in BL Lacertae objects and identified variability timescales ranging from < 1 day to several years. These data are consistent with the frequent occurrence of weak internal shocks in BL Lac jet flows; such shocks may result from the development of Kelvin-Helmholtz instabilities (possibly due to only mildly-relativistic bulk motion), and by reflections from the jet/ambient medium interface. The observed spectral flattening into the millimeter band, apparent from multifrequency monitoring measurements, is consistent with a flatter-than-canonical electron energy distribution of the radiating particles and suggests a class-dependent difference in the underlying flow compared to QSOs. Spatial variations in opacity as well as jet curvature, identified by VLBI/P measurements, play significant roles in source emission and must be accounted for in properly interpreting variability data. Long-term temporal changes in the flow in some class members, found from analyses of tri-decade-long monitoring measurements, are only now beginning to be explored. |
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