7 2009 Fermi Symposium, Washington, D.C., Nov. 2-5 Figure 3: Results of the model-fitting based analysis of BL Lacertae (2200+420). The contours represent the CLEAN image at 43 GHz, the beam size and the linear scale are indicated at the lower left panel. The source structure was approximated with Gaussian components (see Table II). The fitting was conducted in the the uv -plane. The two inner components (C1 its position is marked by the red arrow, C2 marked by the blue arrow) are unresolved while the third component (B1 marked by two black arrows) is resolved at 43 GHz. Fig. 1x presents the spectrum of this source obtained in the course of pixel-based analysis. This spectrum corresponds to the combined spectrum (black) of components C1 (red) and C2 (blue) presented in the upper right panel of the current figure. The peak frequency for B1 is not well constrained, however, following [13] we can put an upper limit on the magnetic field strength in this component B < 0 . 06 G (ob- server’s frame). Table II: BL Lacertae model component parameters 1 mas BL Lac 43 GHz Component Distance ν$_{m}$ S$_{m}$ α [mas] [GHz] [Jy] C1 (core) 0 > 43 > 1 . 02 . . . C2 0 . 26 6 . 4 1 . 43 − 0 . 58 B1 1 . 47 < 4 . 1 > 0 . 51 − 0 . 17 ν$_{m}$ — synchrotron self-absorption peak frequency. S$_{m}$ — synchrotron self-absorption peak flux density. α — spectral index in the optically thin part of the spectrum. eConf C091122