Images of Mrk 421 at 15 GHz (left panel) and 24 GHz (right panel). These two images were obtained by stacking all of the images of the twelve epochs, at the respective frequency. The restoring beam for the 15 GHz image is 0.9 mas  ×  0.55 mas and the peak flux density is 307.2 mJy/beam. For the 24 GHz image, the restoring beam is 0.6 mas  ×  0.35 mas and the peak is 251.9 mJy/beam. The first contour is 0.35 mJy/beam, which corresponds to three times the off-source noise level. Contour levels are drawn at (− 1, 1, 1.4, 2, 2.8, 4...) in steps of .