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The properties of compact extragalactic radio sources are investigated using a sample of 27 sources extensively observed at millimeter frequencies. It is suggested that the radio variability of all AGN, independent of their classification, can be understood in terms of growing and decaying shocks in a jet.

BL Lacertae has been the target of four observing campaigns by the Whole Earth Blazar Telescope (WEBT) collaboration. In this paper we present UBVRI light curves obtained by theWEBT from 1994 to 2002, including the last, extended BL Lac 2001 campaign. A total of about 7500 optical observations performed by 31 telescopes from Japan to Mexico have been collected, to be added to the ∼15 600 observations of the BL Lac Campaign 2000. All these data allow one to follow the source optical emission behaviour with unprecedented detail. The analysis of the colour indices reveals that the flux variability can be interpreted in terms of two components: longer-term variations occurring on a fewday time scale appear as mildly-chromatic events, while a strong bluer-when-brighter chromatism characterizes very fast (intraday) flares. By decoupling the two components, we quantify the degree of chromatism inferring that longer-term flux changes imply moving along a ∼0.1 bluerwhen- brighter slope in the B − R versus R plane; a steeper slope of ∼0.4 would distinguish the shorter-term variations. This means that, when considering the long-term trend, the B-band flux level is related to the R-band one according to a power law of index ∼1.1. Doppler factor variations on a “convex” spectrum could be the mechanism accounting for both the long-term variations and their slight chromatism. Reig Torres, Pablo, Pablo.Reig@uv.es

Quasars have been monitored with the Metsahovi Radio Telescope since 1980 at 22 GHz and 37 GHz. During these years we have recorded numerous outbursts in our sample. Some of the outbursts are simple in nature, while others also seem to have fine structure. Can the outbursts be modelled in a simple way?

Quasars have been monitored with the Metsähovi Radio Telescope since 1980 at 22 GHz and 37 GHz. During these years we have recorded numerous outbursts in our sample. Some of the outbursts are simple in nature, while others also seem to have fine structure. Can the outbursts be modelled in a simple way?

We analyze the time variability of compact radio sources at 22 and 37 GHz by using a structure function analysis. Our monitoring program in Metsahovi, Finland, includes a complete sample of bright, flat-spectrum extragalactic compact radio sources. For the structure function analysis we have selected 42 best-observed sources from this sample. In this subsample we have 14 BL Lac objects, 13 high-polarization quasars, 12 low-polarization quasars, two galaxies, and one empty field source. The structure function analysis exhibits very similar slopes for all sources, which is consistent with there being one basic physical mechanism of millimeter variability for all compact radio sources