Millimeter VLBI and Variability in AGN Jets

Millimeter-VLBI images probe as deep as never before the nuclei of AGN. VLBI at 147GHz yields transatlantic fringes for the first time. Now we can begin to study the relation between jet kinematics and spectral activity with a few ten micro-arcsecond resolution. 1. Present Status of 3mm-VLBI The main advantage of VLBI observations at short millimeter wavelengths (mmVLBI) is the very high angular resolution and the ability to study emission regions, which are self-absorbed at longer wavelengths. The small observing beam (up to ∼ 50 μas at 86GHz) not only provides a spatial resolution of a few hundred to thousand Schwarzschild radii for a typical z > 0.1 quasar, but also facilitates to locate and trace the motion of moving jet components with unprecedented accuracy and at early times after initial flux density outbursts. To date, global VLBI observations at 86GHz are performed regularly, however, with regard to the relatively fast motion seen in many quasars (0.1 − 1mas/yr), not yet with dense enough time sampling (typically 2 observations per year). The worldwide combination of stations in the US – like the VLBA (presently 7 antennas equipped with receivers) and Haystack (37m) – and sensitive antennas in Europe (Effelsberg 100m, Pico Veleta 30m Onsala 20m and Metsahovi 14m) forms the most powerful global mm-VLBI array. With a typical single baseline detection sensitivity (7σ) of typically ∼ 0.2 Jy between Effelsberg and Pico Veleta and ∼ 0.5 Jy on VLBA baselines, sources with S86GHz ≥ 1−2 Jy can be reliably imaged with a dynamic range of up to a few hundred. In Figure 1 we show examples of 3mm-VLBI images for the quasar 3C 345 and for BLLac. 2. VLBI at Higher Frequencies Although sensitivity limitations and remaining calibration uncertainties still restrict the imaging capabilities of VLBI in the 3mm band a bit, such VLBI observations nowadays can be regarded as more or less standard.