We discuss the present performance and the future perspectives of VLBI in the 3 mm to 0.85 mm observing bands (so called mm-VLBI). The availability of new telescopes and the recent technical development towards larger observing bandwidth and higher d
ata-rates now allow to image with 3mm-VLBI hundreds of sources with high dynamic range. As an example we show new images of the jets of Cygnus A. At 1.3 mm, pilot VLBI studies have proven detectability of the brightest AGN, and the existence of ultra-compact regions therein. In the next few years global VLBI imaging will be established also at 1.3 mm and 0.85 mm wavelength. With an angular resolution in the 10-20 micro-arcsecond range, future 1.3 mm- and 0.8 mm VLBI will be an extraordinarily powerful astronomical observing method, allowing to image the enigmatic `central engines and the foot-points of AGN-jets in greater detail than ever possible before. A sufficiently large number of telescopes is a prerequisite for global aperture synthesis imaging. Therefore a strong effort is needed to make more telescopes available for VLBI at short millimeter and sub-millimeter wavelengths. In this context, the further VLBI upgrade of both IRAM telescopes and the outfit of the APEX telescope in Chile, in preparation for later mm-/sub-mm VLBI with ALMA, is of high scientific importance. With a sufficiently large mm-VLBI network, the micro-arcsecond scale imaging of the post-Newtonian emission zone around the event horizon/ergosphere of nearby super-massive Black Holes (such as e.g. Sgr A*, M87, ...) should become possible within the next few years.
Our previous studies revealed a good kinematic model for the jet of Cygnus A, but the counter-jet speed is still not well constrained. The central engine and part of the counter-jet of Cyg A are likely to be obscured by free-free absorbing material,
presumably a thick torus. At mm-wavelengths, the absorber becomes optically thin, which provides a more detailed view into the inner nuclear region. Knowing the speed of jet and counter-jet and their flux density ratio allows to determine the jet Lorentz factors and orientation. Therefore we started to monitor Cyg A with global VLBI at 43GHz in Oct. 2007. Our first epoch reveals a previously unseen gap between both jets. This could be either a sign for a new counter-jet component that is slowly separating or we start to see the very inner acceleration region of the jet which is not efficiently radiating at radio wavelengths. Further more the image shows transversely resolved jet structures at distances beyond ~0.5pc which facilitate more detailed investigations addressing jet stratification. Analysis of the resolved jet structure shows that the initially wide jet (opening angle ~10deg) collimates within the first parsec into a edge-brightened jet with an opening angle of ~3deg.
To locate and image the compact emission regions in quasars, which are closely connected to the phenomenon of IntraDay Variability (IDV), space VLBI observations are of prime importance. Here we report on VSOP observations of two prominent IDV source
s, the BL Lac objects S5 0716+714. To monitor their short term variability, these sources were observed with VSOP at 5 GHz in several polarisation sensitive experiments, separated in time by one day to six days, in autumn 2000. Contemporaneous flux density measurements with the Effelsberg 100m radio telescope were used to directly compare the single dish IDV with changes of the VLBI images. A clear IDV behaviour in total intensity and linear polarization was observed in 0716+714. Analysis of the VLBI data shows that the variations are located inside the VLBI core component of 0716+714. In good agreement with the single-dish measurements, the VLBI ground array images and the VSOP images, both show a decrease in the total flux density of ~20 mJy and a drop of ~5 mJy in the linear polarization of the VLBI core. No variability was found in the jet. From the variability timescales we estimate a source size of a few micro-arcseconds and brightness temperatures exceeding 10^15 K. Independent of whether the interpretation of the IDV seen in the VLBI core is source intrinsic or extrinsic a lower limit of T_B > 2x10^12 K is obtained by model fitting of the VLBI-core. Our results show that future VSOP2 observations should be accompanied by a single dish monitoring not only to discriminate between source-extrinsic and source-intrinsic effects but to allow also a proper calibration and interpretation of ultra-high resolution VSOP2 images.
NRAO 150 -a compact and bright radio to mm source showing core/jet structure- has been recently identified as a quasar at redshift z=1.52 through a near-IR spectral observation. To study the jet kinematics on the smallest accessible scales and to com
pute the first estimates of its basic physical properties, we have analysed the ultra-high-resolution images from a new monitoring program at 86 GHz and 43 GHz with the GMVA and the VLBA, respectively. An additional archival and calibration VLBA data set, covering from 1997 to 2007, has been used. Our data shows an extreme projected counter-clock-wise jet position angle swing at an angular rate of up to ~11 deg./yr within the inner ~31 pc of the jet, which is associated with a non-ballistic superluminal motion of the jet within this region. The results suggest that the magnetic field could play an important role in the dynamics of the jet in NRAO 150, which is supported by the large values of the magnetic field strength obtained from our first estimates. The extreme characteristics of the jet swing make NRAO 150 a prime source to study the jet wobbling phenomenon.
