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Up: 50 as resolution VLBI


1 Introduction

The 1988 session was the first epoch where we could produce hybrid maps using intercontinental baselines to obtain a resolution of 50 $\mu$as. This could be achieved because of the development of new receivers, improved phase stability of local oscillators, and new data reduction techniques (Bååth et al. 1992). Since 1988 until and including the epochs presented here we have observed four epochs, in March 1988, March 1989, April 1990, and April 1993. These observations involved telescopes in Europe, Japan, Chile, and U.S.A. The 1988 and 1989 epochs have been previously published (Bååth et al. 1991 and Bååth et al. 1992). This paper presents the results from the 1990 and 1993 epochs. Our main aim with these observations is to obtain the highest possible resolution to be able to image the vicinity of the AGN core and the base of the jet emerging from the core.

Our observations so far have shown that the radio cores of the most powerful of the AGN's are very small, on the order of 1016 - 1017cm, which is only 5-500 times larger than the Schwarzschild radius of a $10^9~M_\odot$ black hole and of the same size scale as the non-thermal source observed by Band & Malkan (1989). A special and characteristic feature of the sub-milliarcsecond scale structures is that the curvature observed with cm-VLBI seems to continue, but is further enhanced, closer to the core.

Typical for the major outbursts we observe from AGN's is that they first start with a rapid increase in intensity simultaneously (Courvoisier et al. 1988) at optical-IR and later at high radio frequencies (86 GHz and higher). High frequency single dish monitoring shows that outbursts tend to emerge almost simultaneously over the range 86 - 300 GHz with turnover frequency reaching 86 GHz within a month and then 22 GHz within 4-5 months (Lainela et al. 1993; Stevens et al. 1996). Similar time scales are observed in 3C279 (Litchfield et al. 1995). This has been explained (Marscher & Gear 1985; Marscher et al. 1993) by a thin shock which is formed close to the central engine and then moves down the jet. After some time the shock will expand adiabatically and the spectral turnover (due to synchrotron self-absorption) will move towards lower frequencies. The outburst will therefore be observed at lower frequencies after the expansion has started. Observations of the shocks at their early stages of development are fundamental to our understanding of how they are formed, how they emerge from the core, and how they develop on their way through the radio jet. It must be emphasized that such observations can only be done with very high angular resolution at high frequencies.

It is clear that at present our $\lambda$3mm observations suffers from lack of uv-coverage and has sensitivity problems, but we see this as the first step towards obtaining high dynamic range maps.


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Up: 50 as resolution VLBI

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