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We present the initial results of The University of Michigan Radio Astronomy Observatory's `flicker' program. The `flicker' sources are compact or very compact QSOs and BL Lac objects that have been found to vary by 10 -- 20\% in flux, on time scales of hours to days (Quirrenbach, et al., Astr. Ap., 226, L1 1989), in contrast to the month to year time scale of variability traditionally thought to characterize these objects. The short timescales of this variability can help to provide us with information on structure currently beyond the reach of VLBI. This variability must also be taken into account when making synthesis maps of sources which may vary significantly in flux over the time period when they are observed.
We have made closely spaced observations at 4.8, 8.0 and 14.5 GHz using The University of Michigan's 26-meter radio telescope so as to extend the variability program started in the 1960s down to time scales of hours to days. The emphasis in our program has been to confirm the variability on time scales of hours to days as well as to better define variability on scales of days to weeks. The goal is to attempt to determine if the nature of the variations on these intermediate time scales is different for the `flicker' sources (where the variations appear to peak at lower frequencies) as compared to the traditional sources (where variations peak at higher frequencies and can be explained by a shocked jet model). We have also done Monte Carlo type simulations to simulate flicker phenomena. We hope that the simulations will help us to better understand what features of the light curves are due to small scale turbulent structure, and determine to what extent single dish observations can be used to test quantitatively models of such flows.
This research has been supported in part by NSF grant AST-9120224
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