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The NRAO VLA and VLBA were used from 1988 November to 1998 June to monitorthe radio continuum counterpart to the optical broad line region (BLR) in theSeyfert galaxy NGC 5548. Photometric and astrometric observations were obtainedat 21 epochs. The radio nucleus appeared resolved, so comparisons were limitedto observations spanning 10-60 days and 3-4 yr, and acquired at matchedresolutions of 1210 mas = 640 pc (9 VLA observations), 500 mas = 260 pc (9 VLAobservations), or 2.3 mas = 1.2 pc (3 VLBA observations). The nucleus isphotometrically variable at 8.4 GHz by $33\pm5$% and $52\pm5$% between VLAobservations separated by 41 days and 4.1 yr, respectively. The 41-day changesare milder ($19\pm5$%) at 4.9 GHz and exhibit an inverted spectrum ($\alpha\sim +0.3\pm0.1$, $S\propto \nu ^{+\alpha}$) from 4.9 to 8.4 GHz. The nucleusis astrometrically stable at 8.4 GHz, to an accuracy of 28 mas = 15 pc betweenVLA observations separated by 4.1 yr and to an accuracy of 1.8 mas = 0.95 pcbetween VLBA observations separated by 3.1 yr. Such photometric variability andastrometric stability is consistent with a black hole being the ultimate energysource for the BLR, but is problematic for star cluster models that treat theBLR as a compact supernova remnant and, for NGC 5548, require a new supernovaevent every 1.7 yr within an effective radius $r_e =$ 42 mas = 22 pc. A deepimage at 8.4 GHz with resolution 660 mas = 350 pc was obtained by adding datafrom quiescent VLA observations. This image shows faint bipolar lobesstraddling the radio nucleus and spanning 12 arcsec = 6.4 kpc. Thesesynchrotron-emitting lobes could be driven by twin jets or a bipolar wind fromthe Seyfert 1 nucleus.Comment: with 9 figures, to appear in the Astrophysical Journal, 2000 March  10, volume 53

Photometric Variability and Astrometric Stability of the Radio Continuum Nucleus in the Seyfert Galaxy NGC 5548