LONG-TERM RADIO OBSERVATIONS OF THE NUCLEUS OF NGC-5128 (CENTAURUS-A)

We report 22- and 43-GHz observations of the nucleus of the radio gala xy NGC 5128 (Centaurus A) obtained during the last 12 years. We detect ed periods of low activity, when the flux density was low and had quas i-periodic fluctuations with time-scales of a few months, and periods of high activity, in which the flux density was high and the fluctuati ons were not so well defined. The fluctuations were larger at the high er frequency. The time behaviour of the flux densities was very simila r to those of quasars and BL Lac objects, although the total luminosit y was much lower in the radio galaxy. A very strong and short burst wa s detected in 1985 September at 22 GHz, probably associated with a lar ge increase in X-rays that occurred three months earlier. We interpret the quasi-periodic variability at radio frequencies as the result of synchrotron radiation from adiabatically expanding sources, which grad ually become optically thin at the observed frequencies. These sources are probably located in the milliarcsec jet detected by very long-bas eline interferometry (VLBI). The X-ray emission from the variable sour ce can be either synchrotron or inverse self-Compton radiation. In the first case, a continuous injection of relativistic electrons is requi red.