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 galaxy NGC 5128 (Centaurus A) obtained during the last 12 years. We detected periods of low activity, when the flux density was low and had quasi-periodic fluctuations with time-scales of a few months, and periods of high activity, in which the flux density was high and the fluctuations were not so well defined. The fluctuations were larger at the higher frequency. The time behaviour of the flux densities was very similar to those of quasars and BL Lac objects, although the total luminosity was much lower in the radio galaxy. A very strong and short burst was detected in 1985 September at 22 GHz, probably associated with a large 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 gradually become optically thin at the observed frequencies. These sources are probably located in the milliarcsec jet detected by very long-baseline interferometry (VLBI). The X-ray emission from the variable source can be either synchrotron or inverse self-Compton radiation. In the first case, a continuous injection of relativistic electrons is required.