OPTICAL-EMISSION IN THE RADIO LOBES OF CYGNUS-A

Deep optical imaging and spectroscopy of stellar objects in the radio lobes of Cygnus A have been carried out. Brighter optical objects clos e to the radio hotspots are either confirmed or suspected Galactic sta rs. Very faint optical emission is found coinciding with hotspot D (R similar to 23.1) and hotspot B (V similar to 25.4) The implications of these observations are discussed in the context of a double twin-jet model. In this model the radio lobes are assumed to contain a supermas sive black hole each, and the twin jets emanating from the black holes are responsible for the radio emission. Assuming that the compact X-r ay sources in the lobes arise from the accretion disks, the level of e xpected optical emission from the accretion disks is estimated. The pr esent observations can rule out the model if the accretion disk is ext ensive, but they are not sensitive enough to exclude a truncated accre tion disk, such as is expected in the slingshot ejection process. An a pproximate estimate for the cutoff frequency in hotspot D yields a mag netic field that is close to values obtained by other authors using mi nimun energy considerations.