Optical Emission from Jets

Abstract

Until a few years ago the only information available about extragalactic radio sources came from measurements of the radio emission itself and of the optical objects from which the sources were presumed to originate. Relatively few source parameters could be determined. This was depressing for theorists but attractive to people like me who find detailed physics difficult! I am afraid that the situation has changed. First, it was demonstrated that some, maybe all, radio jets have M87-type optical/X-ray counterparts (Butcher et al., 1980; Schreier et al., 1979; Feigelson et al., 1981). It followed that the spectra of synchrotron jets can extend over at least seven decades of frequency. These discoveries of optical and X-ray emission provided compelling arguments that localized particle acceleration takes place in jets. It would be impossible to transport the particles down the jet within the extremely short lifetimes (few years) of particles which radiate in the optical and X-rays. Moreover the evidence for the occurrence of in situ particle acceleration has weakened severely if not finally laid to rest those ritualized arguments which we all used to make about the ages of radio sources. If localized acceleration occurs in some places, can it be excluded anywhere in extended radio lobes?