Time Variable Extragalactic Radio Sources with Peaked Spectra

This dissertation presents an investigation of two variable Giga-Hertz peaked spectrum (GPS) sources: 4C 39.25 and PKS 0528+134, in order to determine why their properties are intermediate between two distinct classes of compact radio sources. Multi-frequency, multi-epoch VLBI and VLA imaging experiments are used to determine the angular structure of each source over milliarcsecond and arcsecond scales, respectively. Spectral variations as well as total and polarized radio light curves are investigated together with structural changes on milliarcsecond scales. Concurrent X-ray spectra at two epochs are analyzed to search for correlations of source variability between radio and X -ray wavelengths. In 4C 39.25, we find that an apparently superluminally moving feature between stationary components continues eastward, in rough alignment with the arcsecond-scale structure. A radio flare in progress since 1984 is visible concurrently from 5 GHz to at least 250 GHz. Both the frequency and flux density of spectral peaks decreased between 1973 and 1984 and have been increasing since then. We detect no X-ray variability on timescales of 10^3 to 10^6 sec, while variability on timescales of 10^8-10 ^9 seconds corresponds quantitatively with that of the optically thin radio flux density. A relativistic jet bent by about 4^circ at two locations and containing a relativistically moving shock explains the source characteristics remarkably well. The inverse Compton scattered X-ray Aux density expected according to this model is confirmed observationally. Our VLBI images reveal, for the first time, a bent milliarcsecond-scale jet in PKS 0528+134. Our ROSAT observations, taken before and after the onset of a recent flare visible from gamma-ray to centimeter wavelengths, leads us to conclude that the X-ray spectrum has become significantly flatter while the flux density has doubled. Excess X-ray absorption about two times the Galactic value is detected. The ROSAT X-ray flux density, compared with the inverse Compton prediction, indicates that relativistic bulk motion with Doppler factor delta_sp{~}>4 is required in PKS 0528 + 134. We suggest that we observe in PKS 0528 + 134 a rare case of sporadic nonthermal activity with a short duty cycle in a quasar. The source may be in the process of generating a more fully developed core-jet structure. We note that a bent jet on parsec scales is a common phenomenon in compact radio sources. The combination of bulk relativistic motion and differential Doppler enhancement provides an effective solution to the problem of over-population of superluminal radio sources as well as a viable scheme to suppress the flat spectrum core component that is missing in 4C 39.25 and PKS 0528+134 (prior to mid-1991). In this scheme, a variable GPS source results from either (i) a mis-directed core (narrow end) of the jet or (ii) periods during which energy injection into the jet is at a relatively low rate.