3C-279 MULTIWAVELENGTH MONITORING .2. THE GROUND-BASED CAMPAIGN

The optically violently variable quasar 3C 279 was monitored simultane ously from radio to gamma-ray frequencies in 1992 December-1993 Januar y. We report a detailed study of the ground-based results from radio t o optical wavelengths. These data show that 3C 279 has a typical blaza r spectrum, slightly rising at radio frequency and then progressively steeper above a first turnover frequency between 37 and 90 GHz. In the millimeter wavelength region, a simple power law is not an adequate d escription of the spectrum. We suggest that the millimeter ''shoulder' ' corresponds to an additional emission component, self-absorbed betwe en 150 and 375 GHz, possibly associated with the detachment of a new V LBI knot and with the start of radio flare. A flux increase of 20% ove r 20 days was observed at 37 and 90 GHz, while contemporaneously the R -band flux doubled in about two weeks. The lack of strong variability in contemporaneous X-ray light curves (possible X-ray variations are l ess than 30%) implies no direct (i.e., zero lag) correlation between t he optical and X-ray fluxes. If X-rays are produced by inverse-Compton scattering of relativistic electrons on some seed photons, the above results exclude that the observed optical photons are the seeds and/or that the relativistic electrons radiating via synchrotron in the opti cal band are responsible for the scattering to X-ray energies. We sugg est that the X-rays are instead produced through the inverse-Compton p rocess by electrons of lower energy, which radiate via synchrotron in the radio to millimeter wave bands and which scatter either on the syn chrotron photons themselves or on external photons.