Multifrequency observations of the superluminal quasar 3C 345

Abstract

We have investigated the continuum properties of the superluminal quasar 3C 345 with monitoring studies at radio, optical, infrared, and X-ray frequencies as well as with simultaneous multifrequency spectra extending from the radio through the X-ray bands. Variability occurs more rapidly and with greater amplitude toward shorter wavelengths in the infrared-optical region (0.4-100 μm). Radio outbursts, which appear to follow infrared-optical outbursts by about a year, occur first at the highest frequencies, as expected from optical depth effects, although the peak flux is often reached at several frequencies at once. The beginning of outbursts as defined by millimeter measurements corresponds to the appearance of the three known "superluminal" components. An increase in the X-ray flux during 1979-1980 corresponds to increased radio flux, while the infrared flux changes in the opposite sense. The multifrequency spectra show that the nearly flat radio continuum steepens at 10^(11)-10^(12) Hz and has a power-law slope of -0.91 ± 0.04 from 350 to 20 μm that steepens to -1.40 ± 0.02 at 20 μm-1200 A. A "blue bump" is detected at rest wavelengths 4000-1500 A. The X-ray emission has a flatter slope (-0.7) than the infrared-ultraviolet continuum and lies above an extrapolation of that continuum to X-ray energies. This supports the finding from variability that the X-ray emission is not simply connected to the optical emission. Although the shape of the infrared-ultraviolet continuum is generally preserved during flux variations, slope variations occur and are most common in the ultraviolet region. Most of the total power (3 x 10^(47) ergs s ^(-1)) emerges in the submillimeter-optical region but about one-sixth of the power is emitted at X-ray and radio.