Coordinated X-Ray, Ultraviolet, and Optical Observations of 3C 120

Abstract

The results of 14 observations with EXOSAT and 10 quasi-simultaneous observations with IUE of the Seyfert galaxy 3C 120 are reported. On few epochs, quasi-simultaneous optical spectrophotometry was also obtained, allowing us to derive the energy distribution from the optical to the X-ray band in different intensity states. The X-ray spectra are well-represented by a power-law model with constant interstellar absorption, with slopes varying between 1.00 +/- 0.04 and 0.57 +/- 0.05. The spectral slope is positively correlated with the soft (0.3-2 keV) X-ray intensity, but anticorrelated with the medium energy (3.5-6 keV) intensity, suggesting a pivoting of the spectrum around an energy of ~2 keV. Variability with time scales shorter than 1 day is not observed in either band. The average amplitude of variability in the short UV wavelength range is larger than in the X- ray bands. The quasi-simultaneous UV observations exclude a strong correlation of the X-ray and UV fluxes with short lags, but some correlation is indicated between the flux in the highest frequency ultraviolet band (132-1420 A) and the flux in the soft X-ray bands (<= 1 keV). Six observations at intervals of days seem to indicate that the medium X-ray emission lags the soft one. The implications of the results for models of production of the continuum are discussed.