1995A&A...293..293A

With the aim of better understanding the physical processes that produce the continuous emission in active galactic nuclei (AGNs), a snapshot of the overall continuous energy distribution of NGC 3783, from γ-ray to radio wavelengths, has been obtained within the framework of the World Astronomy Days. The data collected in this campaign are from GRO, ROSAT, Voyager 2, IUE, HST, ESO, CTIO, SAAO and the VLA. Great care has been taken in disentangling the genuine AGN continuous emission from other contributions: depending on the waveband, the latter might be (i) unrelated contaminating sources in cases where the instrument field of view is large, (ii) components within which the AGN is embedded, such as the stellar bulge population which accounts for a significant fraction of the optical continuum, and free-bound and FeII blends which contribute to the ultraviolet flux. After correction for these other contributions, the continuous emission of the isolated AGN appears to be rather flat (i.e., approximately equal energy per unit logarithmic frequency) from soft γ-ray to infrared wavelengths. At high energies (0.1MeV to 0.1keV), the AGN continuum can be fitted by a power law F_ν∝ν^–α with a spectral index α≃1. At longer wavelengths, two excesses above this power law ("bumps") appear: in the ultraviolet, the classical big blue bump, which can be interpreted as thermal emission from the accretion disc surrounding a massive black hole, and in the infrared a second bump which can be ascribed to thermal emission from dust in the vicinity of the AGN, heated by ultraviolet radiation from the central source. By fitting accretion-disc models to the observed AGN spectral energy distribution, we find values for the accretion disc innermost temperature, accretion rate, and black hole mass, with some differences that depend on whether or not we extrapolate the high-energy power law up to infrared wavelengths. A fit to the IR bump above the extended α=1 power law suggests the presence of a dust component covering the region from a distance r ≃80 light days (hot grains at a temperature T≃1500K) to r≃60 light years (cool grains at T≃200K). The total mass of dust is around 60M_☉.