NONSTANDARD EXTINCTION BY CIRCUMNUCLEAR DUST IN ACTIVE GALACTIC NUCLEI

Observational data imply that Active Galactic Nuclei are likely to hav e a substantial amount of dust within some 0.1-100 pc. This dust modif ies the observed spectrum of the nucleus by reprocessing a significant fraction of its intrinsic bolometric luminosity. The details of the e merging spectrum in the infrared and in the ultraviolet are dependent on the composition of such dust. It has been argued that the compariso n of such models with the IR data indicates strong depletion of silica tes (Czerny, Loska and Szczerba 1991, Laor and Draine 1993). In this p aper we discuss several theoretical extinction curves. We show that th e empirical extinction curve for the Small Magellanic Cloud correspond s well to extinction by dust dominated by amorphous carbon grains. We study the extinction by dust in X-ray band and suggest that in some so urces this extinction can be mistaken for an absorption by neutral gas with cosmic abundances and an accompanying soft X-ray excess below 0. 4 keV. Finally, we show that an application of a pure carbon dust mode l to the well-studied Seyfert galaxy NGC 6814 reveals a particularly s imple form of the underlying starlight-subtracted continuum, where the spectrum (in F-v) is slightly rising with frequency. Although our det ermination of the intrinsic spectrum is not necessarily unique, it cle arly shows the importance of further studies of the circumnuclear dust .