On the ionizing continuum in active galactic nuclei: Clues from the Infrared Space Observatory

The Infrared Space Observatory (ISO) coronal line spectrum of the brightest Seyfert galaxies from the CfA sample is presented and modeled. ISO observa tions of [O IV] 25.9 mu m, [Ne V] 14.3 mu m, [Mg VIII] 3.02 mu m, and [Si I X] 2.58 mu m lines are presented their relationship with the soft part of t he ionizing spectrum from 50 to 300 eV is investigated. Pure photoionizatio n models reproduce the line ratios, setting ranges for the ionization param eter and the optical depth of the emitting clouds, On the basis of the avai lable data alone it is not possible to distinguish between a power-law or a blackbody distribution as the intrinsic shape of the W-ionizing spectrum. However, for the brightest Seyfert galaxies analyzed, namely, NGC 1068, Cir cinus, and NGC 4151, a blackbody UV continuum is favored.