Uncovering the Spectral Energy Distribution in Active Galaxies Using High-Ionization Mid-Infrared Emission Lines

The shape of the spectral energy distribution (SED) of active galaxies in the extreme-ultraviolet (EUV)–soft X-ray band (13.6 eV–1 keV) is uncertain because obscuration by dust and gas can hamper our view of the continuum. To investigate the shape of the SED in this energy band, we have generated a set of photoionization models which reproduce the small dispersion found in correlations between high-ionization mid-infrared emission lines in a sample of hard X-ray-selected active galactic nuclei (AGNs). Our calculations show that a broken power-law continuum model is sufficient to reproduce the [Ne v ] 14.32 μm /[Ne iii ], [Ne v ] 24.32 μm /[O iv ] 25.89 μm , and [O iv ] 25.89 μm /[Ne iii ] ratios and does not require the addition of a "big bump" EUV model component. We constrain the EUV–soft X-ray slope, α i , to be between 1.5 and 2.0 and derive a best fit of α i ∼ 1.9 for Seyfert 1 galaxies, consistent with previous studies of intermediate-redshift quasars. If we assume a blue bump model, most sources in our sample have derived temperatures between T BB = 10 5.18 K and 10 5.7 K, suggesting that the peak of this component spans a large range of energies extending from ∼600 Å to 1900 Å. In this case, the best-fitting peak energy that matches the mid-infrared line ratios of Seyfert 1 galaxies occurs between ∼700 and 1000 Å. Despite the fact that our results do not rule out the presence of an EUV bump, we conclude that our power-law model produces enough photons with energies >4 Ry to generate the observed amount of mid-infrared emission in our sample of Burst Alert Telescope AGNs.