The Low-Frequency Spectra of Accretion Disks in Active Galactic Nuclei: The Dark Side of the Big Blue Bump

Abstract

The flux density of most Seyfert 1 galaxy and quasar continua has an average slope of ~−1 (i.e., f _v ∝ v ^-1) from the visual to a far-infrared turnover (around 80 μm). Relative to this red “power law”, there is generally excess flux in the blue and ultraviolet, a.k.a. the “Big Blue Bump” (BBB). Much effort has gone into measuring the shape of the high-frequency side of this BBB. (It is unfortunate that comparable effort has not been made to find a better name for it). These observations are difficult because of our lack of very powerful instrumentation in the ultraviolet, and absorption from gas along the line-of-sight, in our Galaxy and in intergalactic clouds (see Malkan, Alloin and Shore 1987 for a review). But fortunately the BBB component usually dominates the observed ultraviolet continuum, and may well extend out to the soft X-rays. The very steep soft X-ray continuum below 1 keV-i.e.the “Soft X-ray Excess” frequently detected by Exosat (Pounds and Turner 1989) and indirectly by the Einstein IPC (Wilkes and Elvis 1988)- appears to be the high-frequency tail of the BBB (Czerny and Elvis 1987). Observational progress is being made on closing the spectral gap between the far-UV and soft X-rays. Reimers et al. (1989) recently detected a quasar at z=2.75 down to the short-wavelength limit covered by IUE, corresponding to a rest wavelength of 330 A. Unfortunately, the observed far-UV fluxes must be multiplied by a large, but highly uncertain factor (6) to correct for absorption by intergalactic HI clouds along the line-of-sight. Detailed spectroscopy by IIST with high resolution and high signal/noise ratio will be needed to determine column densities for the absorbing clouds, for accurate de-blanketing of the continuum.