Hard X-ray selection is , arguably , the optimal method for defining a representative sample of active galactic nuclei ( AGN ) .
Hard X-rays are unbiased by the affects of obscuration and re-processing along the line-of-sight intrinsic/external to the AGN which result in unknown fractions of the population being missed from traditional optical/soft-X-ray samples .
We present the far-infrared ( IR ) observations of 21 hard X-ray selected AGN from the HEAO-1 A2 sample observed with ISO .
We characterize the far-infrared ( IR ) continua of these X-ray selected AGN , compare them with those of various radio and optically selected AGN samples and with models for an AGN-heated , dusty disk .
The X-ray selected AGN show broad , warm IR continua covering a wide temperature range ( \sim 20 - 1000 K in a thermal emission scenario ) .
Where a far-IR turnover is clearly observed , the slopes are < 2.5 in all but three cases so that non-thermal emission remains a possibility , although the presence of cooler dust resulting in a turn-over at wavelengths longwards of the ISO range is considered more likely .
The sample also shows a wider range of optical/UV shapes than the optically/radio-selected samples , extending to redder near-IR colors .
The bluer objects are type 1 Seyferts , while the redder AGN are mostly intermediate or type 2 Seyferts .
This is consistent with a modified unification model in which obscuration increases as we move from a face-on towards more edge-on line-of-sight ( l.o.s . )
However , this relation does not extend to the mid-infrared as the 25 \mu m/60 \mu m ratios are similar in Seyferts with differing type and optical/UV reddening .
The resulting limits on the column density of obscuring material through which we are viewing the redder AGN ( { N _ { H } } Â \sim 10 ^ { 22 } cm ^ { -2 } ) are inconsistent with standard optically thick torus models ( { N _ { H } } Â \sim 10 ^ { 24 } cm ^ { -2 } ) and simple unification models .
Instead our results support more complex models in which the amount of obscuring material increases with viewing angle and may be clumpy .
Such a scenario , already suggested by differing optical/near-IR spectroscopic and X-ray AGN classifications , allows for different amounts of obscuration of the continuum emission in different wavebands and of the broad emission line region which , in turn , results in a mixture of behaviors for AGN with similar optical emission line classifications .
The resulting decrease in the optical depth of the obscuring material also allows the AGN to heat more dust at larger radial distances .
We show that an AGN-heated , flared , dusty disk with mass \sim 10 ^ { 9 } { M _ { \bigodot } } Â and size \sim few hundred pc is able to generate optical - far-IR spectral energy distributions ( SEDs ) which reproduce the wide range of SEDs present in our sample with no need for an additional starburst component to generate the long-wavelength , cooler part of the IR continuum .