Frying doughnuts: what can the reprocessing of X-rays to IR tell us about the AGN environment?

Abstract

Active galactic nuclei (AGN) produce vast amounts of high-energy radiation deep in their central engines. X-rays either escape the AGN or are absorbed and re-emitted mostly as infrared (IR). By studying the dispersion in the ratio of observed mid-IR luminosity to observed 2–10 keV X-ray luminosity (RIR/X) in AGN, we can investigate the reprocessing material (possibly a torus or donut of dust) in the AGN central engine, independent of model assumptions. We studied the ratio of observed mid-IR and 2–10 keV X-ray luminosities in a heterogeneous sample of 245 AGN from the literature. We found that when we removed AGN with prominent jets, ∼90 per cent of type I AGN lay within a very tight dispersion in luminosity ratio (1 < RIR/X < 30). This implies that the AGN central engine is extremely uniform and models of the physical AGN environment (e.g. cloud cover, turbulent disc, opening angle of absorbing structures such as dusty tori) must span a very narrow range of parameters. We also found that the far-IR (100 μm) to mid-IR (12 μm) observed luminosity ratio is an effective discriminator between heavily obscured AGN and relatively unobscured AGN.