paper

A growth-rate indicator for Compton-thick active galactic nuclei

arXiv:1606.09265 · doi:10.3847/0004-637X/826/1/93

Abstract

Due to their heavily obscured central engines, the growth rate of Compton-thick (CT) active galactic nuclei (AGN) is difficult to measure. A statistically significant correlation between the Eddington ratio, λ, and the X-ray power-law index, Γ, observed in unobscured AGN offers an estimate of their growth rate from X-ray spectroscopy (albeit with large scatter). However, since X-rays undergo reprocessing by Compton scattering and photoelectric absorption when the line-of-sight to the central engine is heavily obscured, the recovery of the intrinsic Γ is challenging. Here we study a sample of local, predominantly Compton-thick megamaser AGN, where the black hole mass, and thus Eddington luminosity, are well known. We compile results on X-ray spectral fitting of these sources with sensitive high-energy (E> 10 keV) NuSTAR data, where X-ray torus models which take into account the reprocessing effects have been used to recover the intrinsic Γ values and X-ray luminosities, L. With a simple bolometric correction to L to calculate λ, we find a statistically significant correlation between Γ and λ (p = 0.007). A linear fit to the data yields Γ = (0.410.18)logλ+(2.38 0.20), which is statistically consistent with results for unobscured AGN. This result implies that torus modeling successfully recovers the intrinsic AGN parameters. Since the megamasers have low-mass black holes (M M) and are highly inclined, our results extend the Γ-λ relationship to lower masses and argue against strong orientation effects in the corona, in support of AGN unification. Finally this result supports the use of Γ as a growth-rate indicator for accreting black holes, even for Compton-thick AGN.

Accepted for publication in ApJ

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