A Growth-rate Indicator for Compton-thick Active Galactic Nuclei

Abstract

Due to their heavily obscured central engines, the growth rate of Compton-thick (CT) active galactic nuclei (AGNs) is difficult to measure. A statistically significant correlation between the Eddington ratio, λ <SUB>Edd</SUB>, and the X-ray power-law index, Γ, observed in unobscured AGNs 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 CT megamaser AGNs, where the black hole mass, and thus Eddington luminosity, are well known. We compile results of the 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 <SUB>X</SUB>. With a simple bolometric correction to L <SUB>X</SUB> to calculate λ <SUB>Edd</SUB>, we find a statistically significant correlation between Γ and λ <SUB>Edd</SUB> (p = 0.007). A linear fit to the data yields Γ = (0.41 ± 0.18)log<SUB>10</SUB> λ <SUB>Edd</SUB> + (2.38 ± 0.20), which is statistically consistent with results for unobscured AGNs. This result implies that torus modeling successfully recovers the intrinsic AGN parameters. Since the megamasers have low-mass black holes (M <SUB>BH</SUB> ≈ 10<SUP>6</SUP>-10<SUP>7</SUP> M <SUB>☉</SUB>) and are highly inclined, our results extend the Γ-λ <SUB>Edd</SUB> 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 CT AGNs.