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

Journal
Date Issued
2016
Author(s)
Brightman, M.
•
Masini, A.
•
Ballantyne, D. R.
•
Baloković, M.
•
Brandt, W. N.
•
Chen, C. -T.
•
•
Farrah, D.
•
Gandhi, P.
•
Harrison, F. A.
•
Ricci, C.
•
Stern, D.
•
Walton, D. J.
DOI
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 Edd, 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 X. With a simple bolometric correction to L X to calculate λ Edd, we find a statistically significant correlation between Γ and λ Edd (p = 0.007). A linear fit to the data yields Γ = (0.41 ± 0.18)log10 λ Edd + (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 BH ≈ 106-107 M ☉) and are highly inclined, our results extend the Γ-λ Edd 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.
Volume
826
Issue
1
Start page
93
Uri
Url
Issn Identifier
0004-637X
Ads BibCode
2016ApJ...826...93B
Rights
open.access
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