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

M. Brightman, A. Masini, D. R. Ballantyne, M. BalokoviC, W. N. Brandt, C. T. Chen, A. Comastri, D. Farrah, P. Gandhi, F. A. Harrison, C. Ricci, D. Stern, D. J. Walton

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26 Scopus citations


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, AEdd, and the X-ray power-law index, T, 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 T 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 > 10keV) 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, LX. With a simple bolometric correction to LX to calculate λEdd, we find a statistically significant correlation between Γ and λEdd (p = 0.007). A linear fit to the data yields Γ = (0.41 ± 0.18)log10 λEdd + (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 (MBH ≈ 106-107 M) and are highly inclined, our results extend the Γ-λEdd 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.

Original languageEnglish (US)
Article number93
JournalAstrophysical Journal
Issue number1
StatePublished - 2016

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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