A GROWTH-RATE INDICATOR FOR COMPTON-THICK ACTIVE GALACTIC NUCLEI

• Published in: The Astrophysical journal Vol. 826; no. 1; p. 93
• Main Authors: Brightman, M., Masini, A., Ballantyne, D. R., Balokovi, M., Brandt, W. N., Chen, C.-T., Comastri, A., Farrah, D., Gandhi, P., Harrison, F. A., Ricci, C., Stern, D., Walton, D. J.
• Format: Journal Article
• Language: English
• Published: United States The American Astronomical Society 20.07.2016
• Subjects: ABSORPTION; Active galactic nuclei; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; black hole physics; BLACK HOLES; BOLOMETERS; COMPTON EFFECT; COMPUTERIZED TOMOGRAPHY; CORRECTIONS; Correlation; CORRELATIONS; COSMIC X-RAY SOURCES; Engines; GALAXIES; galaxies: general; galaxies: nuclei; galaxies: Seyfert; GALAXY NUCLEI; Indicators; KEV RANGE; LUMINOSITY; MASERS; MASS; Mathematical models; REPROCESSING; SIMULATION; X RADIATION; X rays; X-RAY SPECTROSCOPY
• ISSN: 0004-637X; 1538-4357
• 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 (AGNs) is difficult to measure. A statistically significant correlation between the Eddington ratio, λEdd, 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, 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)log10λEdd + (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.