A New Population of Compton-thick AGNs Identified Using the Spectral Curvature above 10 keV
Journal
Date Issued
2016
Author(s)
Koss, Michael J.
•
Assef, R.
•
Baloković, M.
•
Stern, D.
•
Gandhi, P.
•
Lamperti, I.
•
Alexander, D. M.
•
Ballantyne, D. R.
•
Bauer, F. E.
•
Berney, S.
•
Brandt, W. N.
•
•
Gehrels, N.
•
Harrison, F. A.
•
Lansbury, G.
•
Markwardt, C.
•
Ricci, C.
•
Rivers, E.
•
Schawinski, K.
•
Trakhtenbrot, B.
•
Treister, E.
•
Urry, C. Megan
Abstract
We present a new metric that uses the spectral curvature (SC) above 10 keV to identify Compton-thick active galactic nuclei (AGNs) in low-quality Swift/Burst Alert Telescope (BAT) X-ray data. Using NuSTAR, we observe nine high SC-selected AGNs. We find that high-sensitivity spectra show that the majority are Compton-thick (78% or 7/9) and the remaining two are nearly Compton-thick (N H ≃ (5-8) × 1023 cm-2 ). We find that the SC BAT and SC NuSTAR measurements are consistent, suggesting that this technique can be applied to future telescopes. We tested the SC method on well-known Compton-thick AGNs and found that it is much more effective than broadband ratios (e.g., 100% using SC versus 20% using 8-24 keV/3-8 keV). Our results suggest that using the >10 keV emission may be the only way to identify this population since only two sources show Compton-thick levels of excess in the Balmer decrement corrected [O III] to observed X-ray emission ratio ({F}[{{O}{{III}}]}/{F}2{--10 {keV}}{obs}\gt 1) and WISE colors do not identify most of them as AGNs. Based on this small sample, we find that a higher fraction of these AGNs are in the final merger stage (<10 kpc) than typical BAT AGNs. Additionally, these nine obscured AGNs have, on average, ≈4× higher accretion rates than other BAT-detected AGNs (< {λ }{Edd}> \=\0.068+/- 0.023 compared to < {λ }{Edd}> \=\0.016+/- 0.004). The robustness of SC at identifying Compton-thick AGNs implies that a higher fraction of nearby AGNs may be Compton-thick (≈22%) and the sum of black hole growth in Compton-thick AGNs (Eddington ratio times population percentage) is nearly as large as mildly obscured and unobscured AGNs.
Volume
825
Issue
2
Start page
85
Issn Identifier
0004-637X
Ads BibCode
2016ApJ...825...85K
Rights
open.access
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