A New Population of Compton-thick AGNs Identified Using the Spectral Curvature above 10 keV

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 <SUB>H</SUB> ≃ (5-8) × 10<SUP>23</SUP> cm<SUP>-2</SUP> ). We find that the SC<SUB> BAT </SUB> and SC<SUB> NuSTAR </SUB> 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}<SUB>[{{O</SUB>}{{III}}]}/{F}<SUB>2{--</SUB>10 {keV}}<SUP>{obs</SUP>}\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 }<SUB>{Edd</SUB>}> \=\0.068+/- 0.023 compared to < {λ }<SUB>{Edd</SUB>}> \=\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.