A New Population of Compton-Thick AGN Identified Using the Spectral
  Curvature Above 10 keV
  We present a new metric that uses the spectral curvature (SC) above 10 keV to
identify Compton-thick AGN in low-quality Swift BAT X-ray data. Using NuSTAR,
we observe nine high SC-selected AGN. We find that high-sensitivity spectra
show the majority are Compton-thick (78% or 7/9) and the remaining two are
nearly Compton-thick (NH~5-8x10^23 cm^-2). We find the SC_bat and SC_nustar
measurements are consistent, suggesting this technique can be applied to future
telescopes. We tested the SC method on well-known Compton-thick AGN and find it
is much more effective than broad band ratios (e.g. 100% using SC vs. 20% using
8-24/3-8 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 OIII to X-ray emission ratio (F_OIII/F_2-10 keV>1) and
WISE colors do not identify most of them as AGN. Based on this small sample, we
find that a higher fraction of these AGN are in the final merger stage than
typical BAT AGN. Additionally, these nine obscured AGN have, on average, four
times higher accretion rates than other BAT-detected AGN (Edd ratio=0.068
compared to 0.016). The robustness of SC at identifying Compton-thick AGN
implies a higher fraction of nearby AGN may be Compton-thick (~22%) and the sum
of black hole growth in Compton-thick AGN (Eddington ratio times population
percentage), is nearly as large as mildly obscured and unobscured AGN.
