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 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 (N H 5 − 8 × 10 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 wellknown 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 Balmer decrement corrected [O III] to observed X-ray emission ratio (F [O III] /F obs 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 (<10 kpc) than typical BAT AGN. Additionally, these nine obscured AGN have, on average, ≈ 4× higher accretion rates than other BAT-detected AGN ( λ Edd = 0.068 ± 0.023 compared to λ Edd = 0.016 ± 0.004). 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.

The NuSTAR Serendipitous Survey: Hunting for the Most Extreme Obscured AGN at >10 keV

“…The unified model of AGNs (e.g., Antonucci 1993; Urry & Padovani 1995;Netzer 2015), which largely succeeds at describing AGNs in the local universe, posits that unobscured, obscured, and CT systems have intrinsically similar nuclear structures but are simply viewed from different inclination angles. In tension with this model (at least in its simplest form) are observational results that find possible evidence for high merger fractions in highly obscured AGN samples (e.g., Kocevski et al 2015;Del Moro et al 2016;Koss et al 2016a;Ricci et al 2017). Furthermore, observations of the clustering of AGNs find that obscured and unobscured AGNs may inhabit different large-scale environments (e.g., Allevato et al 2011Allevato et al , 2014DiPompeo et al 2014DiPompeo et al , 2016Donoso et al 2014; but see also Mendez et al 2016;Ballantyne 2017).…”

“…First, we select the objects with the highest NuSTAR band ratios, implying very hard spectral shapes and hence the likely presence of heavy absorption. Although band ratios only give a crude estimate of absorption, they are nevertheless an effective way to isolate the most extreme outliers (e.g., Koss et al 2016a). Second, we perform a detailed analysis of the X-ray and multiwavelength properties of these extreme objects and discuss how their properties compare to those of the general AGN population.…”

“…In the lower-redshift universe, however, hard X-ray telescopes become essential. Large (e.g., all-sky) surveys with nonfocusing hard X-ray missions (e.g., SwiftBAT and INTEGRAL) have been important for the identification of highly obscured AGNs in the very local universe ( < z 0.05; e.g., Burlon et al 2011;Vasudevan et al 2013;Ricci et al 2015;Akylas et al 2016;Koss et al 2016a). Now, with the first focusing hard X-ray mission (NuSTAR; Harrison et al 2013) it is possible to study source populations that are approximately two orders of magnitude fainter, thus extending to lower luminosities and higher redshifts.…”

BAT AGN Spectroscopic Survey. VIII. Type 1 AGN with Massive Absorbing Columns

“…Hard X-ray emission (>10keV) appears to be ubiquitous, albeit reduced due to absorption in all but the most highly obscured AGNs (Compton thick; N H 10 24 cm −2 ; e.g., Koss et al 2016), and is thought to originate from a compact corona near the SMBH (e.g., Haardt et al 1994). Intervening neutral gas along our line of sight absorbs X-rays up to an energy cutoff that is dependent on the column of gas.…”

“…Many of the studies previously mentioned have relied on AGN selection in the 2-10 keV band, which can be heavily affected by even moderate X-ray absorption (N H ∼10 23 Koss et al 2016) and thereby bias the sample against X-ray absorbed objects. Furthermore, definitions of optically obscured and unobscured, as well as X-ray absorbed and unabsorbed, have either changed or relied on less reliable methods such as SED fitting to define optical obscuration or hardness ratios to define the level of X-ray absorption.…”