Compton-thick AGN in the NuSTAR Era. XI. Analyzing 11 CT-AGN Candidates Selected with Machine Learning

Abstract

This work discusses the broadband X-ray spectral analysis of 11 candidate heavily-obscured active galactic nuclei (AGN) selected based on their infrared and X-ray properties by a recently published machine learning algorithm. This paper is part of a larger work to identify and characterize all AGN in the local universe (z < 0.1) with the largest line-of-sight (los) column densities (NH), the so-called Compton-thick (CT-, NH,los >= 1024 cm-2) AGN. We modeled the X-ray spectra using two physically- motivated models, UXClumpy and RXTorusD. Of the 11 AGN in our sample, we found three to be obscured with 22.7 < LogNH,los <= 23.0, five have 23.0 < LogNH,los <= 23.25, and three have 23.4 < LogNH,los <= 23.9, according to UXClumpy. Meanwhile, according to RXTorusD, we found three AGN to be obscured with 22.7 < LogNH,los <= 23.0, four with 23.0 < LogNH,los <= 23.4, and four with 23.85 < LogNH,los <= 23.96. Additionally, this work served as a comparison between UXClumpy and RXTorusD. We found broad agreement between the two, with 8/11 sources agreeing on the value of the photon index Gamma, while only 5/11 sources agreeing on the NH,los value within the 90% confidence level.

Figures (15)

• Figure 1: The line-of-sight column density for all 11 sources predicted by two different techniques versus the best-fit UXClumpy N$_{\rm H,los}$ presented in this work. The blue circles represent the values predicted by the machine learning algorithm presented in Silver2023 and the orange stars are the N$_{\rm H,los}$ values predicted by the relation in Asmus2015. The dashed diagonal line represents the 1:1 relationship between the predicted and X-ray-measured values. The dash-dotted horizontal line represents the boundary where any source above it was predicted to be CT by the Silver2023 algorithm.
• Figure 2: Histograms of the LogN$_{\rm H,los}$ predicted by the machine learning (ML) algorithm subtracted by the UXClumpy best-fit LogN$_{\rm H,los}$. The top left shows the predictions with all parameters used and the top right shows the predictions without the WISE colors. The bottom left shows the predictions without the Swift-XRT hardness ratios and the bottom right shows the predictions without the IR $-$ X-ray flux ratio. The two vertical lines in each plot designate the region where the difference is $< \pm 0.5$. In the legend of each plot, we list the average of the absolute value of LogN$_{\rm H,ML} -$ LogN$_{\rm H,XR}$.
• Figure 3: Left: Scatter plot of the best-fit line-of-sight column densities from UXClumpy and RXTorusD. The black dotted line represents where N$_{\rm H,los,UXC}$ = N$_{\rm H,los,RXT}$. The grey dash-dotted lines signify the lower boundary of the CT regime. Right: Scatter plot of the best-fit photon indices for all 11 sources from UXClumpy and RXTorusD. The black dotted line represents where $\Gamma_{\rm UXC}$ = $\Gamma_{\rm RXT}$.
• Figure 4: Comparison of column density values between the UXClumpy$N_{H, l.o.s.}$ (blue circles), RXTorusD$N_{H, l.o.s.}$ (orange squares), and RXTorusD$N_{H, eq.}$ (green stars). If no green star appears, the $N_{H, eq.}$ was fixed to $N_{H, l.o.s.}$ for this source. The dashed horizontal line represents the boundary where any source above it is CT.
• Figure 5: UXClumpy best fit of IGR J19118$-$1707.