Nature of Compton-thick Active Galactic Nuclei in "Non-merging''
  Luminous Infrared Galaxies UGC 2608 and NGC 5135 Revealed with Broadband
  X-ray Spectroscopy
  We have analyzed the broadband X-ray spectra of active galactic nuclei (AGNs)
in two "non-merging" luminous infrared galaxies (LIRGs) UGC 2608 and NGC 5135,
utilizing the data of NuSTAR, Suzaku, XMM-Newton, and Chandra. Applying the
X-ray clumpy-torus model (XCLUMPY: Tanimoto et al. 2019), we find that both
sources have similar spectra characterized by Compton-thick (CT) absorption
($N_{\rm H} \sim$ 5-7 $\times$ $10^{24}$ cm$^{-2}$) and small torus angular
width ($\sigma$ $<$ 20$^{\circ}$). The intrinsic 2-10 keV luminosities are
$3.9^{+2.2}_{-1.7}$ $\times$ $10^{43}$ erg s$^{-1}$ (UGC 2608) and
$2.0^{+3.3}_{-1.0}$ $\times$ $10^{43}$ erg s$^{-1}$ (NGC 5135). The [O
IV]-to-nuclear-12 $\mu$m luminosity ratios are larger than those of typical
Seyferts, which are consistent with the torus covering factors ($C_{\rm T}
\lesssim$ 0.7) estimated from the torus angular widths and column densities by
X-ray spectroscopy. The torus covering factors and Eddington ratios
($\lambda_{\rm Edd} \sim$ 0.1) follow the relation found by Ricci et al.
(2017c) for local AGNs, implying that their tori become geometrically thin due
to significant radiation pressure of the AGN that blows out some part of the
tori. These results indicate that the CT AGNs in these "non-merger" LIRGs are
just a normal AGN population seen edge-on through a large line-of-sight column
density. They are in contrast to the buried CT AGNs in late-stage mergers that
have large torus covering factors even at large Eddington ratios.
