Shedding Light on the Compton-thick Active Galactic Nucleus in the
  Ultra-luminous Infrared Galaxy UGC 5101 with Broadband X-ray Spectroscopy
  We report the broadband X-ray spectra of the ultra-luminous infrared galaxy
(ULIRG) UGC 5101 in the 0.25-100 keV band observed with Swift/Burst Alert
Telescope (BAT), NuSTAR, Suzaku, XMM-Newton, and Chandra. A Compton-thick AGN
obscured with a hydrogen column density of $\approx 1.3\times10^{24}$ cm$^{-2}$
is detected above 10 keV. A spectral fit with a numerical torus model favors a
large half opening angle of the torus, $>41$ degrees, suggesting that the
covering fraction of material heavily obscuring the X-ray source is moderate.
The intrinsic 2-10 keV luminosity is determined to be $\approx 1.4\times
10^{43}$ erg s$^{-1}$, which is $\approx$2.5 times larger than the previous
estimate using only data below 10 keV with a simple spectral model. We find
that UGC 5101 shows the ratio between the [O IV] 26 $\mu$m line and 2-10 keV
luminosities similar to those of normal Seyfert galaxies, along with other
ULIRGs observed with NuSTAR, indicating that a significant portion of local
ULIRGs are not really "X-ray faint" with respect to the flux of forbidden lines
originating from the narrow line region (NLR). We propose a possible scenario
that (1) the AGN in UGC 5101 is surrounded not only by Compton-thick matter
located close to the equatorial plane but also by Compton-thin ($N_\mathrm{H}
\sim 10^{21}$ cm$^{-2}$) matter in the torus-hole region and (2) it is
accreting at a high Eddington rate with a steep UV to X-ray spectral energy
distribution. Nevertheless, we argue that AGNs in many ULIRGs do not look
extraordinary (i.e., extremely X-ray faint), as suggested by recent works,
compared with normal Seyferts.
