Shedding Light on the Compton-thick Active Galactic Nucleus in the Ultraluminous Infrared Galaxy UGC 5101 with Broadband X-Ray Spectroscopy

We report the broadband X-ray spectra of the ultraluminous infrared galaxy (ULIRG) UGC 5101 in the 0.25-100 keV band observed with the Swift/Burst Alert Telescope (BAT), Nuclear Spectroscopic Telescope Array (NuSTAR), Suzaku, XMM-Newton, and Chandra. A Compton-thick active galactic nucleus (AGN) obscured with a hydrogen column density of ≈ 1.3× {10}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°, 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 ≈ 1.4× {10}⁴³ erg s^-1, which is ≈ 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 μm 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. 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}_{{H}}∼ {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.