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.Comment: 11 pages, 7 figures, accepted for publication in Ap

Broadband X-ray Spectral Analysis of the Double-Nucleus Luminous Infrared Galaxy Mrk 463

We present a broadband (0.4–70 keV) X-ray spectral analysis of the luminous infrared galaxy (LIRG) system Mrk 463 observed with Nuclear Spectroscopic Telescope Array (NuSTAR), Chandra, and XMM-Newton, which contains double active galactic nuclei (AGNs; Mrk 463E and Mrk 463W) with a separation of ~3.8 kpc. Detecting their transmitted hard X-ray >10 keV continua with NuSTAR, we confirm that Mrk 463E and Mrk 463W have AGNs with intrinsic luminosities of (1.6–2.2) × 10^(43) and (0.5–0.6) × 10^(43) erg s^(−1) (2–10 keV) obscured by hydrogen column densities of 8 × 10^(23) and 3 × 10^(23) cm^(−2), respectively. Both nuclei show strong reflection components from cold matter. The luminosity ratio between X-ray (2–10 keV) and [O IV] 25.89 μm of Mrk 463E is ~5 times smaller than those of normal Seyfert galaxies, suggesting that the intrinsic SED is X-ray weak relative to the UV luminosity. In fact, the bolometric AGN luminosity of Mrk 463E estimated from L'-band (3.8 μm), [O IV] 25.89 μm, and [Ne V] 14.32 μm lines indicate a large bolometric-to-X-ray luminosity ratio, κ_(2–10 keV) ≈ 110–410, and a high Eddington ratio, λ_(Edd) ~ 0.4–0.8. We suggest that the merger triggered a rapid growth of the black hole in Mrk 463E, which is not yet deeply "buried" by circumnuclear dust. By contrast, the L'-band luminosity of Mrk 463W is unusually small relative to the X-ray luminosity, suggesting that the Eddington ratio is low (<10^(−3)) and it might be still in an early phase of merger-driven AGN activity