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Register a new userNature of Compton-thick Active Galactic Nuclei in Non-merging Luminous Infrared Galaxies UGC 2608 and NGC 5135 Revealed with Broadband X-ray Spectroscopy
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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.
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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 $approx 1.3times10^{24}$ 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 $approx 1.4times 10^{43}$ 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.
We present X-ray bolometric correction factors, $kappa_{Bol}$ ($equiv L_{Bol}/L_X$), for Compton-thick (CT) active galactic nuclei (AGN) with the aim of testing AGN torus models, probing orientation effects, and estimating the bolometric output of the most obscured AGN. We adopt bolometric luminosities, $L_{Bol}$, from literature infrared (IR) torus modeling and compile published intrinsic 2--10 keV X-ray luminosities, $L_{X}$, from X-ray torus modeling of NuSTAR data. Our sample consists of 10 local CT AGN where both of these estimates are available. We test for systematic differences in $kappa_{Bol}$ values produced when using two widely used IR torus models and two widely used X-ray torus models, finding consistency within the uncertainties. We find that the mean $kappa_{Bol}$ of our sample in the range $L_{Bol}approx10^{42}-10^{45}$ erg/s is log$_{10}kappa_{Bol}=1.44pm0.12$ with an intrinsic scatter of $sim0.2$ dex, and that our derived $kappa_{Bol}$ values are consistent with previously established relationships between $kappa_{Bol}$ and $L_{Bol}$ and $kappa_{Bol}$ and Eddington ratio. We investigate if $kappa_{Bol}$ is dependent on $N_H$ by comparing our results on CT AGN to published results on less-obscured AGN, finding no significant dependence. Since many of our sample are megamaser AGN, known to be viewed edge-on, and furthermore under the assumptions of AGN unification whereby unobscured AGN are viewed face-on, our result implies that the X-ray emitting corona is not strongly anisotropic. Finally, we present $kappa_{Bol}$ values for CT AGN identified in X-ray surveys as a function of their observed $L_X$, where an estimate of their intrinsic $L_{X}$ is not available, and redshift, useful for estimating the bolometric output of the most obscured AGN across cosmic time.
Due to their heavily obscured central engines, the growth rate of Compton-thick (CT) active galactic nuclei (AGN) is difficult to measure. A statistically significant correlation between the Eddington ratio, {lambda}$_{Edd}$, and the X-ray power-law index, {Gamma}, observed in unobscured AGN offers an estimate of their growth rate from X-ray spectroscopy (albeit with large scatter). However, since X-rays undergo reprocessing by Compton scattering and photoelectric absorption when the line-of-sight to the central engine is heavily obscured, the recovery of the intrinsic {Gamma} is challenging. Here we study a sample of local, predominantly Compton-thick megamaser AGN, where the black hole mass, and thus Eddington luminosity, are well known. We compile results on X-ray spectral fitting of these sources with sensitive high-energy (E> 10 keV) NuSTAR data, where X-ray torus models which take into account the reprocessing effects have been used to recover the intrinsic {Gamma} values and X-ray luminosities, L$_X$. With a simple bolometric correction to L$_X$ to calculate {lambda}$_{Edd}$, we find a statistically significant correlation between {Gamma} and {lambda}$_{Edd}$ (p = 0.007). A linear fit to the data yields {Gamma} = (0.41$pm$0.18)log$_{10}${lambda}$_{Edd}$+(2.38$pm$ 0.20), which is statistically consistent with results for unobscured AGN. This result implies that torus modeling successfully recovers the intrinsic AGN parameters. Since the megamasers have low-mass black holes (M$_{BH}approx10^6-10^7$ M$_{sol}$) and are highly inclined, our results extend the {Gamma}-{lambda}$_{Edd}$ relationship to lower masses and argue against strong orientation effects in the corona, in support of AGN unification. Finally this result supports the use of {Gamma} as a growth-rate indicator for accreting black holes, even for Compton-thick AGN.
Heavily obscured active galactic nuclei (AGNs) play an important role in contributing to the cosmic X-ray background (CXRB). However, the AGNs found in deep X-ray surveys are often too weak to allow direct measurement of the column density of obscuring matter. One method adopted in recent years to identify heavily obscured, Compton-thick AGNs under such circumstances is to use the observed mid-infrared to X-ray luminosity ratio as a proxy for the column density. This is based on the supposition that the amount of energy lost by the illuminating X-ray continuum to the obscuring matter and reprocessed into infrared emission is directly related to the column density and that the proxy is not sensitive to other physical parameters of the system (aside from contamination by dust emission from, for example, star-forming regions). Using Monte Carlo simulations, we find that the energy losses experienced by the illuminating X-ray continuum in the obscuring matter are far more sensitive to the shape of the X-ray continuum and to the covering factor of the X-ray reprocessor than they are to the column density of the material. Specifically we find that it is possible for the infrared to X-ray luminosity ratio for a Compton-thin source to be just as large as that for a Compton-thick source even without any contamination from dust. Since the intrinsic X-ray continuum and covering factor of the reprocessor are poorly constrained from deep X-ray survey data, we conclude that the mid-infrared to X-ray luminosity ratio is not a reliable proxy for the column density of obscuring matter in AGNs even when there is no other contribution to the mid-infrared luminosity aside from X-ray reprocessing. This conclusion is independent of the geometry of the obscuring matter.
We present an exhaustive methodology for fitting Compton-thick X-ray reprocessor models to obscured AGNs and for interpreting the results. We focus on the MYTORUS model but also utilize other models. We apply the techniques to Suzaku, BeppoSAX, and Swift BAT spectra of the Sy 2 galaxy NGC 4945, but the methods are applicable to other AGNs including Compton-thin sources. The models overcome a major restriction of disk-reflection models, namely the assumption of an infinite column density. Finite column-density models produce a richer variety of spectral shapes and characteristics, even for Compton-thin AGNs. Although NGC 4945 is one of the brightest AGNs above 10 keV, the models span nearly a factor of 3 in column density (~2 to 6 x 10^{24} cm^{-2}) and 2 orders of magnitude in the intrinsic 2-195 keV luminosity. Models in which the continuum above 10 keV is dominated by the direct (unscattered) continuum or Compton-scattered continuum give the highest and lowest intrinsic luminosities respectively. Variability properties favor solutions in which the unscattered continuum dominates above 10 keV. The data require that the Compton-scattered continuum and Fe Kalpha line emission come predominantly from the illuminated surfaces of the X-ray reprocessor, implying a clumpy medium with a global covering factor that is small enough that the Compton-scattered continuum does not dominate the spectrum above 10 keV. This can be identified with the ~30 pc region spatially resolved by Chandra. The implied intrinsic bolometric luminosity is close to, or greater than, the Eddington luminosity. However, a strongly beamed AGN embedded in a shell of Compton-thick (but clumpy) matter requires less fine-tuning of the covering factor. Beaming is consistent with recent radio and Fermi results. Such beamed Compton-thick AGNs would be preferentially selected in surveys over unbeamed Compton-thick AGNs.
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