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

Abstract

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 ≈ 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 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 ≈ 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 (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 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.

XCLUMPY: X-Ray Spectral Model from Clumpy Torus and Its Application to the Circinus Galaxy

“…All these models assume a uniform density of the gas, and hence we refer to them as "smooth torus models". These models have also been widely used to fit the observed X-ray spectra of AGNs (Ikeda model: e.g., Awaki et al 2009;Eguchi et al 2011;Tazaki et al 2011Tazaki et al , 2013Kawamuro et al 2013Kawamuro et al , 2016aTanimoto et al 2016Tanimoto et al , 2018Oda et al 2017Oda et al , 2018Yamada et al 2018, MYTorus model: e.g., Yaqoob 2012Yaqoob et al 2015Yaqoob et al , 2016Koss et al 2013Koss et al , 2015Koss et al , 2016Koss et al , 2017Ricci et al 2014aRicci et al ,b, 2015Ricci et al , 2016aRicci et al ,b, 2017a,b, Torus model: e.g., Brightman & Nandra 2011bBrightman et al 2013Brightman et al , 2014Brightman et al , 2015Brightman et al , 2016Brightman et al , 2017Buchner et al 2014Buchner et al , 2015Gandhi et al 2014Gandhi et al , 2015Gandhi et al , 2017.…”

Suzaku Observations of Heavily Obscured (Compton-thick) Active Galactic Nuclei Selected by the Swift/BAT Hard X-Ray Survey

“…Even with an apparently large opening angle (e.g., > 65 degrees), we do not rule out the possibility that Compton-thin material (N H ≤ 10 24 cm −2 ) is present in the torus-hole region (i.e., at inclinations lower than this half-opening angle) and "buried" the nucleus, because it would not affect significantly the broadband spectra except for the scattered component. In fact, such a geometry is proposed for the CTAGN in UGC 5101 (Oda et al 2017) and possibly for NGC 4945. Figure 7 shows the distribution of the (a) line-of-sight hydrogen column density, (b) photon index, (c) scattered fraction, (d) 10-50 keV luminosity and (e) Eddington ratio, all based on the Ikeda2 model fit.…”

Broadband X-Ray Spectral Analysis of the Double-nucleus Luminous Infrared Galaxy Mrk 463

“…Since such mid-IR lines emitted from the NLR are less attenuated by dust in the host galaxy and are less contaminated by star-formation activities, they are generally thought to be better indicators of the intrinsic AGN luminosity than the widely used [O III] λ5007 line (Heckman et al 2005). Since these MIR lines are produced by irradiation of UV photons from the AGN, their luminosity ratios to X-rays reflect the intrinsic AGN SED in the UV to X-ray band, unless the nucleus is deeply "buried" by dust (i.e., the obscurer has a large covering factor) and hence the NLR is underdeveloped (Oda et al 2017). Figure 4 plots the relation between the de-absorbed 2-10 keV luminosity and the [O IV] 25.89 µm luminosity for Mrk 463E (Armus et al 2004).…”