Evidence for Relativistic Disk Reflection in the Seyfert 1h Galaxy/ULIRG IRAS 05189-2524 Observed by NuSTAR and XMM-Newton

TL;DR

This study analyzes X-ray spectra from NuSTAR and XMM-Newton observations of IRAS 05189-2524, revealing relativistic disk reflection features, a rapidly spinning black hole, and a compact corona close to the black hole.

Contribution

It provides the first detailed spectral evidence for relativistic disk reflection and high black hole spin in IRAS 05189-2524 using broadband X-ray data.

Findings

Detection of a broad Fe Kα emission line extending down to 3 keV

High reflection fraction indicating a compact corona near the black hole

Rapid black hole spin suggested but not tightly constrained

Abstract

We present a spectral analysis of the NuSTAR and XMM-Newton observations of the Seyfert 1h galaxy/ULIRG IRAS 05189-2524 taken in 2013. We find evidence for relativistic disk reflection in the broadband X-ray spectrum: a highly asymmetric broad Fe K$\alpha$ emission line extending down to 3 keV and a Compton scattering component above 10 keV. Physical modeling with a self-consistent disk reflection model suggests the accretion disk is viewed at an intermediate angle with a super-solar iron abundance, and a mild constraint can be put on the high-energy cutoff of the power-law continuum. We test the disk reflection modeling under different absorption scenarios. A rapid black hole (BH) spin is favored, however we cannot place a model-independent tight constraint on the value. The high reflection fraction ($R_{\rm ref} \simeq$ 2.0-3.2) suggests the coronal illuminating source is compact and close to the BH (lying within 8.7 $R_{\rm g}$ above the central BH), where light-bending effects are important.

Tables

Table 1. Table 1 XSPEC Models

Model	XSPEC Components	Spectral Range
1	zpcfabs*powerlaw + mekal	0.3–30 keV
2	zpcfabs*(pexrav + zgauss) + mekal	0.3–30 keV
3a	zpcfabs*relxilllp	2–30 keV
3b	(partcov*tbnew_feo)*relxilllp	2–30 keV
4a	zpcfabs*XSTAR*relxilllp	2–30 keV
4b	(partcov*tbnew_feo)*XSTAR*relxilllp	2–30 keV

Table 2. Table 2 Spectral Fitting of IRAS 05189–2524 : Part I

Component	Parameter	Model 1	Model 2
zpcfabs	$N_H$ ($\times {10}^{22}{\mathrm{cm}}^{-2}$)	${8.82}_{-0.26}^{+0.27}$	$8.30\pm 0.21$
	$f_{\mathrm{abs}}(\%)$	${98.89}_{-0.09}^{+0.08}$	${99}^{\star }$
power-law	$\Gamma$	$2.29\pm 0.04$	$\cdots$
mekal	kT (keV)	${0.092}_{-0.005}^{+0.010}$	${0.087}_{-0.009}^{+0.006}$
pexrav	$\Gamma$	$\cdots$	$2.47\pm 0.04$
	$R_{\mathrm{ref}}$	$\cdots$	${1.48}_{-0.44}^{+0.53}$
zgauss	LineE (keV)	$\cdots$	${6.40}^{\star }$
	$\sigma$ (keV)	$\cdots$	${0.71}_{-0.11}^{+0.13}$
	${\chi }^2/d.o.f$	1425.8/1049	1194.8/1047

Table 3. Table 3 Spectral Fitting of IRAS 05189–2524 : Part II Self-consistent Disk Reflection Models

Component	Parameter	Model 3a	Model 3b	Model 4a	Model 4b
zpcfabs	$N_H$ ($\times {10}^{22}{\mathrm{cm}}^{-2}$)	${7.17}_{-0.16}^{+0.17}$	${4.20}_{-0.27}^{+0.21}$	${7.15}_{-0.53}^{+0.62}$	${4.33}_{-0.33}^{+0.98}$
	$f_{\mathrm{abs}}(\%)$	${99}^{\star }$	${99}^{\star }$	${99}^{\star }$	${99}^{\star }$
	$A_{\mathrm{Fe}}$	$\cdots$	${5.5}_{-1.1}^{+2.5}$	$\cdots$	${5.0}_{-0.7}^{+u}$
xstar	$N_H$ ($\times {10}^{22}{\mathrm{cm}}^{-2}$)	$\cdots$	$\cdots$	${0.27}_{-0.27}^{+0.37}$	${1.44}_{-1.44}^{+1.56}$
	log $\xi$ ($\mathrm{erg}\mathrm{cm}{s}^{-1}$)	$\cdots$	$\cdots$	${3.20}_{-0.68}^{+1.26}$	${3.17}_{-0.39}^{+0.28}$
	$V_{\mathrm{out}}$ (c)	$\cdots$	$\cdots$	${0.13}_{-0.06}^{+0.02}$	${0.12}_{-0.01}^{+0.02}$
	$A_{\mathrm{Fe}}$	$\cdots$	$\cdots$	${10.0}_{-3.3}^{+u}$	${5.0}_{-0.7}^{+u}$
relxilllp	h (GM/c2)	${2.39}_{-0.32}^{+0.80}$	${4.70}_{-1.99}^{+4.04}$	${3.00}_{-0.87}^{+1.48}$	${4.08}_{-1.60}^{+2.16}$
	a (cJ/GM2)	${0.96}_{-0.03}^{+0.02}$	${0.96}_{-0.94}^{+u}$	${0.96}_{-0.07}^{+u}$	${0.98}_{-0.61}^{+u}$
	$\theta$ (∘)	$42\pm 2$	${44}_{-5}^{+2}$	${41}_{-2}^{+5}$	${51}_{-5}^{+4}$
	$\Gamma$	${2.04}_{-0.03}^{+0.02}$	${1.79}_{-0.14}^{+0.17}$	${2.00}_{-0.19}^{+0.17}$	${1.74}_{-0.17}^{+0.18}$
	log $\xi$ ($\mathrm{erg}\mathrm{cm}{s}^{-1}$)	$3.02\pm 0.02$	${3.12}_{-0.11}^{+0.22}$	$3.06\pm 0.07$	${3.23}_{-0.18}^{+0.14}$
	$A_{\mathrm{Fe}}$	${10.0}_{-1.3}^{+u}$	${5.5}_{-1.1}^{+2.5}$	${10.0}_{-3.3}^{+u}$	${5.0}_{-0.7}^{+u}$
	$E_{\mathrm{cut}}$ (keV)	${55}_{-7}^{+10}$	${34}_{-10}^{+21}$	${59}_{-24}^{+111}$	${33}_{-10}^{+21}$
	${\chi }^2/d.o.f$	862.0/859	860.6/859	854.8/856	854.7/856

Full text

1. Introduction

The hard X-ray continuum emission of active galactic nuclei (AGNs) is believed to be produced in the corona via thermal Comptonization of ultraviolet (UV) radiation from the accretion disk (e.g., Haardt & Maraschi, 1993). This continuum emission is subsequently reprocessed by dense material either from the accretion disk or distant structures such as the dusty torus or the broad line region (BLR), producing the characteristic reflected X-ray spectrum (e.g., George & Fabian, 1991). Fluorescent spectral lines, line edges and a Compton hump are typical features of the reflected X-ray spectrum. Fe K$\alpha$ is the most prominent spectral line in the X-ray band due to its high cosmic abundance and fluorescent yield. The line emission from the innermost regions of the accretion disk is relativistically broadened by the combined effects of Doppler shifts and gravitational redshift (Fabian et al., 1989; Laor, 1991), which skew the line profile into an extended red wing. Assuming the accretion disk extends down to the innermost stable orbit (ISCO), which is determined by the spin of the black hole (BH), a relativistically broadened Fe K$\alpha$ line could be used as a probe of the BH spin.

So far, broad Fe K$\alpha$ lines have been detected in many AGNs (e.g. Tanaka et al., 1995; Nandra et al., 2007; Risaliti et al., 2013; Walton et al., 2013); these are mostly Seyfert 1–1.5 galaxies with only a few absorbed Seyfert 2 galaxies showing such features: e.g., IRAS 18325–5926 (Iwasawa et al., 1996), IRAS 00521–7054 (Tan et al., 2012). *XMM-Newton *and *Suzaku *observations of IRAS 00521–7054 revealed a possibly strong and broad Fe K$\alpha$ line favoring a high BH spin, however the broad line could also be artificially produced by a combination of partial covering absorbers (Tan et al., 2012; Ricci et al., 2014). With high-quality broadband X-ray data and multi-epoch observations, BH spins have been robustly constrained in some nearby obscured Seyferts (e.g., Seyfert 1.9 galaxy NGC 1365, Risaliti et al., 2013; Walton et al., 2014), but the measurement is still difficult for these obscured objects.

IRAS 05189–2524 is an ultraluminous infrared galaxy (ULIRG) with log (L_{\rm IR}/\mbox{L_{\mathord{\odot}}})=12.16 (U et al., 2012) and hosts a Compton-thin AGN at redshift $z=0.0426$. While originally classified as Seyfert 2 (Veilleux et al., 1995), broad polarized Balmer lines were detected (Young et al., 1996) and near-infrared spectroscopy revealed broad Paschen lines (Veilleux et al., 1999b; Severgnini et al., 2001), indicating the existence of a hidden BLR. Therefore, it has been more accurately classified as a hidden broad line Seyfert 1 galaxy (Seyfert 1h galaxy/S1h) (Véron-Cetty & Véron, 2006), which defines the kind of Seyfert 2s having the spectra of Seyfert 1s in the polarized light.

In the X-ray band, IRAS 05189–2524 is one of the most luminous ULIRGs in the sky, which are typically merger systems undergoing rapid star-forming activities. IRAS 05189–2524 is likely a late stage merger of two spiral galaxies (Sanders et al., 1988). Its X-ray emission is dominated by the central AGN rather than starburst activities (Veilleux et al., 1999b). Iron emission has been detected in IRAS 05189–2524 from previous X-ray observations by Beppo-SAX (Severgnini et al., 2001) and Suzaku (Teng et al., 2009), but the general line profile was not resolved. The source went through a major spectral change during the 2006 *Suzaku *observation when the 2–10 keV flux dropped by a factor of $\sim$30, which could be explained by an increase in the absorption column density or the change of the intrinsic AGN luminosity (Teng et al., 2009). At a galactic scale, high-velocity outflows of ionized (Westmoquette et al., 2012; Bellocchi et al., 2013), neutral (Rupke et al., 2005; Teng et al., 2013; Rupke & Veilleux, 2015) and molecular (Veilleux et al., 2013) gas have been observed.

Outflows have been recognized as a signature of AGN feedback, which is believed to play an important role in the co-evolution of AGNs and their host galaxies. Evidence for multi-phase outflowing gas has been observed in AGNs on a wide rage of spatial scales from that of the central accretion disk to large-scale molecular outflows. If outflows at different scales are all AGN-driven, linking them could help unveil the expansion mechanism of the wind, such studies have been conducted on ULIRGs IRAS 11119+3257 (Tombesi et al., 2015) and Mrk 231 (Feruglio et al., 2015). In the X-ray band, there is growing evidence for ultra-fast outflows (UFOs) with the typical velocities of $\sim$0.1–$0.2\,c$, detected by fitting blue-shifted Fe K-shell absorption lines between 7 and 10 keV (e.g., Pounds et al., 2003; Tombesi et al., 2010; Pounds & King, 2013). In the case of PDS 456, a P-Cygni-like profile was observed, demonstrating a powerful wide angle outflow (Nardini et al., 2015).

In this paper, we report evidence for relativistic disk reflection in IRAS 05189–2524 based on analysis of the broadband X-ray spectrum from *NuSTAR *and XMM-Newton. In Section 2, we describe the observations and the data reduction. Section 3 provides the details of our spectral fitting. We present a discussion of the results in Section 4 and summarize our results in Section 5.

2. DATA REDUCTION

IRAS 05189–2524 was observed with NuSTAR (Harrison et al., 2013) and XMM-Newton (Jansen et al., 2001) in 2013 as a part of the *NuSTAR *ULIRG survey (Teng et al., 2015) and the *NuSTAR *survey of Swift/BAT AGN (Baloković et al., in prep.). *NuSTAR *observed the source at two epochs: on 2013 February 20 (OBSID 60002027002) and on 2013 October 2 (consecutive OBSIDs 60002027004 and 60002027005) with the exposures of 23.1 ks, 25.4 ks and 8.3 ks, respectively. The *XMM-Newton *observation (OBSID 0722610101) with an exposure time of 37.8 ks was coordinated with the second *NuSTAR *observation. The dataset was previously analysed in the *NuSTAR *ULIRG survey (Teng et al., 2015), with the focus on uncovering the obscuration levels and intrinsic luminosities of several nearby ULIRGs. We reduced the data following the data processing description in Teng et al. (2015).

2.1. NuSTAR

The *NuSTAR *data were processed using v.1.6.0 of the NuSTARDAS pipeline with *NuSTAR *CALDB v20160731. For each observation, the source spectra were extracted at the position of IRAS 05189–2524 within the radius of 60$\arcsec$. Corresponding background spectra were extracted from source-free areas on the same chip using polygonal regions. As discussed in Teng et al. (2015), the *NuSTAR *data show minor count rate variability ($\sim$20%) and no obvious change in spectral shape between the two epochs. Therefore, we combined the three *NuSTAR *observations using the ADDSPEC script in HEASoft v6.19 to maximize the signal-to-noise ratio (S/N) of the spectra. We binned the data taking into account the background level to provide a nearly constant S/N over the NuSTAR bandpass, with a minimum S/N of 3, and median S/Ns of 5.5 and 5.3 for the two focal plane modules (FPMA and FPMB).

2.2. XMM-Newton

The *XMM-Newton *observation was reduced with the XMM-Newton Science Analysis System v14.0.0 following standard procedures. The raw event files were filtered using EPCHAIN and EMCHAIN to produce cleaned event lists for each of the EPIC-pn (Strüder et al., 2001) and EPIC-MOS (Turner et al., 2001) detectors, respectively. Science products were then produced using XMMSELECT, considering only single and double events for EPIC-pn, and single to quadruple events for EPIC-MOS. We extracted the source from a circular region of radius 40′′, while background was estimated from a large region of blank sky on the same detector as IRAS 05189–2524. Instrumental response files were generated with RMFGEN and ARFGEN for each detector. The good exposure is $\sim$31 ks for EPIC-pn and $\sim$36 ks for each of the two EPIC-MOS detectors. After performing the reduction separately for MOS1 and MOS2, we combined these data into a single spectrum using ADDASCASPEC. The MOS and PN spectra were grouped to have 25 counts per bin.

3. SPECTRAL ANALYSIS

We model the time-averaged *NuSTAR *and *XMM-Newton *spectra jointly in XSPEC v12.9.0 (Arnaud, 1996) using the $\chi^{2}$ statistics. All uncertainties of the spectral parameters are reported at the 90% confidence level. Cross-normalization constants are allowed to vary freely for *NuSTAR *FPMB and XMM-Newton MOS and PN, and assumed to be unity for FPMA. The values are within $\sim$10% of unity, as expected from Madsen et al. (2015). For all the spectral fitting, we take into account the Galactic absorption by modifying the spectrum with a TBabs absorption model (Wilms et al., 2000). The Galactic column density is set to $N_{\rm H,gal}=1.66\times 10^{20}$ cm-2 (Kalberla et al., 2005).

We begin our spectral analysis of the broadband X-ray spectrum considering the band from 0.3 keV to 30 keV, as it becomes background-dominated above 30 keV. We first fit the spectrum with a power-law model subject to one partial covering neutral absorber modeled with zpcfabs, plus a mekal line emission model with its elemental abundances fixed at solar values (model 1, XSPEC models are listed in Table 1). The mekal component is used to model the thermal plasma contribution from the host galaxy. We find the best fit (reduced chi-square $\chi^{2}/{\rm d.o.f.}=1425.8/1049$) with a photon index $\Gamma=2.29\pm 0.04$ and a mekal temperature $kT=0.092^{+0.010}_{-0.005}$ keV. The covering fraction of the neutral absorber is well constrained as 98.89%, therefore contribution from the scattered power-law component is negligible above $\sim$2 keV. For simplicity, we fix the value at 99% from here on.

The data/model ratio plot shows strong signatures of disk reflection (Figure 1, panel b): a broad iron line peaking around 6.7 keV in the source rest-frame with a skewed shape extending down to $\sim$3 keV and residuals above 10 keV from the Compton reflection. In addition, there are possible absorption features of blue-shifted Fe K lines in both the *NuSTAR *and the *XMM-Newton *spectra, with the most evident absorption troughs lying in the vicinity of the Fe K-edge around 7.3 keV (Figure 2).

3.1. Phenomenological Reflection Model

We then test fitting the reflection features with a phenomenological model by adding a neutral reflection component pexrav (Magdziarz & Zdziarski, 1995) and one Gaussian emission line with variable width using the model zgauss (model 2, see table 1). We freeze the centroid of the Gaussian emission line at 6.4 keV, the disk inclination angle at the default value (cos i = 0.45), the iron abundance at solar and include no high-energy cutoff (foldE = 0), as this simple model is not sensitive to these parameters. The fit is improved considerably with $\Delta\chi^{2}/\Delta{\rm d.o.f}=-231/-2$. A broad Gaussian line with line width $\sigma=0.71^{+0.13}_{-0.11}$ keV and equivalent width (EW) $\simeq 0.60$ keV is required to provide a decent fit for the iron line emission feature in the spectrum. However, a clear excess still remains in the residuals between 3 and 6 keV as would be expected from the red wing of a relativistically broadened iron line (Figure 1, panel c). Model 2 also gives a very steep power-law of $\Gamma=2.47\pm 0.04$. We do not attempt to interpret the intrinsic continuum steepness at this point, as this is only a phenomenological model.

We note that for both the simple absorbed power-law and the phenomenological reflection model, at least two extra emission features are present in the residuals around 0.9 and 1.9 keV. The 0.9 keV excess might be associated with O Ly$\alpha$ or Fe–L shell emissions, and the feature around 1.9 keV could be caused by ionized Si emission lines. The excesses cannot be adequately fitted with simple Gaussian emission line models, as they could be a complicated combination of ionized absorption, over-abundant metal lines and X-ray contribution from the high mass X-ray binary population in the star-forming regions. X-ray point-source emission correlates with galaxy star forming rate (SFR) in star-forming galaxies (Lehmer et al., 2010). At an SFR of \sim 80\,\mbox{M_{\mathord{\odot}}}\,\rm\,yr^{-1} (Westmoquette et al., 2012), the 2–10 keV luminosity from the binary population is estimated to be $1.4\times 10^{41}\rm\,ergs\,s^{-1}$, about 1% of the total luminosity, which is enough to dominate the soft emission of IRAS 05189–2524. Therefore, the soft part of the spectrum likely helps little to constrain the X-ray continuum from the AGN. In order to avoid the possible situation where the soft emission from the host galaxy is driving our modeling of the disk reflection features, we ignore the data below 2 keV from here on.

3.2. Self-consistent Disk Reflection Models

In Teng et al. (2015), the authors attempted to explain the reflection features observed in IRAS 05189–2524 by neutral reflection from the torus, which is commonly the case for Compton-thick AGNs. However, they found neither the MYTorus (Murphy & Yaqoob, 2009) nor BNTorus model (Brightman & Nandra, 2011) could provide an adequate fit for the broadband spectrum, which ruled out the possibility of distant reflection. In order to explore the relativistic reflection hypothesis, we replace the pexrav and zgauss components in the phenomenological model with the self-consistent relativistic reflection model relxill (Dauser et al., 2014; García et al., 2014) (model 3a, see Table 1). We use relxilllp in the relxill model family, instead of using an empirical emissivity law, the emissivity is calculated directly in the lamp post geometry (Dauser et al., 2013). Relxilllp calculates a self-consistent reflection fraction. This could help us constrain the BH spin by shrinking the parameter space, as results with unphysical solutions of low BH spins with high reflection fractions can be ruled out by the assumption of a lamp-post geometry (for discussion, see Dauser et al. 2014). In order to measure the BH spin, we assume the inner disk radius extends down to the ISCO. We fix the outer radius at the default value of 400 $R_{\rm g}$ ($R_{\rm g}\equiv$ GM/c2 is the gravitational radius) as the model is not sensitive to this parameter, and the cutoff energy is allowed to vary in the fitting.

The relxilllp model yields a good fit to the 2–30 keV spectrum with $\chi^{2}/$d.o.f.$=862.0/859$ and no obvious excess in the residuals (Figure 3, panel b). The intrinsic X-ray continuum is steep, with the power-law photon index $\Gamma=2.04^{+0.02}_{-0.03}$. It is in the upper range of the photon index distribution observed in local Seyfert galaxies (e.g. Nandra & Pounds, 1996; Winter et al., 2009; Rivers et al., 2013), but less extreme than the previously published values for IRAS 05189–2524: e.g, $\Gamma=2.68^{+0.30}_{-0.13}$ (Teng et al., 2009), $\Gamma=2.51\pm 0.02$ (Teng et al., 2015). Albeit lacking high quality data above 30 keV, the model is able to put a modest constraint on the high-energy cutoff $E_{\rm cut}=55^{+10}_{-7}$ keV, as discussed in García et al. (2015a), such sensitivity could come partly from the low energy part of the reflection spectrum ($<$ 3 keV). The location of the primary radiation source is found to be close to the disk $h=2.39^{+0.80}_{-0.32}$ $R_{\rm g}$, with a rapid BH spin (dimensionless spin parameter $a=0.96^{+0.02}_{-0.03}$), and the self-consistent reflection fraction derived by the model is high ($R_{\rm ref}$ = 3.24). The best-fit results also find an intermediate inclination of $\theta=42\pm 2$ degrees and an ionization parameter111The ionization parameter $\xi=4\pi F_{\rm x}/n$, where $F_{\rm x}$ is the ionizing flux and $n$ is the gas density. log~{}\xi$$=3.02\pm 0.02~{}\rm\,erg~{}cm~{}s^{-1} for the accretion disk. We note that the iron abundance $A_{\rm Fe}$ is unusually high with best-fit value hitting the upper bound of the model of 10. But if we fix the iron abundance at solar in the relxilllp model, the fitting degrades considerably ($\Delta\chi^{2}\sim 130$) with a visible excess in the Fe K band.

A super-solar metal abundance is not uncommon among ULIRGs (e.g., Rupke et al., 2008). In the case of IRAS 05189–2524, based on the diagnostics of [N II]/H$\alpha$, [S II]/H$\alpha$ and [O I]/H$\alpha$ line ratios, an uncommonly high metallicity ($>4{Z_{\odot}}$)222Only an upper limit was put on the metallicity because the line ratios were outside the range predicted by the photoionization, shock or AGN models used in the paper. is estimated for the outer galactic disk, or else some peculiar ionization effects are needed to explain the line ratio measurements (Westmoquette et al., 2012). If the metallicity is indeed high on the galactic scale in IRAS 05189–2524, it is more physically consistent to use neutral absorption models with super-solar abundances in our spectral fitting, as neutral absorbers normally reside far from the central BH.

In an attempt to further constrain the iron abundance, we tie the iron abundances of the neutral absorber and the disk reflection component by replacing the zpcfabs model with an improved model for neutral absorption tbnew_feo333http://pulsar.sternwarte.uni-erlangen.de/wilms/research/tbabs/ convolved with the partial covering model partcov (model 3b, see Table 1). Both Fe and O abundances are variable parameters in the tbnew_feo model. We freeze the O abundance at solar, since it would not have a significant influence on our spectral fitting above 2 keV. As a result, the model finds a best-fit iron abundance $A_{\rm Fe}=5.5^{+2.5}_{-1.1}$, which is consistent with the measurement from Westmoquette et al. 2012. This model provides a slightly better fit for the data ($\chi^{2}/$d.o.f.$=860.6/859$), with a higher coronal height $h=4.70^{+4.04}_{-1.99}$, a less steep power-law continuum ($\Gamma=1.79^{+0.17}_{-0.14}$) and a cutoff at $34^{+21}_{-10}$ keV, even lower than in the previous model. As a result, the reflection fraction is reduced to $R_{\rm ref}=1.99$. We note here the cutoff energy is uncommonly low for an AGN, even compared with the lowest *NuSTAR *measurements (e.g., Ursini et al., 2015; Tortosa et al., 2016). This model generally has more uncertainty in the parameters, most notably, places no constraint on the BH spin. Although the metallicities in the whole galaxy should be related, there is no solid justification to assume the metallicities are the same in the BH vicinity and in the outer disk of the galaxy. However, we believe it is a reasonable simplification given the data quality, untying the iron abundances brings no obvious improvement to the fitting.

Given the possible ionized absorption features in the Fe K band and motivated by the presence of large-scale outflows in the host galaxy (e.g., Westmoquette et al., 2012; Bellocchi et al., 2013; Rupke & Veilleux, 2015), we add an ionized absorption component to account for any outflowing ionized absorbers. We modify the spectrum with an ionized absorption table model calculated by the XSTAR code (Kallman & Bautista, 2001) and tie its iron abundance with that of the relxilllp component. It is a physically reasonable requirement, since if the wind has extreme velocity, it most likely arises from the accretion disk, thus the same chemical abundances would naturally be expected. We first assume the partial covering neutral absorber has a solar iron abundance (model 4a, see Table 1). The fitting results reveal an ionized absorber with a velocity $v_{\rm out}=0.13^{+0.02}_{-0.06}\,c$ in the source rest-frame, which is well above the typical outflowing velocities of warm absorbers, about the common value found for UFOs (e.g., Tombesi et al., 2010). With an ionization parameter log$~{}\xi=3.06\pm 0.07~{}\rm erg~{}cm~{}s^{-1}$, the absorption should be dominated by Fe XXV. However the detection of a high-velocity ionized outflow is not significant, as it only brings marginal improvement to the fitting $\Delta\chi^{2}/\Delta{\rm d.o.f}=-7.2/-3$ and the difference not visually evident in the ratio plot. Other best-fit parameters are consistent with model 3a (without the ionized absorber). A rapidly spinning BH with $a>0.89$ is required to provide an adequate fit for the data.

We then reintroduce the neutral absorption model with a variable iron abundance (model 4b, see Table 1), and force the iron abundances of the disk reflection, the outflowing wind and the neutral absorber to be the same. The model brings no further improvement for the fitting. But again, it reduces the iron abundance $A_{\rm Fe}$ to $\sim$5, and since the iron abundance and the column density in the XSTAR model are degenerate parameters, the column density of the ionized outflow is estimated to be higher. Best-fit parameters of the disk reflection agree well with those given by model 3b, except for a slightly higher inclination of $\theta=51^{+4}_{-5}$ degrees. Only a low BH spin can be ruled out at the 90% confidence level by this model.

4. DISCUSSION

We test four models under different absorption scenarios to physically fit for the disk reflection features in IRAS 05189–2524, which provide good and comparable fits to the 2–30 keV broadband spectrum (for details, see Table 3 and Figure 3). With the covering fraction of the neutral absorber fixed at 99%, there is also no large discrepancy in the continuum below 2 keV. Based on the limited S/N of the spectrum, we cannot distinguish which model better describes the data. A super-solar neutral absorber is a reasonable requirement by the metallicity measurement of the galaxy from the literature, and it reduces the disk iron abundance to a less extreme value. However, detailed modeling of the intrinsic absorption causes apparent difficulty in disentangling the parameter degeneracies. Adding an ionized absorber modeled by XSTAR helps to account for the possible features of a high-velocity outflow in the Fe K band, but is not statistically strongly required. For three out of the four models, we can rule out a slow or retrograde BH spin at the 90% confidence level. In model 3a and 4a, a rapidly spinning BH is favored (for the spin constraints, see Figure 4). In addition, we consider the possible contribution from distant reflection produced in the torus or BLR, but including an extra xillver component does not improve the fitting.

4.1. Super-solar Iron Abundance

In our spectral analysis of IRAS 05189–2524, all acceptable fits require a super-solar iron abundance for the disk reflection component, but the value is not very well constrained. The primary effect of the iron abundance parameter is to change the relative strength of the iron line and the Compton hump. Because of the low net number counts at the high energy end of the *NuSTAR *spectra, the shape of the Compton hump is not well constrained, which could make it difficult to obtain a tight constraint of the iron abundance. We note that a similar over-abundance in iron has been reported in a number of AGNs (e.g., Risaliti et al., 2009; Patrick et al., 2011) and stellar-mass BHs (e.g., García et al., 2015b), with the examples of well-known narrow-line Seyfert 1 galaxies (NLSy1s) 1H 0707–495 (Dauser et al., 2012; Kara et al., 2015) and IRAS 13224–3809 (Fabian et al., 2013). In the case of IRAS 05189–2524, the galaxy is undergoing intense star formation, so it is expected to be iron enriched. Since super-solar metallicities were indicated via spatially resolved optical spectroscopy in the outer disk and the nuclear region of the galaxy (Westmoquette et al., 2012), it is physically reasonable for the iron abundance to be high in the accretion disk.

4.2. Absorption-dominated Models

Absorption from material lying relatively distant from the BH has frequently been proposed as an alternative interpretation of relativistic disk reflection (e.g., Miller et al., 2008, 2009). To investigate whether the excesses in the Fe K band and above 10 keV observed in IRAS 05189–2524 can be fully accounted by absorption-dominated models (without the reflection component), we test modeling the 2–30 keV spectrum with single and dual partial covering neutral absorption models with variable iron abundances. For the single-absorber model444(partcovtbnew_feo)powerlaw, we fix the covering faction at 99% to avoid large deviation from the data below 2 keV, the same way as in all the previous modeling. With $\Delta\chi^{2}=215.6$ compared to model 3b, this scenario can be easily ruled out. The dual-absorber model555(partcovtbnew_feo)(partcovtbnew_feo)powerlaw with an iron abundance $A_{\rm Fe}=2.2^{+0.2}_{-0.4}$ provides a better fit for the data, which requires one full covering absorber with the column density $N_{\rm H,abs\,1}=6.01^{+0.14}_{-0.93}\times 10^{22}$ cm-2 and another of the coverage fraction $f_{\rm abs\,2}=64\pm 5$% with $N_{\rm H,abs\,2}=1.92^{+0.58}_{-0.18}\times 10^{23}$ cm*-2*. However, it is still statistically worse than the reflection-dominated model with $\Delta\chi^{2}=75.6$. The model leaves a broad excess in the Fe K band and also fails to produce the shape of the Compton hump above 10 keV (see Figure 4, panel b), resembling the case in NGC 1365 (Risaliti et al., 2013) .

4.3. Eddington Ratio and Outflow

The time-averaged 2–30 keV X-ray flux observed by NuSTAR is $7.1\times 10^{-12}$ ergs cm-2 s*-1*, corresponding to an intrinsic luminosity of $3.0\times 10^{43}$ ergs s*-1* during the 2013 observation. Using the central velocity dispersions measured from the Ca II Triplet line widths (Rothberg et al., 2013) and the $M_{\rm BH}$-$\sigma$ relation (Tremaine et al., 2002), the BH mass of IRAS 05189–2524 is estimated to be M_{\rm BH}=4.2\times 10^{8}~{}\mbox{M_{\mathord{\odot}}}. We note the BH mass derived from the Ca II Triplet here is more than ten times larger than the value measured from CO band heads in the near infrared (Dasyra et al., 2006). As discussed in Dasyra et al. (2006), CO band heads are often representative of young stellar populations, which could lead to systematically lower BH masses for ULIRGs. Combined with the bolometric luminosity inferred from the infrared (Teng et al., 2015), IRAS 05189–2524 is measured to be accreting at an Eddington rate $\lambda_{\rm Edd}=L_{\rm bol}/L_{\rm Edd}$ of 0.12. Based on the correlation between the photon indexes and the Eddington ratios of AGNs (Shemmer et al., 2008; Brightman et al., 2013, 2016), a photon index of $\Gamma\sim 2.0$ would be expected. The photon index obtained by our disk reflection modeling with a low cutoff energy is $\Gamma\simeq 1.7-2.0$, which is consistent with the expected value given the scatter of the relation.

In our spectral modeling, we did not detect significant disk winds. Fitting the possible absorption features in the spectrum with a simple Gaussian absorption line using the gabs model would reach similar $\chi^{2}$ as the physical XSTAR grid, with the best-fit line centroid at $\sim$7.33 keV. Although the effect of this ionized absorber on the continuum is subtle, we include it in the spectral fitting, as even mild absorption lying around the Fe K-edge could influence the BH spin measurement, which is sensitive to the profile of the broad iron line. The power of an outflow scales with the amount of matter being ejected (King & Pounds, 2015). With the rather small column density measured for the ionized absorption compared with that of massive disk winds found in other AGNs (e.g., IRAS 11119+3257, Tombesi et al., 2015), even if an UFO indeed exist in IRAS 05189–2524 at the epoch of the observation, it is probably not energetic enough to to have a large impact on its galactic environment.

5. SUMMARY AND CONCLUSION

We detect disk reflection features of a broad iron line and a Compton reflection hump in IRAS 05189–2524, a Seyfert 1h galaxy and ULIRG, from the *NuSTAR *and *XMM-Newton *data taken in 2013. With the broadband X-ray spectrum, an alternative dual partial covering absorber explanation can be ruled out. Physically modeling with the self-consistent relativistic reflection model relxilllp finds that the reflection is likely to be generated from the inner range of a rapidly spinning BH. The high reflection fraction measured ($R_{\rm ref}\simeq$ 2.0–3.2) indicates the reflection area is close to the BH, where relativistic light bending effects boost the reflection signature (Miniutti & Fabian, 2004), suggesting the coronal illuminating source is compact. However, due to the complex effects from neutral and ionized absorption and the limited S/N data, parameter degeneracies preclude putting a tight, model-independent estimation on the BH spin.

As a merger system, IRAS 05189–2524 is an interesting target for the spin measurement, as BH spins are believed to encode information about the galaxy evolution history (Berti & Volonteri, 2008), and ULIRGs have been proposed as an important transition phase in the hierarchical evolution model of galaxies (Hopkins et al., 2006; Sanders et al., 1988). Simulations predict it is uncommon for mergers to yield large spins, unless alignment of the spins of the merging BHs with the orbital angular momentum is very efficient (Berti & Volonteri, 2008).

With Fe K band absorption features marginally evident in the residuals, the detection of a high-velocity ionized wind is not statistically significant. Also, we find the iron abundance of the neural absorber is strongly degenerated with the BH spin. Future observations with higher S/N data could help disentangle the reflection component from the underlying continuum and the various absorption effects. Assuming the source remains at the flux level observed in 2013, if the ionized absorption indeed exists in IRAS 05189–2524, simulations indicate the XSTAR component would be significantly required by datasets with more than $\sim$2 times of exposure, leading to a $>3\sigma$ detection of the ionized absorption features. With longer observations (500 ks *NuSTAR *and 300 ks *XMM-Newton *exposures), it would be possible to obtain the BH spin to within $\sim$30% of uncertainty under all the different absorption scenarios we have discussed above.

We thank the anonymous referee for helpful comments that improved the paper. M. B. acknowledges support from NASA Headquarters under the NASA Earth and Space Science Fellowship Program, grant NNX14AQ07H. This work was supported under NASA Contract No. NNG08FD60C, and made use of data obtained with NuSTAR, a project led by Caltech, funded by NASA and managed by NASA/JPL and has utilized the NUSTARDAS software package, jointly developed by the ASDC (Italy) and Caltech (USA). We thank the *NuSTAR *Operations, Software and Calibration teams for support with the execution and analysis of these observations. This research has also made use of data obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States.