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A possible ultra strong and broad Fe Ka emission line in Seyfert 2 galaxy IRAS 00521-7054

We present XMM-Newton spectra of the Seyfert 2 Galaxy IRAS 00521-7054. A strong feature at ~6 keV (observer's frame) can be formally fitted with a strong (EW=1.3+-0.3 keV in the rest frame) and broad Fe Ka line, extending down to 3 keV. The underlying X-ray continuum could be fitted with an absorbed powerlaw (with Gamma = 1.8+-0.2 and N_H = 5.9 x 10^22 cm^-2) plus a soft component. If due to relativistically smeared reflection by an X-ray illuminated accretion disk, the spin of the supermassive black hole is constrained to be 0.97^+0.03_-0.13 (errors at 90% confidence level for one interesting parameter), and the accretion system is viewed at an inclination angle of 37+-4 degree. This would be the first type 2 AGN reported with strong red Fe Ka wing detected which demands a fast rotating SMBH. The unusually large EW would suggest that the light bending effect is strong in this source. Alternatively, the spectra could be fitted by a dual absorber model (though with a global chi^2 higher by ~ 6 for 283 d.o.f) with N_H1 = 7.0 x 10^22 cm^-2 covering 100% of the X-ray source, and N_H2 = 21.7 x 10^22 cm^-2 covering 71%, which does not require an extra broad Fe Ka line.


Introduction
The broad Fe Kα fluorescent emission line in Active Galactic Nuclei (AGNs), first clearly detected by ASCA (Tanaka et al. 1995), provides an important tracer to probe the physical conditions and geometrical distribution of matter in the vicinity of supermassive black hole. The line is believed from the accretion disk by reflecting the illuminating X-ray continuum, and the unique broad and skewed (toward the red) line profile is a consequence of the relativistic Doppler effect and strong gravitational redshift (e.g. Fabian et al. 1989;Laor 1991). Such broad Fe Kα emission lines have been detected in a significant number of AGNs (Nandra et al. 1997;Wang, Zhou & Wang 1999;Fabian et al. 2002;Nandra et al. 2007;Miller 2007;Guainazzi 2010;de La Calle Pérez et al. 2010). More interestingly, in a few sources the red wing of the Fe Kα emission line appears too broad to be produced from accretion disk surrounding Schwarzschild black holes (Wilms et al. 2001;Fabian et al. 2002;Patrick et al. 2011a). In these cases, assuming the accretion disk extends to the inner-most stable circular orbit (ISCO, determined by the spin of black hole), fast rotating Kerr black holes are required, for which the accretion disk could extend closer to the horizon thus stronger gravitational redshift could be produced (Reynolds & Fabian 2008). Therefore, the Fe Kα line profile enables astronomers to measure the spin of SMBHs, which is otherwise extremely difficult or even impossible (Reynolds & Nowak 2003). Till now, rotating SMBHs have been claimed in a number of type 1 AGNs (e.g. Brenneman & Reynolds 2006, 2009Patrick et al. 2011a;Brenneman et al. 2011). However, strong debates exist on the nature of the ultra broad red wings, challenging the base of such measurements. An alternative scenario is that the observed broad and skewed line profile is artificial due to continuum curvature produced by more complex absorption (e.g. partial covering ionized absorber, Reeves et al. 2004;Miller et al. 2008Miller et al. , 2009). Although various efforts have been made to distinguish two scenarios, including performing spectral fitting over wider band to better determine the absorption and the underlying continuum (Patrick et al. 2011b), studying bare Seyferts with no or very weak warm absorption (Patrick et al. 2011a), and monitoring the lag between reflection component and the continuum (Fabian et al. 2009), a clear picture has not been obtained.
Meanwhile, type 2 AGNs could provide a different insight into the nature of broad Fe Kα emission. According to unification model, type 2 AGNs are viewed at higher inclination angle comparing with type 1 sources. Furthermore, while ionized absorption, which plays the key role in the alternative scenario to the broad Fe Kα line, is common in type 1 AGNs (e.g. Reynolds 1997), X-ray absorption in type 2 sources is dominated by cold matter. Till now, broad Fe Kα lines were detected only in a few type 2 AGNs (see Guainazzi et al. 2010 for a summary of broad Fe Kα line detections in obscured AGNs, mostly type 1.8 and 1.9 sources; Iwasawa et al. 1996;Dewangan et al. 2003;Shu et al. 2010a). However, neither of the Fe Kα line red wing in these type 2 AGNs is broad enough to demand a rotating SMBH.
IRAS 00521-7054 is a Seyfert 2 galaxy at z = 0.0689. In this letter we present new XMM-Newton observation on IRAS 00521-7054 taken in 2006, in which we detect an ultra strong and broad Fe Kα line. In §2 we describe the XMM observations and data reduction. §3 provides our detailed spectral fitting to the continuum and the emission line, followed by discussion in §4.  (April 22, 2006) respectively. We reduced the raw data with the XMM-Newton Scientific Analysis System (SAS) (Gabriel et al. 2004) version 10.0.0. The exposures were processed using the SAS pipeline "epchain" and "emchian". We filtered the event files to include only those events with XMM-SAS quality flag FLAG==0 (#XMMEA EM) for PN (MOS1 and MOS2), and extracted single-and double-pixel events (PATTERN ≤ 4) for PN, and singleto quadruple-pixel events (PATTERN ≤ 12) for MOS1 and MOS2. After excluding high background intervals, the net exposures for two observations are 5.7 ks and 7.9 ks for PN, and 7.4 ks and 11.0 ks for MOS respectively. Source spectra were extracted from circles with radius of 30" for PN (to avoid nearby chip edge) and 40" for MOS centered on the source position. For both PN and MOS data, the background events were extracted from source-free areas on the same chip using four circular circles with a combined area 4 times larger than the source region. We note that adopting different background regions would not alter any scientific results presented in this work. The response matrix and ancillary response file were generated using the RMFGEN and ARFGEN tools with the SAS software. The extracted net source count rate is ∼ 0.26 cts/s for PN, and ∼ 0.08 cts/s for MOS, thus the exposures are free from pile-up effect.

Spectral Fitting
We examined the light curves of each exposure, and found no significant variation within or between two exposures. Through a preliminary spectral fitting, we also found no spectral variations between two observations. We therefore combined the spectra from two exposures to increase the S/N ratio in the spectra, and obtained one composite PN spectrum and one MOS spectrum (co-added from MOS1 and MOS2) 1 . Both spectra were binned to have at least 20 counts per bin to allow the use of χ 2 statistics with XSPEC V12.6.0. All fitting errors are of 90% confidence level for one parameter (∆χ 2 = 2.71), if not otherwise stated. The adopted cosmological parameters are H 0 = 70 km s −1 Mpc −1 , Ω Λ = 0.73 and Ω m = 0.27. During spectral fitting, the Galactic column density along the line of sight to IRAS 00521-7054 is always taken into account (N H = 4.44 × 10 20 cm −2 , Dickey & Lockman 1990).
In Figure 1 we present XMM spectra of IRAS 00521-7054 for both PN and the summed MOS1 and MOS2 data. It can be seen that the MOS data are consistent with the PN data within the statistical errors. We first fit the 0.5 -10 keV spectra with an absorbed powerlaw plus an extra soft component. The soft component could be scattered emission of the intrinsic X-ray continuum, or from the host galaxy. We find that either a powerlaw free from intrinsic absorption (with Γ = 2.4±0.5), a blackbody (with T = 0.21±0.03 keV) or a bremss plasma emission could provide adequate fit to the soft component, and these models are statistically indistinguishable based on current data. Hereafter, we simply choose a blackbody model to fit the soft component. We note that choosing different soft component models would not alter the fitting results in this work. This simple continuum model provides a poor fit to the data, with χ 2 =368.3 for 287 degrees of freedom. The data to model ratio plot ( Figure  1b) shows that there is clear residual emission between 3 -7 keV that could be interpreted as the existence of a relativistic Fe Kα emission line. We then exclude data bins between 3 -7 keV to fit the continuum, and present the plot of data to best-fit model ratio in Fig. 1 (Panel c) to demonstrate the line profile.
The line appears rather broad, and the red wing extends to as low as 3 keV. We then add a relativistic disk line model (laor in XSPEC, Laor 1991) to fit the broad line (see Fig.  1), and the fitting is significantly improved (∆χ 2 = 58 for 4 more free parameters). The best-fit line and continuum parameters are listed in Table 1. During the fit, we fixed the line energy at 6.4 keV in the source rest frame, since it is otherwise poorly constrained by the data. The small best-fit inner radius (R in = 1.8 +0.7 −0.8 r g ) suggests that the central SMBH is fast rotating (fitting with a diskline model for a non-rotating SMBH instead yields larger χ 2 , ∆χ 2 = 9). Furthermore, the line EW is remarkably large (EW = 1.3±0.3 keV in the rest frame). The best-fit inclination of the disk is 37 +6 −5 . Since narrow Fe Kα cores are often seen in the X-ray spectra of AGNs (Yaqoob & Padmanabhan 2004;Shu et al. 2010bShu et al. , 2011, we added a narrow Gaussian line (with central energy fixed at 6.4 keV in the rest frame, and line width at zero, i.e., unresolved by XMM PN and MOS spectra) to the fit, and found such a narrow component is statistically not required in additional to the broad component (∆χ 2 < 1, EW < 56.0 eV). Reflection component from neutral material in additional to the underlying powerlaw (pexrav in XSPEC, Magdziarz & Zdziarski 1995) is statistically not required either (∆χ 2 = 0, R < 0.8), the normalization of which thus has little effect on the broad Fe Ka line intensity.
We also apply a self-consistent model (kerrconv × ref lionx, Brenneman & Reynolds 2006;Ross & Fabian 2005) to fit the broad line (see Fig. 2), and the best-fit parameters are presented in Table 1. The fitting also suggests that the central SMBH is rotating, with α = 0.97 +0.03 −0.13 . This model produces similar residual χ 2 /dof comparing with the simple laor line model (310.13/283 vs 311.54/280), and consistent physical parameters, including inclination angle θ, inner radius and emissivity index q.

Discussion
We present the detection of an ultra strong and broad Fe Kα line in Seyfert 2 galaxy IRAS 00521-7054. The X-ray continuum is fitted with a powerlaw plus a soft blackbody emission. The observed 0.5 -2 keV and 2 -10 keV fluxes are 3.8 × 10 −14 ergs cm −2 s −1 and 2.4 × 10 −12 ergs cm −2 s −1 respectively. Absorption corrected 2 -10 keV rest frame luminosity is 3.9 × 10 43 ergs s −1 . The Fe Kα line is rather strong with EW =1.3±0.3 keV, larger than most broad Fe Kα lines detected in other AGNs (with typical EW of several 100 eV). Spectral fitting suggests that the SMBH in IRAS 00521-7054 in rotating with a spin of α = 0.97 +0.03 −0.13 . This is the first measurement of SMBH spin in a type 2 AGN.

Alternative models
In this sub-section we investigate whether such an ultra strong and broad Fe Kα line could be in fact due to curvature in underlying continuum caused by more complex absorption. Below we applied different absorption models to the spectra without including an additional broad emission line component.
Single warm absorber (absori in XSPEC), a partially covering warm absorber, or a partial covering cold absorber model can be easily ruled out because of the much larger χ 2 reduced (∆χ 2 > 50, comparing with the laor line model).
We then adopted a dual cold absorber model (with N H1 covering 100% of the central X-ray source, and N H2 partially covering the continuum). Interestingly, the dual-absorber model fits the spectra much better (χ 2 /dof = 319.09/285), however it is still statistically worse than the laor line model (∆χ 2 = 9). In Figure 3(b), we can still see clear residuals at ∼ 6 keV in the data to model ratio plot, comparing with the the residuals of best-fit laor and ref lionx model (see Fig. 3d and 3e). Adding a reflection component (pexrav) and/or a narrow Gaussian at 6.4 keV to the dual-absorber model does not change the produced χ 2 . Replacing either one or both of the cold absorbers with ionized absorption yields consistent results and best-fit ionization parameters at zero, indicating more complex warm absorption models do not improve the fitting. The best-fit parameters of the dual-absorber model are N H1 = 7.0±0.8 × 10 22 cm −2 with unit coverage, and N H2 = 21.7 +5.6 −5.4 × 10 22 cm −2 with a covering factor f = 0.71 +0.06 −0.09 . We also note that the best-fit powerlaw slope (Γ = 2.8±0.1) appears considerably steep, greater than that of 99.7% of AGNs in a large XMM sample (Scott et al. 2011).
We note that in our dual absorber model, all element abundances are fixed to the solar values. By adopting the zvphabs model in XSPEC, we tried to fit the spectra by allowing the Fe and Ni abundance to be free parameters, which could likely produce better fit to the strong edge feature at ∼ 7 keV in the spectra. We found that an overabundance of Fe/Ni (by a factor of ∼ 2.6) could slightly improves the fit (∆χ 2 = 2.5), but is statistically not required, thus the abundance of Fe/Ni can not be well constrained. This model however yields a flatter powerlaw index (Γ = 2.3 +0.6 −0.3 ), consistent with most AGNs. Figure 3c shows the data to model ratio plot for this model, in which weak residuals at ∼ 6 keV is still visible, comparing with the laor and ref lionx model. This model (by allowing the Fe and Ni abundance to be free parameters) is still statistically slightly worse than the laor line model (the difference of χ 2 is 6.5 with the same degrees of freedom).
We note that, while the relativistic reflection scenario yields formally a slightly better fit than the double absorption scenario we have presented in this paper, only follow-up observations with higher spectral resolution and/or larger sensitive energy bandpass would allow us to discriminate between them.

The nature of the ultra strong Fe Kα line
Spectral fitting to the broad Fe Kα line suggests that the accretion system is viewed at an intermediate inclination angle of 37±4 • , in agreement with studies of other type 2 AGNs ). It's interesting to note that Frogel & Elias (1987) reported the detection of broad Hα emission line (with full width at zero intensity of > 6000 km/s) in its optical spectrum. This suggests that IRAS 00521-7054 is a Seyfert 1.9 galaxy, consistent with the intermediate inclination angle obtained through X-ray spectral fitting.
The measured broad line shows superb large line EW (1.3±0.3 keV in the rest frame, fitted with a laor line), much larger than most of those previously detected in either type 1 or type 2 AGNs. This is however a different situation comparing with large narrow Fe Kα line EW (> 1 keV) reported in Compton-thick AGNs (see Levenson et al. 2006;Liu & Wang 2010). In Compton-thick AGNs, the large Fe Kα line EW is due to the strong attenuation to the underlying continuum, and the narrow Fe Kα line, which is produced at a much larger scale comparing with the continuum, suffers no or little obscuration. In IRAS 00521-7054, there exists only moderate obscuration to the continuum. Furthermore, the detected Fe Kα line is ultra broad, and is believed from the innermost region of the accretion disk, thus strong attenuation (if there is any) could not enlarge the line EW.
Theoretical calculations have shown that reflection from a constant density slab which is illuminated with a powerlaw spectrum could produce Fe Kα line with EW no larger than ∼ 800 eV (e.g. Ballantyne et al. 2002). One possible explanation to the observed large EW in IRAS 00521-7054 is light bending effect.  reported a similar large line EW (1.4 keV if fitted with a laor line) in MCG 6-30-15 during a low-flux interval, and it can be simply attributed to the strong light bending effect which could (with the X-ray continuum source located closer to the central SMBH) yield weaker observed flux in continuum, but almost constant reflection component. We note that the fitted emissivity index q (see table 1) is also consistent with that reported in MCG 6-30-15 during a lowflux interval (3.3±0.3). However, it is still puzzling that the line EW in IRAS 00521-7054 remains stably large during two XMM exposures, while such large EW was rather rare in other AGNs and was only detected in low flux state in MCG 6-30-15 , and in 1H0707-495 (Fabian et al. 2009;Zoghbi et al. 2010;de La Calle Pérez et al. 2010) and IRAS13224-3809 (Ponti et al. 2010).
The ref lionx model also yields an over abundance of iron (Fe/solar = 3.4±1.3). We note that similar over abundance in iron was also reported in other AGNs (e.g. Ballantyne et al. 2003;Risaliti et al. 2009;Patrick et al. 2011a). The best-fit spin of the SMBH is α = 0.97 +0.03 −0.13 (90% confidence level), suggesting that the black hole may be maximally spinning. However, a Schwarzschild solution is still consistent with the data if the 99% confidence level uncertainties are considered (see Figure 4). Higher S/N spectra are therefore desired to understand the nature of the ultra strong and broad Fe Kα line in IRAS 00521-7054.