The weak Fe fluorescence line and long-term X-ray evolution of the Compton-thick active galactic nucleus in NGC 7674

Abstract

We present NuSTAR X-ray observations of the active galactic nucleus (AGN) in NGC 7674. The source shows a flat X-ray spectrum, suggesting that it is obscured by Compton-thick gas columns. Based upon long-term flux dimming, previous work suggested the alternate possibility that the source is a recently switched-off AGN with the observed X-rays being the lagged echo from the torus. Our high-quality data show the source to be reflection-dominated in hard X-rays, but with a relatively weak neutral Fe Kα emission line (equivalent width [EW] of ≈ 0.4 keV) and a strong Fe XXVI ionised line (EW ≈ 0.2 keV). We construct an updated longterm X-ray light curve of NGC 7674 and find that the observed 2-10 keV flux has remained constant for the past ≈ 20 years, following a high flux state probed by Ginga. Light travel time arguments constrain the minimum radius of the reflector to be ∼ 3.2 pc under the switchedoff AGN scenario, ≈ 30 times larger than the expected dust sublimation radius, rendering this possibility unlikely. A patchy Compton-thick AGN (CTAGN) solution is plausible, requiring a minimum line-of-sight column density (N H ) of 3 × 10 24 cm −2 at present, and yields an intrinsic 2-10 keV luminosity of (3-5) × 10 43 erg s −1 . Realistic uncertainties span the range of ≈ (1-13) × 10 43 erg s −1 . The source has one of the weakest fluorescence lines amongst bona fide CTAGN, and is potentially a local analogue of bolometrically luminous systems showing complex neutral and ionised Fe emission. It exemplifies the difficulty of identification and proper characterisation of distant CTAGN based on the strength of the neutral Fe Kα line.

X-Ray Spectral Analyses of AGNs from the 7Ms Chandra Deep Field-South Survey: The Distribution, Variability, and Evolutions of AGN Obscuration

“…Although strong Fe Kα lines are commonly detected in local bonafide Compton-thick AGNs, for some Compton-thick AGNs, the Fe Kα line might be weak because of ahigh ionization state or alow elemental abundance. For example, NGC7674, which is identified as Compton-thick by NuSTAR, has a weak neutral Fe Kα line of EW≈0.4keV (Gandhi et al 2017), likely because of its high bolometric luminosity (high ionization state). Our 22 Compton-thick sources have highredshifts, 20 of them with z 0.7 > and 6 with z 2 > .…”

The NuSTAR Serendipitous Survey: Hunting for the Most Extreme Obscured AGN at >10 keV

“…The observed X-ray-to-MIR luminosity ratio of a source can therefore give an independent, albeit indirect, assessment of the degree of obscuration (e.g., see Alexander 2017, for a recent review); the observed X-ray luminosity for any significantly absorbed AGN will be suppressed with respect to the intrinsic luminosity, causing it to deviate from the X-ray-to-MIR luminosity relation. This diagnostic has been utilized for other NuSTAR studies of obscured AGNs (e.g., Baloković et al 2014;Lansbury et al 2014Lansbury et al , 2015Stern et al 2014;Annuar et al 2015Annuar et al , 2017Gandhi et al 2017;LaMassa et al 2016). Figure 6 shows the observed X-ray versus intrinsic 6μm luminosities for the eight extreme NuSTAR serendipitous survey sources.…”

“…Flat G eff values (e.g., 0.5) give empirical evidence for high or CT absorption. Further empirical evidence for CT absorption can be obtained from the detection of a strong fluorescent FeKα emission line at »6.4 keV (with an equivalent width of > a EW 1k e V Fe K , although lower values do not necessarily rule out CT absorption; e.g., Della Ceca et al 2008;Gandhi et al 2017). This reflection feature becomes more prominent with increasing levels of absorption (e.g., Risaliti 2002).…”

The NuSTAR Extragalactic Survey: Average Broadband X-Ray Spectral Properties of the NuSTAR-detected AGNs

“…Due to the low counting statistics of the spectra, the iron emission line could not be constrained, as its EW resulted in a limit of EW<0.7 keV. This is not necessarily in contrast with the presence of CT absorption, as some CT AGNs have been reported to have unusually weak Fe lines (e.g., Gandhi et al 2017). To allow for the spectral complexity expected for these heavily obscured sources, we used the TORUS model, with an inclination angle fixed at 80 ; filled symbols) sources.…”