The weak neutral Fe fluorescence line and long-term X-ray evolution of the Compton-thick AGN 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 K$\alpha$ emission line (equivalent width [EW] of $\approx$ 0.4 keV) and a strong Fe XXVI ionised line (EW $\approx$ 0.2 keV). We construct an updated long-term X-ray light curve of NGC 7674 and find that the observed 2-10 keV flux has remained constant for the past $\approx$ 20 years, following a high flux state probed by $Ginga$. Light travel time arguments constrain the minimum radius of the reflector to be $\sim$ 3.2 pc under the switched-off AGN scenario, $\approx$ 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 ($N_{\mathrm{H}}$) of 3 $\times$ 10${}^{24}$ cm${}^{-2}$ at present, and yields an intrinsic 2-10 keV luminosity of (3-5) $\times$ 10${}^{43}$ erg s${}^{-1}$. Realistic uncertainties span the range of $\approx$ (1-13) $\times$ 10${}^{43}$ erg s${}^{-1}$. The source has one of the weakest fluorescence lines amongst {\em 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 K$\alpha$ line.

MNRAS accepted version

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