Abstract
I have studied the variability of the FeKα emission line flux in response to changes in the 2–10 keV unabsorbed flux in a sample of Compton thin type-II active galactic nuclei. All of the 20 sources in the sample studied by Laha et al., exhibit the presence of a narrow FeKα emission line. Except for one source (NGC 2992), all other sources show no correlations between the FeKα emission line flux and 2–10 keV unabsorbed flux. This implies that although most of the sources have shown significant variations in their 2–10 keV flux, the FeKα emission line did not get enough time to respond to those changes. This helps us put lower limits on the distance of the FeKα emitters to >10 lt-yr from the central engine, equivalent to ∼3 pc which is consistent with the standard torus/narrow-line-region distance. I conclude that the FeKα emission lines of these sources arise from reflection of the hard X-ray photons off neutral materials located beyond pcscale.
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1. Introduction
Active galactic nuclei (AGNs) are powerful emitters in the panchromatic wavelength range: Radio to Gamma-rays. However, the most crucial wavelength bands probing the physics of matter in the vicinity of the supermassive black hole (SMBH) are the optical, UV and X-rays. The primary X-ray spectra from an AGN arises from an optically thin, and hot (∼109 K) corona. These X-ray photons strike the circumnuclear material, including the ionized accretion disk, the neutral/lowly ionized torus, the broad line region and the narrow line region to create several reprocessed spectral features (Fabian et al. 2009; García et al. 2014; Ghosh et al. 2016, 2018; Pal et al. 2016, 2020, 2021; Laha et al. 2018a, 2019a; Pal 2018; Ghosh & Laha 2020, 2021). Thus, the primary X-ray emission and the reprocessed features give us a rare glimpse of the hitherto spatially unresolved regions in the sub-pc to a few 10 s of pc scale region around the super massive black hole. Some AGN show ionized absorption in their X-ray spectra (Tombesi et al. 2010; Laha et al. 2011, 2013, 2014, 2016a, 2016b, 2017, 2018b, 2019b, 2021; Reeves et al. 2020) while some show neutral absorption with a range in column density, from Compton-thick to Compton-thin (Markowitz et al. 2014; Ricci et al. 2015; Civano et al. 2019; Lopez-Rodriguez et al. 2019; Laha et al. 2020). One of the main reprocessed features in X-rays is the narrow FeKα emission line which is commonly found in AGNs. It is believed to arise when hard X-ray power law photons strike the surrounding neutral matter, such as the torus or the narrow line region gas. Sometimes the line shows a broad profile, due to gravitational and/or Doppler broadening when it arises from the inner regions of the accretion disk which is in close vicinity of the SMBH (Fabian et al. 2009; Pal & Dewangan 2013). In this work I focus on the origin of the narrow FeKα emission lines in a sample of Compton-thin type-II AGN, by studying the presence/absence of any correlated variability between the narrow FeKα emission line flux and the primary power law emission.
2. Results and Conclusions
I have collated the narrow FeKα emission line flux and the unabsorbed 2–10 keV flux from Laha et al. (2020) for all the 20 sources in the sample. The Compton-thin type-II sample of Laha et al. (2020) consists of 20 nearby bright AGN with a line of sight absorption column density of ∼1021–23.5 cm−2. All the sources have high signal-to-noise spectra from X-ray telescopes XMM-Newton, Chandra and Suzaku, covering the broad band energy range 0.3–10 keV (and 0.6–40 keV in case of Suzaku). Most sources have shown significant variability in their 2–10 keV emission line fluxes, hence it is important to find if there is any correlated response of the narrow FeKα line to the primary flux. Except for the source NGC 2992, which has been reported in a separate paper (Ghosh & Pal 2021), none of the other 19 sources show any correlation in their 2–10 keV versus FeKα fluxes. The observations for all sources probe timescales ranging from ∼10 to 20 yr, implying that the FeKα emitting line region lies beyond this distance from the central source, or else it would have responded to the primary flux. Figure 1 left and right panel shows examples of no correlation between these quantities, for the sources NGC 5506 and NGC 7582 respectively. I have obtained similar results for all the sources, except NGC 2992. The Spearman correlation coefficient for all the 19 sources in the sample show that there is no statistically significant correlation. Similarly the best fit linear regression slope is consistent with zero in all cases. I have assumed for this discussion that variability in the narrow Fe line flux is a direct response to variability in the illuminating hard X-ray continuum, modified only by a light-travel time delay. For each of these sources, I rule out the notion that the bulk of the narrow Fe line-emitting gas is located at distances smaller than the light-travel time spanned by the average sampling time. That is, in these sources, the bulk of the line-emitting gas must be located at least a few light-years from the corona, which is >1–3 pc consistent with previous estimates of the distance of the torus distance and narrow line region (Ricci et al. 2014).
Figure 1. Left: the FeKα flux plotted against the 2–10 keV flux of the source NGC 5506. I find that there is no significant correlation between the two quantities. Right: same as in left, except for the source which is NGC 7582.
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