Suzaku Monitoring of the Iron K Emission Line in the Type 1 AGN NGC 5548

We present 7 sequential weekly observations of NGC 5548 conducted in 2007 with the \textit{Suzaku} X-ray Imaging Spectrometer (XIS) in the 0.2-12 keV band and Hard X-ray Detector (HXD) in 10-600 keV band. The iron K$\alpha$ line is well detected in all seven observations and K$\beta$ line is also detected in four observations. In this paper, we investigate the origin of the Fe K lines using both the width of the line and the reverberation mapping method. With the co-added XIS and HXD spectra, we identify Fe K$\alpha$ and K$\beta$ line at 6.396${-0.007}^{+0.009}$ keV and 7.08${-0.05}^{+0.05}$ keV, respectively. The width of line obtained from the co-added spectra is 38${-18}^{+16}$ eV ($\textrm{FWHM}=4200{-2000}^{+1800}$ km/s) which corresponds to a radius of 20${-10}^{+50}$ light days, for the virial production of $1.220\times10^7$ M${\odot}$ in NCG 5548. To quantitatively investigate the origin of the narrow Fe line by the reverberation mapping method, we compare the observed light curves of Fe K$\alpha$ line with the predicted ones, which are obtained by convolving the continuum light curve with the transfer functions in a thin shell and an inclined disk. The best-fit result is given by the disk case with $i=30^\circ$ which is better than a fit to a constant flux of the Fe K line at the 92.7% level (F-test). We find that the emitting radius obtained from the light curve is 25-37 light days, which is consistent with the radius derived from the Fe K line width. Combining the results of the line width and variation, the most likely site for the origin of the narrow iron lines is 20-40 light days away from the central engine, though other possibilities are not completely ruled out. (abridged)

💡 Research Summary

The authors present a detailed Suzaku monitoring campaign of the Seyfert 1 galaxy NGC 5548, consisting of seven weekly observations carried out in 2007 with the X‑ray Imaging Spectrometer (XIS, 0.2–12 keV) and the Hard X‑ray Detector (HXD, 10–600 keV). All seven pointings detect the Fe Kα fluorescence line, and the Fe Kβ line is also seen in four of them. By co‑adding the XIS and HXD spectra the authors obtain high‑signal‑to‑noise measurements of the two lines: the Kα centroid is 6.396 keV (−0.007 +0.009 keV) with a width of 38 eV (−18 +16 eV), corresponding to a full‑width at half‑maximum (FWHM) of ≈4200 km s⁻¹; the Kβ centroid is 7.08 keV (±0.05 keV) with a relative intensity of ≈0.13 of Kα.

Assuming virial motion around the central black hole, whose mass is taken to be 1.22 × 10⁷ M☉ from reverberation‑mapping studies, the measured velocity width translates into a characteristic radius of 20 light‑days (−10 +50 light‑days). This places the narrow Fe K emission well outside the innermost accretion disc but inside the classical dusty torus, roughly at the outer edge of the broad‑line region (BLR).

To test whether the line originates at this distance, the authors perform a reverberation‑mapping analysis. They construct the 2–10 keV continuum light curve from the seven observations and convolve it with transfer functions representing two simple geometries: (i) a thin spherical shell and (ii) an inclined thin disc. The disc model with an inclination of 30° provides the best fit, improving over a constant‑flux model at the 92.7 % confidence level (F‑test). The disc transfer function yields a response lag corresponding to an emitting radius of 25–37 light‑days, fully consistent with the radius inferred from the line width.

The agreement between the independent width‑based and lag‑based distance estimates strongly supports the conclusion that the narrow Fe Kα (and Kβ) lines in NGC 5548 arise from material located 20–40 light‑days from the central engine. This region could be the outer BLR clouds, a high‑density inner wall of the dusty torus, or a combination of both. The detection of Kβ in multiple epochs further indicates that the same physical region produces both lines.

The study also discusses limitations. The weekly cadence limits sensitivity to very short lags (≲ a few days), and the simple geometric transfer functions cannot capture possible complexities such as clumpiness, anisotropic illumination, or relativistic effects. Future observations with higher temporal resolution and higher spectral resolution (e.g., XRISM, Athena) will be able to refine the geometry, measure the ionisation state of the reflector, and test whether the narrow Fe K emission is truly associated with a flattened disc‑like structure or a more spherical distribution of clouds.

In summary, this work provides a robust, dual‑method determination of the location of the narrow iron fluorescence in NGC 5548, placing it at 20–40 light‑days from the black hole. The result offers an important observational constraint for models of AGN inner structure, the connection between the BLR and the dusty torus, and the role of X‑ray reprocessing in shaping the observed spectra of Seyfert galaxies.