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

Abstract

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$\hat{I}\pm$ line is well detected in all seven observations and K${\hat{I}}^2$ 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$\hat{I}\pm$ and K${\hat{I}}^2$ 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 ($\text{FWHM}=4200_{-2000}^{+1800}$ km/s) which corresponds to a radius of 20${}_{-10}^{+50}$ light days, for the virial production of $1.220\times 10^7$ 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$\hat{I}\pm$ 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)

33 pages, 9 figures, 4 tables, accepted for publication in the Astrophysical Journal