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SUZAKU MONITORING OF THE IRON K EMISSION LINE IN THE TYPE 1 ACTIVE GALACTIC NUCLEUS NGC 5548

We present seven sequential weekly observations of NGC 5548 conducted in 2007 with the Suzaku X-ray Imaging Spectrometer (XIS) in the 0.2–12 keV band and Hard X-ray Detector (HXD) in the 10–600 keV band. The iron Kα line is well detected in all seven observations and Kβ 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 Kα and Kβ line at 6.396+0.009−0.007 keV and 7.08+0.05−0.05 keV, respectively. The width of line obtained from the co-added spectra is 38+16−18 eV (FWHM = 4200+1800−2000 km s−1) which corresponds to a radius of 20+50−10 light days, for the virial production of 1.220 × 107 M☉ in NGC 5548. To quantitatively investigate the origin of the narrow Fe line by the reverberation mapping method, we compare the observed light curves of Fe Kα 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° which is better than a fit to a constant flux of the Fe K line at the 92.7% level (F-test). However, the results with other geometries are also acceptable (P>50%). 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. This radius is larger than the Hβ emitting parts of the broad-line region at 6–10 light days (obtained by the simultaneous optical observation), and smaller than the inner radius of the hot dust in NGC 5548 (at about 50 light days).