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T. Yaqoob, I. M. George, T. J. Turner (NASA/GSFC, UMBC), K. Nandra (NASA/GSFC, USRA), P. J. Serlemitsos, R. F. Mushotzky (NASA/GSFC)
The Fe K emission line in Seyfert 1 galaxies is generally believed to originate in a relativistic disk around a black hole, extending down to a few gravitational radii. There has been some evidence that in some sources there is a separate component to the emission line from a region much further out, such as the BLR and/or a parsec-scale torus structure. We present the first unambiguous detection of such a narrow Fe K emission-line from a Chandra observation of NGC 5548. In the galaxy frame, we measure a center energy of 6.40 ±0.03 keV and an equivalent width of 133 (+62, -54) eV which is a substantial fraction (at least 50%) of the total Fe K line emission measured from previous ASCA observations. The velocity width of the narrow line is 4525 (+3525, -2645) km/s FWHM. This, and all the other measurements of the line parameters are consistent with the bulk of the narrow-line emission originating in the BLR, about a light month from the center. Some contribution to the line intensity from the putative torus is not ruled out and the 90% confidence errors on the velocity width allow the emission region to extend down to ~1 light week from the central black hole. We present evidence that the narrow-line intensity varied in the past. Long-term monitoring and reverberation mapping with future large-collecting area missions will provide much more accurate and reliable determinations of the black-hole mass than the same techniques applied to the optical/UV lines or the broad, relativistic component of the Fe K line.
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