0 PHYSICAL DIAGNOSTICS FROM A NARROW Fe K α EMISSION LINE DETECTED BY CHANDRA IN THE SEYFERT 1 GALAXY NGC 5548

We report the detection of a narrow Fe Kα emission line in the Seyfert 1 galaxy NGC 5548 with the Chandra High-Energy Transmission Grating. In the galaxy frame we measure a center energy of 6.402 −0.025 keV, a FWHM of 4515 −2645 km/s, an intensity of 3.6 +1.7 −1.5 × 10 photons cm s, with an equivalent width of 133 −54 eV (errors are 90% confidence for one parameter). The line is only marginally resolved at the 90% confidence level. The line energy is consistent with an origin in cold, neutral matter, but ionization states up to ∼ Fe XVIII are not ruled out. We cannot constrain the detailed dynamics but assuming Keplerian motion, the velocity width is consistent with the line being produced in the outer optical/UV broad-line region (BLR) at about a light-month from the central X-ray source. We cannot rule out some contribution to the narrow Fe Kα line from a putative, parsec-scale obscuring torus which is postulated to be a key component of AGN unification models. The continuum intensity during the Chandra observation was a factor ∼ 2 less than typical historical levels. If the X-ray continuum was at least a factor of 2 higher in the recent past before the Chandra observation and the narrow Fe Kα intensity had not yet responded to such a change then the predicted line intensity and equivalent width for an origin in the BLR is within the 90% measurement errors. Anisotropic X-ray continuum illumination of the BLR and/or additional line emission from a torus structure would improve the agreement with observation. Two out of three archival ASCA data sets are consistent with the narrow line being present with the same intensity as in the Chandra observation. However, there is strong evidence that the narrow-line intensity varied and was unusually low during one of the ASCA campaigns. In general, inclusion of the narrow line to model the overall broad Fe Kα line profile