Physical diagnostics from a narrow FeK alpha emission line detected by Chandra in the Seyfert 1 galaxy NGC 5548

We report the detection of a narrow Fe K alpha emission line in the Seyfert 1 galaxy NGC 5548 with the Chandra High-Energy Transmission Grating. In th e galaxy frame we measure a center energy of 6.402(-0.025)(+0.027) keV, a F WHM of 4515(-2645)(+3525) km s(-1), an intensity of 3.6(-1.5)(+1.7) x 10(-5 ) photons cm(-2) s(-1), with an equivalent width of 133(-54)(+62) eV (error s are 90% confidence for one parameter). The line is only marginal resolved at the 90% confidence level. The line energy is consistent with an origin in cold, neutral matter, but ionization states up to similar to Fe XVIII ar e not ruled out. We cannot constrain the detailed dynamics but, assuming Ke plerian motion, the velocity width is consistent with the line being produc ed in the outer optical/UV broad-line region (BLR) at about a light month f rom the central X-ray source. We cannot rule out some contribution to the n arrow Fe K alpha line from a putative, parsec-scale, obscuring torus that i s postulated to be a key component of active galactic nuclei (AGNs) unifica tion models. The continuum intensity during the Chandra observation was a f actor similar to2 less than typical historical levels. If the X-ray continu um was at least a factor of 2 higher in the recent past before the Chandra observation and the narrow Fe K alpha intensity had not yet responded to su ch 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 c ontinuum illumination of the BLR and/or additional line emission from a tor us 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 st rong evidence that the narrow-line intensity varied and was unusually low d uring one of the ASCA campaigns. In general, inclusion of the narrow line t o model the overall broad Fe K alpha line profile in terms of a rotating di sk plus black-hole model can have a non-negligible effect on the disk line intensity and variability properties. Variability of the broad disk line in NGC 5548 is difficult to reconcile with the expectations of the simple dis k model, even when the narrow-line component is accounted for. It will be c ritical to ascertain the importance of a similar nondisk Fe K alpha line in other Seyfert 1 galaxies. Future monitoring of the narrow Fe K alpha compo nent with large collecting area and high spectral resolution will enable re verberation mapping the BLR region, complementary to similar studies using the optical/UV lines, and therefore provide independent constraints on the black-hole mass.