ASCAObservations of Seyfert 1 Galaxies. II. Relativistic Iron Kα Emission

We present evidence for widespread relativistic effects in the centralregions of active galactic nuclei. In a sample of 18 Seyfert 1 galaxiesobserved by \asca, 14 show an iron K$\alpha$ line which is is resolved, withmean width $\sigma_{\rm K\alpha}=0.43\pm 0.12$~keV for a gaussian profile (FullWidth at Half Maximum, FWHM$\sim 50,000$~km s$^{-1}$). However, many of theline profiles are asymmetric. A strong red wing is indicative of gravitationalredshifts close to a central black hole and accretion disk models provide anexcellent description of the data. The peak energy of the line is 6.4~keV,indicating that it arises by fluorescence in near-neutral material. Our fitsimply a low inclination for the disk in these Seyfert 1 galaxies, with a meanof 30\deg, consistent with orientation-dependent unification schemes.Differences in the line profiles from source-to-source imply slight variationsin geometry, which cannot be accounted for solely by inclination. In mostcases, we require that the line emission arises from a range of radii. Althougha small contribution to the emission from a region other than the disk is notruled out, it is not generally required and has little effect on ourconclusions regarding the disk line. Our data are fit equally well withrotating (Kerr) and non-rotating (Schwarzschild) black hole models. We find amean spectral index in the 3-10 keV range of $<\Gamma_{3-10}>=1.91\pm 0.07$after accounting for the effects of reflection. Such observations probe theinnermost regions of AGN, and arguably provide the best evidence yet obtainedfor the existence of super-massive black holes in the centers of activegalaxies.