The Cores of the Fe K Lines in Seyfert I Galaxies Observed by the Chandra High Energy Grating

Abstract

We report on the results of 18 observations of the core, or peak, of the Fe K emission line at $\sim 6.4$ keV in 15 Seyfert I galaxies using the {\it Chandra} High Energy Grating (HEG). These data afford the highest precision measurements of the peak energy of the Fe K line, and the highest spectral resolution measurements of the width of the core of the line to date. We were able to measure the peak energy in 17 data sets, and, excluding a very deep observation of NGC 3783, we obtained a weighted mean of $6.404 \pm 0.005$ keV. In all 15 sources the two-parameter, 99% confidence errors on the line peak energy do not exclude fluorescent $K\alpha$ line emission from Fe {\sc i}, although two sources (Mkn 509 and 3C 120) stand out as very likely being dominated by $K\alpha$ emission from Fe {\sc xvii} or so. We were able to measure the line core width in 14 data sets and obtained a weighted mean of 2380 +/- 760 km/s FWHM (excluding the NGC 3783 deep exposure), a little larger than the instrument resolution. However, there is evidence of underlying broad line emission in at least 4 sources. In fact, the width of the peak varies widely from source to source and it may in general have a contribution from the outer parts of an accretion disk {\it and} more distant matter. For the disk contribution to also peak at 6.4 keV requires greater line emissivity at hundreds of gravitational radii than has been deduced from previous studies of the Fe K line.