Fe XXV and Fe XXVI diagnostics of the black hole and accretion disk in active galaxies:: Chandra time-resolved grating spectroscopy of NGC 7314

We report the detection of Fe XXV and Fe XXVI Kalpha emission lines from a Chandra High Energy Transmission Grating Spectrometer observation of the narrow-line Seyfert 1 galaxy NGC 7314, made simultaneously with the Rossi X-Ray Timing Explorer. The lines are redshifted (cz similar to 1500 km s(-1)) relative to the systemic velocity and unresolved by the gratings. We argue that the lines originate in a near face-on (<7°) disk having a radial line emissivity flatter than r(-2). Line emission from ionization states of Fe in the range ∼Fe I up to Fe XXVI is observed. The ionization balance of Fe responds to continuum variations on timescales of less than 12.5 ks, supporting an origin of the lines close to the X-ray source. We present additional, detailed diagnostics from this rich data set. These results identify NGC 7314 as a key source to study in the future if we are to pursue reverberation mapping of spacetime near black hole event horizons. This is because it is first necessary to understand the ionization structure of accretion disks and the relation between the X-ray continuum and Fe Kα line emission. However, we also describe how our results are suggestive of a means of measuring black hole spin without a knowledge of the relation between the continuum and line emission. Finally, these data emphasize that one can study strong gravity with narrow ( as opposed to very broad) disk lines. In fact, narrow lines offer higher precision, given sufficient energy resolution.