Iron K Features in the Quasar E1821+643: Evidence for Gravitationally Redshifted Absorption?

We report a Chandra high-energy grating detection of a narrow, redshiftedabsorption line superimposed on the red wing of a broad Fe K line in the$z=0.297$ quasar E1821+643. The absorption line is detected at a confidencelevel, estimated by two different methods, in the range ~2-3 sigma. Althoughthe detection significance is not high enough to exclude a non-astrophysicalorigin, accounting for the absorption feature when modeling the X-ray spectrumimplies that the Fe-K emission line is broad, and consistent with an origin ina relativistic accretion disk. Ignoring the apparent absorption feature leadsto the conclusion that the Fe-K emission line is narrower, and also affects theinferred peak energy of the line (and hence the inferred ionization state ofFe). If the absorption line (at ~6.2 keV in the quasar frame) is real, we arguethat it could be due to gravitationally redshifted Fe XXV or Fe XXVI resonanceabsorption within ~10-20 gravitational radii of the putative central blackhole. The absorption line is not detected in earlier low-energy gratingobservations, but is not unequivocally ruled out by these data. The Chandrahigh-energy grating Fe K emission line is consistent with an originpredominantly in Fe I-XVII or so. In a previous LEG observation the line wasdouble-peaked, at ~6.4 keV and ~6.9 keV (H-like Fe). Such a wide range inionization state of Fe is not ruled out by the HEG data and an earlier ASCAobservation, and is suggestive of a complex structure for the line-emitter.Comment: To be published in the Astrophysical Journal, 20 April 2005. 18 pages, ten figures, five of them in color. Four tables. Abstract is abridged.Corrected typos in author affiliation superscript