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

We report a Chandra High-Energy Grating (HEG) detection of a narrow, redshifted absorption line superposed 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 confidence level, estimated by two different methods, in the range similar to 2-3 sigma . Although the detection significance is not high enough to exclude a nonastrophysical origin, accounting for the absorption feature when modeling the X-ray spectrum implies that the Fe K emission line is broad and consistent with an origin in a relativistic accretion disk. Ignoring the apparent absorption feature leads to the conclusion that the Fe K emission line is narrower and also affects the inferred peak energy of the line (and hence the inferred ionization state of Fe). If the absorption line (at similar to 6.2 keV in the quasar frame) is real, then we argue that it could be due to gravitationally redshifted Fe XXV or Fe XXVI resonance absorption within similar to 10-20 gravitational radii of the putative central black hole. The absorption line is not detected in earlier ASCA and Chandra Low-Energy Grating (LEG) observations, but the absorption line is not unequivocally ruled out by these data. The Chandra HEG Fe K emission line is consistent with an origin predominantly in Fe I-XVII or so. In an ASCA observation 8 years earlier, the Fe K line peaked at similar to 6.6 keV, closer to the energies of He-like Fe triplet lines. Furthermore, in a Chandra LEG grating observation, the Fe K line profile was double peaked, one peak corresponding to Fe I-XVII or so and the other peak to Fe XXVI Ly alpha . Such a wide range in ionization states of Fe is not ruled out by the HEG and ASCA data either and is suggestive of a complex structure for the line emitter.