Evidence for Gravitational Infall of Matter onto the Supermassive Black Hole in the Quasar PG 1211+143?

We report the detection of redshifted iron K-alpha absorption lines in theChandra LETG spectrum of the narrow-line quasar, PG 1211+143. The absorptionlines are observed at 4.22 keV and 4.93 keV in the quasar spectrum,corresponding to 4.56 keV and 5.33 keV in the rest frame of PG 1211+143. FromMonte Carlo simulations, the chance probability of both lines being falsedetections is low at 1.36e-04. Highly redshifted ionized iron K-alpha (Fe XXVor Fe XXVI) is the most plausible identification for the lines at theirobserved energies. If identified with H-like iron K-alpha at 6.97 keV, then therelativistic velocity shifts required are 0.40c and 0.26c. The extremevelocities can be explained by pure gravitational redshift if the matter existsin a stable orbit within 6 gravitational radii of the black hole. This wouldrequire a Kerr metric for the black hole. Alternatively the absorption may bethe result of matter infalling directly onto the black hole, with a maximumobserved velocity of 0.38c at 6Rg in the Schwarzschild metric. This matter mayoriginate in a failed outflow or jet, which does not escape the gravitationalpotential of the black hole.Comment: 14 pages, including 3 figures. Acccepted for publication in Astrophysical Journal Letter