Discovery of an Ultraviolet Counterpart to an Ultrafast X-Ray Outflow in the Quasar PG 1211+143

We observed the quasar PG 1211+143 using the Cosmic Origins Spectrograph on the Hubble Space Telescope in 2015 April as part of a joint campaign with the Chandra X-ray Observatory and the Jansky Very Large Array. Our ultraviolet spectra cover the wavelength range 912-2100 angstrom. We find a broad absorption feature (similar to 1080 km s(-1)) at an observed wavelength of 1240 angstrom. Interpreting this as H I Ly alpha, in the rest frame of PG 1211+143 (z = 0.0809), this corresponds to an outflow velocity of -16,980 km s(-1) (outflow redshift z(out) similar to -0.0551), matching the moderate ionization X-ray absorption system detected in our Chandra observation and reported previously by Pounds et al. With a minimum H I column density of log N-H (I) > 14.5, and no absorption in other UV resonance lines, this Ly alpha absorber is consistent with arising in the same ultrafast outflow as the X-ray absorbing gas. The Lya feature is weak or absent in archival ultraviolet spectra of PG 1211+143, strongly suggesting that this absorption is transient, and intrinsic to PG 1211+143. Such a simultaneous detection in two independent wavebands for the first time gives strong confirmation of the reality of an ultrafast outflow in an active galactic nucleus.