Data from "A UV Counterpart to an Ultra-Fast X-ray Outflow in PG1211+143"

We observed the quasar PG1211+143 using the Cosmic Origins Spectrograph on the
Hubble Space Telescope in April 2015 as part of a joint campaign with the
{\it Chandra} X-ray Observatory and the Jansky Very Large Array.
Our ultraviolet spectra cover the wavelength range 912--2100 \AA.
We find a broad absorption feature ($\sim 1080~\kmps$) at an observed wavelength
of 1240 \AA. Interpreting this as \ion{H}{1} Ly$\alpha$,
in the rest frame of PG1211+143 ($z=0.0809$), this corresponds to an
outflow velocity of $-16\,980~\rm km~s^{-1}$
(outflow redshift $z_{\rm out} \sim -0.0551$),
matching the moderate ionization X-ray absorption system detected in our
{\it Chandra} observation and reported previously by Pounds et al. (2016).
With a minimum H\,{\sc i} column density of log $\rm N_{HI} > 14.5$,
and no absorption in other UV resonance lines, this Ly$\alpha$ absorber is
consistent with arising in the same ultra-fast outflow as the X-ray
absorbing gas.
The Ly$\alpha$ feature is weak or absent in
archival ultraviolet spectra of PG1211+143, strongly suggesting that this
absorption is transient, and intrinsic to PG1211+143.
Such a simultaneous detection in two independent wavebands
for the first time
gives strong confirmation of the reality of an ultra-fast outflow
in an active galactic nucleus.