Accurate spectroscopic redshift of the multiply lensed quasar PSOJ0147
  from the Pan-STARRS survey
  Context: The gravitational lensing time delay method provides a one-step
determination of the Hubble constant (H0) with an uncertainty level on par with
the cosmic distance ladder method. However, to further investigate the nature
of the dark energy, a H0 estimate down to 1% level is greatly needed. This
requires dozens of strongly lensed quasars that are yet to be delivered by
ongoing and forthcoming all-sky surveys.
  Aims: In this work we aim to determine the spectroscopic redshift of
PSOJ0147, the first strongly lensed quasar candidate found in the Pan-STARRS
survey. The main goal of our work is to derive an accurate redshift estimate of
the background quasar for cosmography.
  Methods: To obtain timely spectroscopically follow-up, we took advantage of
the fast-track service programme that is carried out by the Nordic Optical
Telescope. Using a grism covering 3200 - 9600 A, we identified prominent
emission line features, such as Ly-alpha, N V, O I, C II, Si IV, C IV, and [C
III] in the spectra of the background quasar of the PSOJ0147 lens system. This
enables us to determine accurately the redshift of the background quasar.
  Results: The spectrum of the background quasar exhibits prominent absorption
features bluewards of the strong emission lines, such as Ly-alpha, N V, and C
IV. These blue absorption lines indicate that the background source is a broad
absorption line (BAL) quasar. Unfortunately, the BAL features hamper an
accurate determination of redshift using the above-mentioned strong emission
lines. Nevertheless, we are able to determine a redshift of 2.341+/-0.001 from
three of the four lensed quasar images with the clean forbidden line [C III].
In addition, we also derive a maximum outflow velocity of ~ 9800 km/s with the
broad absorption features bluewards of the C IV emission line. This value of
maximum outflow velocity is in good agreement with other BAL quasars.
