The WISSH quasars project VI. Fraction and properties of BAL quasars in
  the hyper-luminosity regime
  The WISSH quasars project aims at studying the nuclear and host galaxy
properties of the most luminous quasars ($L_{bol}>10^{47}$ erg/s, $1.8<z<4.6$).
Nuclear winds are manifested as UV broad ($\geq$ 2,000 km/s) absorption lines
(BAL) in $\sim$ 15\% of quasars. We aim at studying the incidence and
properties of such winds in the WISSH sample, to investigate possible
differences with respect to lower luminosity AGN regimes. We collected optical
spectra from the SDSS data release 12, and identified those showing absorption
troughs in the region between \siiv and \civ emission lines. We find a higher
observed fraction of \civ BAL quasars in the WISSH sample (24\%) with respect
to previous catalogues (10-15\%). These WISSH BAL quasars are also
characterized by a larger average BI ($\sim$4,000 km/s) and maximum velocity
($\sim$17,000 km/s). Moreover, for two objects we discovered BAL features
bluewards of the \siiv peak, which can be associated to \civ absorption with
velocity of 0.15c. Finally, we updated previous studies on the dependence of
maximum outflow velocity upon bolometric luminosity, showing that BAL winds
have intermediate properties compared to molecular/ionized winds and ultra fast
outflows (UFOs). Finally, the radio properties of the WISSH BAL quasars as a
whole are in line with those of samples at lower luminosities from previous
studies. Our results suggest that the higher $L_{\rm bol}$ of the WISSH quasars
likely favours the acceleration of BAL outflows and that their most likely
driving mechanism is radiation pressure. Furthermore we estimate that the
kinetic power associated to these winds in hyperluminous quasars is sufficient,
for the highest column density and fastest winds, to provide efficient feedback
onto the host galaxy.
