  We report on the first detection of extreme-ultraviolet (EUV) absorption
variability in the NeVIII 770,780 mini-broad absorption line (mini-BAL) in the
spectrum of the quasar (QSO) PG 1206+459. The observed equivalent width (EW) of
the NeVIII doublet show a ~4 sigma variation over a timescale of 2.8 months in
the QSO's rest-frame. Both members of the NeVIII doublet exhibit non-black
saturation, indicating partial coverage of the continuum source. An increase in
the NeVIII covering fraction from f_c = 0.59\pm0.05 to 0.72\pm0.03 is observed
over the same period. The NeVIII profiles are too highly saturated to be
susceptible to changes in the ionization state of the absorbing gas. In fact,
we do not observe any significant variation in the EW and/or column density
after correcting the spectra for partial coverage. We, thus, propose transverse
motions of the absorbing gas being the cause of the observed variability. Using
a simple model of a transiting cloud we estimate a transverse speed of ~1800
km/s. For Keplerian motion, this corresponds to a distance between the absorber
and the central engine of ~1.3 pc, which places the absorber just outside the
BLR region. We further estimate a density of ~5\times10^6 cm^(-3) and a kinetic
luminosity of ~10^43 - 10^44 erg/s. Such large kinetic powers suggest that
outflows detected via EUV lines are potentially major contributors to AGN
feedback.
