On the covering fraction variability in an EUV mini-BAL outflow from PG 1206+459

We report on the first detection of extreme-ultraviolet (EUV) absorption variability in theNe VIII λλ770, 780 mini-broad absorption line (mini-BAL) in the spectrum of the quasar (QSO) PG 1206+459. The observed equivalent width (EW) of the Ne VIII doublet shows a ~4σ variation over a time-scale of 2.8 months in the QSO's rest frame. Both members of the Ne VIII doublet exhibit non-black saturation, indicating partial coverage of the continuum source. An increase in the Ne VIII covering fraction from f_c = 0.59 ± 0.05 to 0.72 ± 0.03 is observed over the same period. The Ne VIII 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 as the cause of the observed variability. Using a simple model of a transiting cloud we estimate a transverse speed of ~1800 km s^-1. 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 broad-line region.We further estimate a density of ~5 × 10⁶ cm^-3 and a kinetic luminosity of ~10⁴³-10⁴⁴ erg s^-1. Such large kinetic powers suggest that outflows detected via EUV lines are potentially major contributors to active galactic nuclei feedback.