2008MNRAS.391L..39H

We report the first discovery of the emergence of a high-velocity broad-line outflow in a luminous quasar, J105400.40+034801.2, at redshift z ∼ 2.1. The outflow is evident in ultraviolet Civ and Siiv absorption lines with velocity shifts v ∼ 26300 km/s and deblended widths FWHM ∼4000 km/s. These features are marginally strong and broad enough to be considered broad absorption lines (BALs), but their large velocities exclude them from the standard BAL definition. The outflow lines appeared between two observations in the years 2002.18 and 2006.96. A third observation in 2008.48 showed the lines becoming ∼40 per cent weaker and 10 to 15 per cent narrower. There is no evidence for acceleration or for any outflow gas at velocities ≲23000 km/s. The lines appear to be optically thick, with the absorber covering just 20 per cent of the quasar continuum source. This indicates a characteristic absorber size of ∼4x10¹⁵ cm, but with a BAL-like total column density logN_H(cm^–2) ≳ 21.2 and average space density n_H≳ 2x10⁵/cm3. We attribute the emergence of the outflow lines to a substantial flow structure moving across our line of sight, possibly near the ragged edge of the main BAL flow or possibly related to the onset of a BAL evolutionary phase.