Context : Broad absorption lines ( BALs ) in the spectra of quasi-stellar objects ( QSOs ) originate from outflowing winds along our line of sight ; winds are thought to originate from the inner regions of the QSO accretion disk , close to the central supermassive black hole ( SMBH ) .
These winds likely play a role in galaxy evolution and are responsible for aiding the accretion mechanism onto the SMBH .
Several works have shown that BAL equivalent widths can change on typical timescales from months to years ; such variability is generally attributed to changes in the covering factor ( due to rotation and/or changes in the wind structure ) and/or in the ionization level of the gas .

Aims : We investigate BAL variability , focusing on BAL disappearance .

Methods : We analyze multi-epoch spectra of more than 1500 QSOs –the largest sample ever used for such a study– observed by different programs from the Sloan Digital Sky Survey-I/II/III ( SDSS-I/II/III ) , and search for disappearing C IV BALs .
The spectra cover a rest-frame time baseline ranging from 0.28 to 4.9 yr ; the source redshifts range from 1.68 to 4.27 .

Results : We detect 73 disappearing BALs in the spectra of 67 unique sources .
This corresponds to 3.9 % of BALs disappearing within 4.9 yr ( rest frame ) , and 5.1 % of our BAL QSOs exhibit at least one disappearing BAL within 4.9 yr ( rest frame ) .
We estimate the average lifetime of a BAL along our line of sight ( \approx 80 - 100 yr ) , which appears consistent with the accretion disk orbital time at distances where winds are thought to originate .
We inspect various properties of the disappearing BAL sample and compare them to the corresponding properties of our main sample .
We also investigate the existence of a correlation in the variability of multiple troughs in the same spectrum , and find it persistent at large velocity offsets between BAL pairs , suggesting that a mechanism extending on a global scale is necessary to explain the phenomenon .
We select a more reliable sample of disappearing BALs on the basis of the criteria adopted by , where a subset of our current sample was analyzed , and compare the findings from the two works , obtaining generally consistent results .

Conclusions :