The spectrum of BAL QSO Q1303+308: intrinsic variability and line-locking stability

A recently taken WHT optical spectrum of the BAL QSO Q1303+308 shows interesting changes in the spectral characteristics of the absorption systems. In particular, the equivalent widths of many of the broad absorption lines have markedly increased with respect to observations taken 15 yr earlier by Foltz et al., with the more blueshifted components showing the greatest increase by a factor of approximate to5 in optical depth. Perhaps the most remarkable feature of the data is that, in the rest frame of the quasar, the line-of-sight outflow velocity of the clouds has only increased by approximate to 55 km s(-1) in a rest-frame time interval of 5-6 yr. Various examples of doublet line locking are still visible in the system, and we present a provisional model analysis of the spectrum showing that theories based on small cloud velocity interactions via line locking can successfully explain the general appearance of the spectrum and its variability. Finally we highlight the need for further ongoing monitoring of this fascinating object.