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Abstract: We carefully reconsider the problem of classifying broad absorption linequasars BALQSOs and derive a new, unbiased estimate of the intrinsic BALQSOfraction from the SDSS DR3 QSO catalogue. We first show that the distributionof objects selected by the so-called ``absorption index- AI is clearlybimodal in logAI, with only one mode corresponding to definite BALQSOs. Thesurprisingly high BALQSO fractions that have recently been inferred fromAI-based samples are therefore likely to be overestimated. We then present twonew approaches to the classification problem that are designed to be morerobust than the AI, but also more complete than the traditional ``balnicityindex- BI. Both approaches yield observed BALQSO fractions around 13.5 percent, while a conservative third approach suggests an upper limit of 18.3 percent. Finally, we discuss the selection biases that affect our observed BALQSOfraction. After correcting for these biases, we arrive at our final estimate ofthe intrinsic BALQSO fraction. This is f BALQSO = 0.17 +- 0.01 stat +- 0.03sys, with an upper limit of f BALQSO = 0.23. We conclude by pointing out thatthe bimodality of the logAI distribution may be evidence that the BAL-formingregion has clearly delineated physical boundaries.

Autor: Christian Knigge 1, Simone Scaringi 1, Michael R. Goad 2, Christopher E. Cottis 2 1 University of Southampton, 2 University of Le



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