ON THE RADIO PROPERTIES OF BROAD-ABSORPTION-LINE QSOS

It is proposed that the observed absence of radio-loud broad-absorption-line QSOs can be naturally understood as an orientation effect, if broad absorption lines are only visible in QSOs in which the line of sight to the continuum source passes close to an accretion disc. Compact, flat-spectrum, radio-loud quasars are generally thought to contain a jet pointing in the direction of the observer. Since the jet will be perpendicular to the accretion disc, the line of sight to the active nucleus will never pass through the broad-absorption-line region (BALR) in such objects. Extended, steep-spectrum, radio-loud quasars are not as strongly beamed, but it is shown that the number of such objects expected in the sample of 68 BALQSOs with good radio observations is less than or similar to 1. It is not as straightforward to account for the absence of BALQSOs from a sample of approximately 60 high-z, steep-spectrum, radio-loud quasars, but it is argued that a moderate amount of beaming in the extended emission can introduce selection effects which explain this observation. The recently discovered high incidence of BALQSOs among radio-moderate QSOs fits into this scheme, if the radio-moderate population is interpreted as consisting of intrinsically radio-loud systems that appear weak because the jet is almost perpendicular to the line of sight. In this case the chance that the line of sight lies close to the accretion disc, and thus passes through the BALR, is significantly enhanced. An association of the BALR with an accretion disc is also attractive on theoretical grounds.