Unusual high-redshift radio broad absorption-line quasar 1624+3758

We present observations of the most radio-luminous broad absorption-line (BAL) quasar known, 1624+3758, at redshift z = 3.377. The quasar has several unusual properties. (1) The Fe ii UV191 1787-angstrom emission line is very prominent. (2) The BAL trough (BALnicity index 2990 km s(-1)) is detached by 21 000 km s(-1) and extends to velocity v = -29 000 km s(-1). There are additional intrinsic absorbers at -1900 and -2800 km s(-1). (3) The radio rotation measure of the quasar, 18 350 rad m(-2), is the second highest known. The radio luminosity is P-1.4 GHz= 4.3 x 10(27) W Hz(-1) (H-0= 50 km s(-1) Mpc(-1), q(0)= 0.5) and the radio loudness is R* = 260. The radio source is compact and the radio spectrum is GHz-peaked, consistent with it being relatively young. The width of the C iv emission line, in conjunction with the total optical luminosity, implies a black hole mass M(BH)similar to 10(9) M-circle dot, L/L(Eddington)approximate to 2. The high Eddington ratio and the radio-loudness place this quasar in one corner of Boroson's two-component scheme for the classification of active galactic nuclei, implying a very high accretion rate, and this may account for some of the unusual observed properties. The v = -1900 km s(-1) absorber is a possible Lyman-limit system, with N(H i) = 4 x 10(18) cm(-2), and a covering factor of 0.7. A complex mini-BAL absorber at v = -2200 to -3400 km s(-1) is detected in each of C iv, N v and O vi. The blue and red components of the C iv doublet happen to be unblended, allowing both the covering factor and optical depth to be determined as a function of velocity. Variation of the covering factor with velocity dominates the form of the mini-BAL, with the absorption being saturated (e(-tau)approximate to 0) over most of the velocity range. The velocity dependence of the covering factor and the large velocity width imply that the mini-BAL is intrinsic to the quasar. There is some evidence of line-locking between velocity components in the C iv mini-BAL, suggesting that radiation pressure plays a role in accelerating the outflow.