APM 08279+5255: an ultraluminous BAL quasar at a redshift z=3.87

We report on the discovery of a highly luminous, broad absorption line quasar at a redshift of $z=3.87$ which is positionally coincident, within one arcsecond, with the IRAS FSC source F08279+5255. A chance alignment of the quasar and the IRAS source is extremely unlikely and we argue that the optical and FIR flux are different manifestations of the same object. With an R-band magnitude of 15.2, and an IRAS 60$\mum$ flux of $0.51\jy$, APM 08279+5255 is (apparently) easily the most intrinsically luminous object known, with $ L_{Bol}\sim5\times10^{15}L_{\odot}}$. Imaging suggests that gravitational lensing may play a role in amplifying the intrinsic properties of the system. The optical spectrum of the quasar clearly reveals the presence of three potential lensing galaxies, \mg absorption systems at $z=1.18$ and $z=1.81$, and a \ly absorption system at $z=3.07$. We estimate the total amplification of the optical component to be $\approx40$, but, due to the larger scale of the emitting region, would expect the infrared amplification to be significantly less. Even making the conservative assumption that all wavelengths are amplified by a factor 40, APM 08279+5255 still possesses a phenomenal luminosity of $\simgt 10^{14L_{\odot}}$, indicating that it belongs to a small, but significant population of high--redshift, hyperluminous objects with copious infrared emission.