APM 08279+5255: An ultraluminous broad absorption line 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 1 ", with the IRAS Faint Source Catalog source F08279+5255. A chance alignment of the quasar and the IRAS source is extremely unlikely, and we argue that the optical and far-infrared flux are different manifestations of the same object. With an R-band magnitude of 15.2 and an IRAS 60 mu m flux of 0.51 Jy, APM 08279 + 5255 is (apparently) easily the most intrinsically luminous object known, with L-Bol similar to 5 x 10(15) L-.. Optical CCD photometry of the system, taken in good seeing, shows evidence that the system is slightly elongated. Although these data are consistent with the superposition of the quasar on a vastly luminous galaxy, we argue that a more likely scenario is that the optical image implies the presence: of two unresolved point sources. Such a configuration suggests that gravitational lensing may play a significant role in amplifying the intrinsic properties of the system. Point-spread function fitting of two discrete sources gives a separation of similar to 0." 4 and an intensity ratio similar to 1.1. The optical spectrum of the quasar clearly reveals the presence of three potential lensing galaxies, Mg II absorption systems at z = 1.18 and z = 1.81, and a damped Ly alpha absorption system at z = 3.07. Additional, as yet unseen, lensing galaxies may also be present. We estimate the total amplification of the optical component to be approximate to 40 but, owing 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 greater than or similar to 10(14) L-., indicating that it belongs to a small but significant population of high-redshift, hyperluminous objects with copious infrared emission.