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 quas ar 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 J y, 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 slig htly elongated. Although these data are consistent with the superposition o f the quasar on a vastly luminous galaxy, we argue that a more likely scena rio 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. P oint-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 spectru m of the quasar clearly reveals the presence of three potential lensing gal axies, Mg II absorption systems at z = 1.18 and z = 1.81, and a damped Ly a lpha absorption system at z = 3.07. Additional, as yet unseen, lensing gala xies may also be present. We estimate the total amplification of the optica l component to be approximate to 40 but, owing to the larger scale of the e mitting region, would expect the infrared amplification to be significantly less. Even making the conservative assumption that all wavelengths are amp lified by a factor 40, APM 08279 + 5255 still possesses a phenomenal lumino sity of greater than or similar to 10(14) L-., indicating that it belongs t o a small but significant population of high-redshift, hyperluminous object s with copious infrared emission.