APM 08279+5255

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APM 08279+5255 is a quasar that is notable for being a particularly good example of a gravitational lens. When originally discovered, the combination of its high redshift and brightness (particularly in the infrared) made it the most luminous object known. High-resolution observations with the NICMOS camera on-board the Hubble Space Telescope revealed that the source was actually comprised of three discrete components, with a maximum separation of 0.4 arcsec. Subsequent observations with the STIS spectrograph (also aboard HST) showed that each component has the same spectral energy distribution and therefore that each is probably an image of a single quasar. The lensing hypothesis greatly reduces the intrinsic luminosity of the lensed quasar as the observed brightness is enhanced by the magnification effect of the lens. Gravitational lens systems with odd numbers of images are extremely rare, most containing two or four.