. . .In this paper, we present the properties of the recently discovered quasar SMSS J052915.80−435152.0, hereafter called J0529−4351, which is a 16th magnitude object at redshift z=3.962 (Fig. 1), and we reveal it to be the most luminous quasar currently known in the Universe

quasar, an astronomical object of very high luminosity found in the centres of some galaxies and powered by gas spiraling at high velocity into an extremely large black hole.

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The black hole in this quasar accretes around one solar mass per day onto an existing mass of ∼17 billion solar masses. In this process, the accretion disk alone releases a radiative energy of 2×1041W. If the quasar is not strongly gravitationally lensed, then its broad-line region is expected to have the largest physical and angular diameter occurring in the Universe and this will allow the Very Large Telescope Interferometer to image its rotation and measure its black-hole mass directly. This will be an important test for broad-line region size–luminosity relationships, whose extrapolation has underpinned common black-hole mass estimates at high redshift.

Two other quasars with a redshift and apparent brightness similar to J0529−4351 are known to be strongly magnified by lensing: the double-image APM 08279+5255 at z=3.91, with a separation of 0.5arcsec (ref. 26), and the quadruply imaged B 1422+231 at z =3.62, with separations up to 1.5arcsec (ref. 27). Estimated magnification factors for these two objects range from 40 to 100, which implies that these quasars are not intrinsically extreme, but are members of the bulk population26,28.J0529−4351, in contrast, shows no sign of strong lensing;