The accretion of a solar mass per day by a 17-billion solar mass black hole

Around a million quasars have been catalogued in the Universe by probing deeper and using new methods for discovery. However, the hardest ones to find seem to be the rarest and brightest specimens. Here we study the properties of the most luminous of all quasars found so far. These have been overlooked until recently, which demonstrates that modern all-sky surveys have much to reveal. 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.