ASASSN-15lh
"The exploding star was first observed back in June last year but is still radiating vast amounts of energy. At its peak, the event was 200 times more powerful than a typical supernova, making it shine with 570 billion times the brightness of our Sun. Researchers think the explosion and ongoing activity have been boosted by a very dense, highly magnetised, remnant object called a magnetar. This object, created as the supernova got going, is probably no bigger than a major city, such as London, and is likely spinning at a fantastic rate - perhaps a thousand times a second".