A mysterious and intense X-ray source near the black hole shines onto the surface layers of the accretion disk, causing iron atoms to radiate characteristic emission -- what astronomers call the iron K line -- at about 6,000 to 7,000 electron volts. The inner part of the disk is orbiting the black hole so fast that the effects of Einstein's relativity come into play -- most notably, how time slows down close to the black hole.