Kun Hu

Light reflecting off the scorching disk around a black hole can reveal more about it than we ever thought. When the disk emits heat, some of that light bounces back onto the disk and scatters, acquiring an unusual energy distribution, as if a new component, resembling a power law, is added to the ordinary glow.

At the very heart of the Milky Way, something is continuously glowing in gamma rays. This light is special: its energy is exactly 511 thousand electronvolts. Such energy is produced when an electron meets its antimatter counterpart, a positron, and they annihilate, leaving behind a flash of light.