X-rays from the vicinity of a black hole don't arrive all at once. Some of them first fall onto a disk of scorching gas, get reflected, and only then travel to the telescope. This delay is like an echo in the mountains: first you hear the direct sound, then the response from the cliff.
In the galaxy IC 4329A, astronomers caught such an echo from the so-called Compton hump — a burst of radiation at energies of 20–30 kiloelectronvolts. The delay was about 1825 seconds at low frequencies and decreased to 195 seconds at higher ones. This suggests that the X-rays are reflecting off the inner regions of the disk, which lie very close to the black hole.
From the shape of the echo, they could recover the black hole's mass: it turned out to be roughly 137 million times the mass of the Sun. They also measured the height of the corona, a cloud of hot particles above the disk, at about 2.45 times the black hole's radius. These numbers matched previous estimates obtained by other methods.
X-ray echoes let us peer into the very central regions, where ordinary telescopes can't discern details. With each new measurement, the picture becomes more reliable: already five galaxies have shown similar behavior. This means the method works and will allow us to weigh black holes without complex assumptions in the future.
A delay of 1825 seconds is about half an hour. In that time, light could circle the Earth at the equator more than 13 times.
In the movie Interstellar, the signal delay near the black hole Gargantua plays a key role, and although it's a different effect, the idea of an 'echo' from a black hole is very close.