At the center of the galaxy Ton S180 lurks a black hole that occasionally flares in X-rays. These flares are like a flashlight beam shining onto a swirling disk of superheated gas. Part of the light heads straight toward us, while another part first hits the disk, bounces off, and only then reaches the telescope. The result is an echo: first we see the direct flash, and a short while later, its reflection.
Using the XMM-Newton space observatory, astronomers noticed that iron emission, think of it as a spectral fingerprint, arrives with a delay of about 117 seconds after the main flare. When flares become more frequent, the delay shrinks to 23 seconds. From these delays, we can calculate how fast the black hole is spinning. It turns out it's not spinning very fast — roughly 30–40% of the maximum possible rate. The black hole's mass is about 29 million solar masses.
This method is like listening to an echo in the mountains, but instead of sound, we use X-rays. It allows us to measure black hole spin independently of other techniques. If we apply this to many galaxies, we can piece together how black holes have grown and evolved over billions of years.
A 117-second delay is like shouting in the mountains and hearing the echo two minutes later, but here the 'echo' is X-rays bouncing off gas falling into the black hole.