When two black holes merge, a new black hole is born. At first, it quivers like a struck bell, and this quivering spreads through space as waves. Detectors on Earth catch these waves — from them, we can learn the mass and spin of the black hole.

But the signal is weak and full of noise. If you take different chunks of the recording, they might give slightly different answers. Which one is correct? The authors came up with a way to select only those chunks that don't depend on minor processing settings and don't drown in noise. It's like picking out only the moments from a concert recording where you can hear the pure sound of the bell, not the coughing in the audience.

They tested the method on the event GW250114—a merger of two black holes caught by the LIGO detectors. It turned out that the reliable chunks lie not immediately after the merger, but a bit later. Too early — too sensitive to exactly how the signal was cut. Too late — already drowned in noise. In the middle — a golden zone where all chunks give the same picture: one black hole with a specific mass and spin.

This approach is important for future observations. When detectors become more sensitive, they will catch more loud events. The method will allow us to quickly and confidently say what kind of black hole was born in each of them.