What type of black-hole merger produces a damped gravitational-wave signal called ringdown after the collision?

A binary black-hole merger produces a damped gravitational-wave signal called ringdown after the collision.

Two black holes orbiting one another lose orbital energy through gravitational waves. Their orbit shrinks until they merge into one distorted black hole. The newborn black hole then settles into a stable rotating state by emitting a final, fading part of the signal known as ringdown.

The ringdown frequencies depend on the remnant’s mass and spin. Measuring them tests whether the remnant behaves as predicted by general relativity. Binary black-hole mergers are especially valuable because they produce strong gravitational waves without requiring light or other electromagnetic radiation. The first direct detection of such a merger was GW150914, announced by LIGO in 2016, but this question concerns the general merger class rather than that individual event.