GW190412

GW 190412 was a gravitational wave signal observed by the LIGO and Virgo detectors on 12 April 2019. In April 2020, it was announced as the first time a collision of a pair of very differently sized black holes has been detected. As a result of this asymmetry, the signal included two measurable harmonics with frequencies approximately a factor 1.5 apart.
The collision took place 2.4 billion light-years away. The heavier of the black holes had a mass of, and the lighter one around. The difference in mass meant that the secondary harmonic in the signal was strong enough to be detected, allowing researchers to perform a test of general relativity and determine that the larger black hole was spinning.