a hushed thud in May 2019 and another 'chirp' in August
In May 2019 the LIGO and Virgo picked up a signal described as a dull thud lasting one-tenth of a second that turned out to be the energy from two large stellar black holes crashing into each other. One was 66 times the mass of our sun and the other a husky 85 times the mass of the sun. The result of the merger was what may be described as an intermediate black hole, 142 times the mass of the sun.[1]
Lost in the collision was an enormous amount of energy in the form of a gravitational wave. Because the detectors allow scientists to pick up the gravitational waves as audio signals, scientists actually heard the collision. The bigger of the two black holes involved could have been the result of an earlier merger.
Previously, astronomers only had observed black holes in two general sizes: "small" ones about the size of small cities called stellar black holes formed when a star collapses, and supermassive black holes that are millions, maybe billions, of times more massive than our sun and around which entire galaxies revolve. Something in between didn’t quite make sense – until now. After deciphering the signal and checking their work, scientists published the results on 2 September 2020 in Physical Review Letters and Astrophysical Journal Letters.
Then on 14 August 2019, astronomers detected another tell-tale 'chirp' as two massive objects collided about 900 million light years away, this time likely caused by a black hole 'gobbling up' a defunct neutron star. The signal was detected by both the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) in the US and the Virgo detector in Italy. The mass of the objects indicated that one of the objects had to be a black hole and the other a neutron star — the dead core of an exploded star. If it wasn't a neutron star, it could be something unknown to science.
[1] This is a condensed version of an article entitled "Hushed thud sheds light on black holes" and alternately "The biggest bang since the Big Bang" by Seth Borenstein published in the Sydney Morning Herald on 4 September 2020.