LIGO-Virgo Detect the Merger of Two Black Holes with Unequal Masses

On April 12, 2019, the twin LIGO detectors and the Virgo detector observed gravitational-waves from the merger of two black holes. While nearly all previous detections originated from binary black holes with almost equal masses, this event (labeled GW190412) displayed clear signatures of an unequal mass binary. A detailed analysis of the gravitational-wave signal indicates that the two black holes had masses of about 30 and 8 times the mass of the sun. General relativity predicts that binary systems with such mass differences will introduce higher “harmonics” into the waveform, and these were in-fact observed for the first time in this event.

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GW190412: The First Observation of an Unequal-Mass Black Hole Merger

What did we find?

On April 12, 2019, the LIGO Scientific Collaboration and Virgo Collaboration observed gravitational waves produced by the inspiral and merger of two black holes. This event, dubbed GW190412, was observed with all three detectors operating in the network: both LIGO detectors (one in Hanford, Washington and one in Livingston, Louisiana) as well as the Virgo detector (located in Cascina, Italy). GW190412 was detected near the beginning of Advanced LIGO and Virgo’s third observing run, known as O3, which started on April 1st 2019 and was suspended on March 27th 2020.

While the masses of the two black holes are consistent with those of previously observed black holes, GW190412 is unique in that it is the first black hole merger where the masses of the two black holes are definitively unequal — one black hole in the system is more than 3 times heavier than the other. This asymmetry in masses modifies the gravitational-wave signal in such a way that we can better measure other parameters, such as the distance and inclination of the system, the spin of the heavier black hole, and the amount that the system is precessing. In addition, the unequal masses of GW190412 enable us to verify a fundamental prediction of Albert Einstein’s General relativity: that gravitational waves “ring” at more than one fundamental frequency, so-called higher multipoles.

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