Gravitational wave discovery shows merger of two black holes with unequal masses

Issued: Mon, 20 Apr 2020 14:00:00 BST

The third observing run of LIGO and Virgo is offering new insights into the late inspiral and merger phase of binary black hole systems. 

The first gravitational-wave event detected back in 2015, GW150914, originated from a binary black hole merger, and since then this class of events has become the most prominent. However, nearly all of the systems observed so far have comprised black holes of nearly equal masses. 

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This trend was broken by the observation of the merger of a never-before-seen binary black hole system on April 12, 2019 at 05:30:44 UTC, just a couple of weeks after the start of the third observing run. 

Researchers from the University of Glasgow’s Institute of Gravitational Research played a key role in the data analysis which underpinned the detection. 

The signal, named GW190412, was detected by the Advanced Virgo and the two Advanced LIGO detectors, and it was produced by a coalescing binary black hole system with markedly unequal masses, one of the black holes being about 3.6 times heavier than the other. 

Analysis indicates that the black holes that merged had masses equal to about 30 and 8 times the mass of the sun. Such mass differences are predicted by theory to produce subtle differences in the gravitational-wave signal. These variations, which appear as higher ‘harmonics’ in the waveform, have been observed for the first time in this event.

Overall, the characteristics of the signal also depend on other parameters of the binary system. For example, the observed higher harmonics enable scientists to constrain the inclination of the plane of the orbit of the binary black hole system with respect to the line of sight and better measure the distance of the source, since these two quantities are otherwise highly entangled. 

Furthermore, the measurement of these harmonics in the signal, as predicted by Einstein’s theory of general relativity, provides yet another confirmation of the validity of that theory.

Dr Chris Messenger, a lecturer in the University’s Institute for Gravitational Research and a LIGO scientists, said: “The third observing run of the advanced gravitational wave observatories has gifted us many tens of binary black hole candidate signals but they are not all just the same.

“The GW190412 event is unique in having one relatively heavy and one relatively light black hole making up the binary. This makes the higher harmonics in the signal easier to identify and from these we can learn a lot more about the system and really put Einstein’s theory of general relativity to the test.”