The new events are known as GW170729, GW170809, GW170818, and GW170823, in reference to the dates they were detected.
GW170729, detected in the second observing run on July 29, 2017, is the most massive and distant gravitational-wave source ever observed.
In this coalescence, which happened roughly 5 billion years ago, an equivalent energy of almost five solar masses was converted into gravitational radiation, researchers said.
From September 12, 2015, to January 19, 2016, during the first LIGO observing run since undergoing upgrades in a program called Advanced LIGO, gravitational waves from three binary black hole mergers were detected.
The second observing run, which lasted from November 30, 2016, to August 25, 2017, yielded one binary neutron star merger and seven additional binary black hole mergers, including the four new gravitational-wave events being reported now.
GW170814 was the first binary black hole merger measured by the three-detector network, and allowed for the first tests of gravitational-wave polarization (analogous to light polarization).
The event GW170817, detected three days after GW170814, represented the first time that gravitational waves were ever observed from the merger of a binary neutron star system.
What is more, this collision was seen in gravitational waves and light, marking an exciting new chapter in multi-messenger astronomy, in which cosmic objects are observed simultaneously in different forms of radiation.
One of the new events, GW170818, which was detected by the global network formed by the LIGO and Virgo observatories, was very precisely pinpointed in the sky.
The position of the binary black holes, located 2.5 billion light-years from Earth, was identified in the sky with a precision of 39 square degrees.
That makes it the next best localised gravitational-wave source after the GW170817 neutron star merger.
"The release of four additional binary black hole mergers further informs us of the nature of the population of these binary systems in the universe and better constrains the event rate for these types of events," said Albert Lazzarini, Deputy Director of the LIGO Laboratory at California Institute of Technology in the US.
"The next observing run, starting in Spring 2019, should yield many more gravitational-wave candidates, and the science the community can accomplish will grow accordingly," said David Shoemaker, spokesperson for the LIGO Scientific Collaboration.
The scientific papers describing the findings, which are being initially published on the arXiv repository of electronic preprints, present detailed information in the form of a catalogue of all the gravitational wave detections and candidate events of the two observing runs.
Almost all black holes formed from stars are lighter than 45 times the mass of the Sun, researchers said.
Thanks to more advanced data processing and better calibration of the instruments, the accuracy of the astrophysical parameters of the previously announced events increased considerably.
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