Gravitational waves were detected for the first time back in 2015, and since then Earth's three detectors, LIGO (which has two) and Virgo, have completed three more observing runs. Now, researchers have released data analysis of the first six months of the third run (O3), more than tripling the detection of these cosmic emissions. Thirty-nine new events have been reported, which together with 11 from the first two runs, takes us to a nice round total of 50 gravitational wave detections so far.
The source of these emissions is collisions between two extremely dense objects: black holes, compact massive objects from which nothing can escape, including light, or neutron stars, the end product of some supernovas, the huge explosions that end the life of massive stars.
Some of the detections in the O3 run have already made the news. There was the second-ever neutron star collision and the first collision between two black holes of wildly different masses. There was a collision between two black holes as they orbited a supermassive black hole, and even a mystery signal that could be either the lightest black hole detected or the heaviest neutron star.
In the new data release, there are two peculiar cases that the collaborative LIGO/Virgo teams think are worth highlighting: GW190426_152155 and GW190924_021846.
“One of our new discoveries, GW190426_152155, could be a merger of a black hole of around six solar masses with a neutron star. Unfortunately, the signal is rather faint, so we cannot be entirely sure,” Dr Serguei Ossokine from AEI Potsdam said in a statement.
“GW190924_021846 certainly is from the merger of the two lightest black holes we’ve seen so far," he added. "One had the mass of six Suns, the other that of nine Suns. There are signals from mergers with less massive objects like GW190814 but we don’t know for sure whether these are black holes.”
Interestingly, these events have even longer numerical names than previous ones because now we have started to spot more than one gravitational wave signal per day, the UTC time of detection has been added to the date.
The O3 run started on April 1, 2019, and was paused for maintenance in October, constituting O3a. It started up again in November 2019 but was suspended in late-March, 2020 (O3b) due to the Covid-19 pandemic. However, because this was a longer run than the prevous two, and the three detectors – LIGO's two in Washington State and Louisiana and Italy's Virgo – were on for a longer period of time during this run, this led to a significant increase in detections.
The effects of gravitational waves on space-time are much smaller than an atom and these large detectors are susceptible to many environmental and seismic conditions, so the number of events detected is remarkable.
The team is currently analyzing data from O3b, with over 20 gravitational events candidates found so far, while the detectors are getting a bit of TLC in preparation for O4 due to start in 2022. By then there will be a fourth observatory joining in, Japan's KAGRA, which could increase both numbers and knowledge of these exciting events.