The largest haul to date from gravitational waves observation runs has been the most recent.
The LIGO-Virgo interferometers registered a staggering 35 gravitational waves events in less than five months. This was between November 2019 and March 2020. That's nearly 1.7 gravitational waves events per week over the course of the run.
This is a significant increase over the 1.5-event weekly mean detected on the previous run. It also means that the total number of events has risen to 90 since the first gravitational wave detection in September 2015, which was historic.
Susan Scott, an Australian National University astrophysicist, stated that these discoveries represent a tenfold rise in gravitational waves detected in LIGO and Virgo since the beginning of their observations.
"We've detected 35 events. This is huge! We made three detections during our initial observing run in 2015-16, which lasted for four months. This is an exciting new era in gravitational wave detections. The growing number of discoveries is providing so much information about stars' lives and deaths throughout the Universe.
32 of the 35 new discoveries are most likely due to mergers between black holes. This happens when two black holes in close orbit collide to form one massive black hole.
The collision creates ripples in space-time like ripples when you drop a rock into a pond. Astronomers can study those ripples to determine properties of black holes.
This infographic shows the mass of all black hole mergers that have been announced so far. (LIGO-Virgo/Aaron Geller/Northwestern University)
Data revealed that there were a variety of black hole masses. The largest was 87 times larger than the Sun. The black hole that merged with the companion was 61 times larger than the Sun. This created a single black spot with a mass of 141 times that of the Sun. This event is called GW200220_061928.
Another merger resulted in a black hole with 104 times the Sun's mass; these black holes are intermediate mass black holes. They have a mass range of 100 to around a million solar mass, and very few black holes were detected.
Interesting is GW200220_061928 as at least one black hole involved in the merger falls within what we call "the upper mass gap". Our models show that black holes with 65 solar masses or more cannot form from one star. However, stellar mass black holes can.
Because the precursor stars are so large, their supernovae (also known as pair-instabilitysupernovae) should completely obliterate star core. This will leave nothing to gravitationally collapse to a black hole.
This suggests that the 87-solar mass black hole may be the result of a prior merger. Although GW200220_061928 may not be the first black hole to be detected in the upper mass gap's upper mass gap, it does indicate that hierarchical black holes mergers are common.
Another event involves an object in lower mass gap, a gap between 2.5 and 5 times Sun's mass. Although we have not been able to find a neutron star that is larger than the first or a smaller black hole, the event GW200210_092254 had an object with a mass of 2.8 solar masses. Astronomers believe it to be a small black hole.
Scott stated, "Looking at these binary systems' masses and spins indicates how they got together."
It also raises some fascinating questions. Is it possible that the system was formed with two stars, which went through their lives together before becoming black holes. Are the black holes pushed together in dense, dynamical environments such as the center of a galaxy?
Three other events were also recorded, one involving a black hole and another less massive event, most likely a neutron-star. Astronomers find these events of great interest because they could reveal the contents of a neutron-star - if one emits light. We can begin to understand how these mergers occur by finding more.
Christopher Berry, an astronomer at the University of Glasgow in Scotland said that "only now are we beginning to appreciate the amazing diversity of black holes e neutron stars."
"Our most recent results show that they can be found in many sizes and combinations. We have solved long-standing mysteries but also discovered new puzzles. These observations will help us to unravel the mysteries surrounding how stars, the building blocks of the Universe, develop.
The paper of the team has been submitted to publication and is available on arXiv's preprint server.
