Five years ago, humanity still had to find gravitational waves.
Now, the observations are moving at an astonishing pace. Over a six-month period last year, the LIGO-Virgin collaboration detected an average of 1.5 gravitational wave events per week.
From 1 April to 1 October 2019, upgraded LIGOs and virgin interferometers found 39 new gravitational wave events: large collisions between shock wires or black holes spread throughout space. In total, the Gravitational-Wave Transient Catalog 2 (GWTC-2) now has 50 such events.
This gives a complete census of black holes in our toolkit, representing a series of black holes that not only have never been discovered before, but can reveal the evolution of binary stars and the subsequent depth of succession.
“Gravitational-wave astronomy is revolutionary – revealing to us the hidden life of black holes and neutron stars,” said Christopher Berry, an astronomer at Northwestern University who is a member of the LIGO Scientific Collaboration (LSC).
“In just five years, we know that binary black holes exist because there are more than 400 catalogs. More discoveries have been made than before the third observation. Combining them with previous discoveries paints a beautiful picture of the rich diversity of the universe. Binary.”
You may have heard of some new discoveries made by Observation Run.
GW 190412 (the gravitational wave events are named for the date of their discovery) was the first collision of black holes in which two black holes were massively unmatched; All of the other previously discovered black hole collisions involved more or less identical mass binary.
GW 190425 is thought to be from a collision between two neutron stars, only one of which was discovered (the first was in August 2017).
GW 190521 finally confirmed the existence of an insidious ‘middleweight’ class of black holes between a constellation of stars and supermassive behemoths.
And GW 190814 was the first collision involving an object in a ‘mass gap’ between a neutron star and a black hole.
“So far, the third observation rate of LIGOs and Virgo has caused many surprises,” said Maya Fishbach, an astronomer at Northwestern University and LSC.
“After the second observation race, I thought we’ve seen the full spectrum of binary black holes, but the landscape of black holes is richer and more diverse than I imagined. I’m excited to see what future observations will teach us.”
It does not need to offer all new data spacing. The other two events, GW 190426_152155 and GW 190924_021846, came out as exceptional. And yes, those names are long: as we find more and more events, the date is not enough to separate them, so the new naming convention has to include time in the UTC.
“One of our new discoveries, GW 190426_152155, could merge about six solar mass black holes with a neutron star. Unfortunately the signal is obscure, so we can’t be absolutely sure,” said Albert astronomer Sergei Osokin. The Einstein Institute in Potsdam, Germany.
“GW 190924_021846 is definitely from the merger of the two lighter black holes we’ve seen so far. One had a mass of six suns, the other was a sun. There are signs of a merger with less massive objects like GW 190814 but we don’t know for sure if these are black holes. “
The new population of black holes and neutron star mergers is described in four preprint papers.
The first paper lists 39 new events. The second paper reconstructs the mass and spin distribution of 47 merger events found in the entire GWTTC-2 catalog, and estimates the rate of black hole and neutron star collisions. The third paper searches diligently for gamma-ray explosions associated with merging events (nothing is found). And the fourth paper evaluates the data against general relativity predictions; Spoiler, possessing general relativity perfectly.
Overall, the new collection of merger events is not the way to study collisions. It gives us a way to study black holes directly, which – they emit any detectable radiation – is notoriously difficult to investigate.
Thanks to gravitational waves, we know more about these objects than we did a year ago. And that’s going to be a snowball from here.
“Merging black holes and neutron star binary is a unique laboratory,” Berry said.
“We can both use it to study gravity – so far Einstein’s general relativity has passed every test – and the astrophysics of how large stars live their lives. LIGO and Virgo have changed our ability to observe these binary, and “As our detectors improve, so does the search rate.”
LIGO has uploaded preprints on its website while awaiting peer-review. They can be found here, here, here and here.
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