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Just over five years ago, humanity still had to detect gravitational waves.
Now, the observations are pouring in at amazing speed. Over a span of six months last year, the LIGO-Virgo collaboration detected, on average, 1.5 gravitational wave events per week.
From April 1 to October 1, 2019, the updated LIGO and Virgo interferometers detected 39 new gravitational wave events – shock waves that propagate through spacetime due to massive collisions between neutron stars or black holes. In total, the Gravitational Wave Transient Catalog 2 (GWTC-2) now boasts 50 of these events.
This gave us the most comprehensive census of black holes in our toolkit, representing a range of black holes that not only had never been detected before, but can reveal depths of the evolution and successive lives of previously binary stars. unexplored.
“Gravitational wave astronomy is revolutionary: it reveals the hidden lives of black holes and neutron stars,” said astronomer Christopher Berry of Northwestern University, a member of the LIGO Scientific Collaboration (LSC).
“In just five years we have gone from not knowing that binary black holes exist to having a catalog of over 40. The third set of observations produced more discoveries than ever. Combining them with previous discoveries paints a beautiful picture of the rich variety of the Universe. of tracks. “
You have already heard of some of the new discoveries made while observing.
GW 190412 (gravitational wave events named after their date of detection) was the first collision of a black hole in which the two black holes had wildly mismatched masses; all the other black hole collisions previously detected had involved more or less binaries of equal mass.
GW 190425 is thought to result from a collision between two neutron stars, only the second ever detected (the first was in August 2017).
GW 190521 has finally confirmed the existence of the elusive class of “medium” black holes, between those of stellar mass and supermassive giants.
And GW 190814 was the first collision involving an object in the “mass gap” between neutron stars and black holes.
“So far, the third round of LIGO and Virgo observation has produced many surprises,” said astronomer Maya Fishbach of Northwestern University and LSC.
“After the second set of observations, I thought I’d seen the full spectrum of binary black holes, but the black hole landscape is much richer and more varied than I imagined. I’m excited to see what future observations will teach us.”
That’s not all the new data stream had to offer. Two other events, GW 190426_152155 and GW 190924_021846, stood out as extraordinary. And yes, those names are longer – as we detect more and more events, the date may not be enough to tell them apart, so the new naming convention is to include the time in UTC.
“One of our new discoveries, GW 190426_152155, could be the merger of a black hole of about six solar masses with a neutron star. Unfortunately the signal is rather weak, so we can’t be entirely sure,” said the astronomer. Serguei Ossokine of the Albert Einstein Institute in Potsdam in Germany.
“GW 190924_021846 certainly comes from the merger of the two lightest black holes we have seen so far. One had the mass of six Suns, the other that of nine Suns. There are signs of mergers with less massive objects like GW 190814 but we don’t know. for sure if it’s black holes. “
The new population of black hole and neutron star mergers has been described in four pre-printed papers.
The first paper catalogs the 39 new events. The second article reconstructs the mass and spin distributions of 47 fusion events found in the entire GWTC-2 catalog and estimates the collision rate between black holes and neutron stars. The third article meticulously searches for gamma-ray bursts associated with fusion events (found none). And the fourth article evaluates the data against the predictions of general relativity; spoiler, general relativity holds up completely.
Overall, the new collection of fusion events isn’t just a way to study collisions. It gives us a way to study black holes directly, which – because they emit no detectable radiation – are notoriously difficult to probe.
Thanks to gravitational waves, we know a lot more about these objects than we did a year ago. And from here it will start to snowball.
“The merger of black hole and binary neutron stars is a unique laboratory,” Berry said.
“We can use them to study both gravity – so far Einstein’s general relativity has passed all tests – and astrophysics of how massive stars live their lives. LIGO and Virgo have transformed our ability to observe these binaries and, as as our detectors improve, the rate of discovery will only accelerate. “
LIGO has uploaded the preprints to their website pending a peer review. They can be found here, here, here and here.
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