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About 870 million years ago, two dead stars became one. Their fusion shook the fabric of space with a gravitational wave that swept the Earth last Wednesday, waving through three pairs of carefully calibrated lasers designed to detect their passage. An automated system sent a preliminary alert 21 seconds later, vibrating smartphones and pinging laptops around the world.
Three years after the first Nobel Prize – winning gravitational wave detection, which stems from two colliding black holes, such warnings have become commonplace. This time, however, astrophysicists immediately knew that the observed event was special. "My jaw dropped when I saw the data," says Geoffrey Lovelace of Fullerton (C.S.F.F.) of California State University, a member of the LIGO (laser interferometer) collaboration.
The wave was detected by LIGO in the United States and by the Observatory of the Virgin in Italy at 14:11 on August 18 at 21:11 (UTC). A first automatic pass indicated that this wave resulted from an unprecedented merger between a pair of bodies too light to be classified, sending scramble to look for additional electromagnetic emissions from the event. Subsequent analysis recategorized the signal as a collision between a black hole and a neutron star, a stellar remain in which gravity compresses the mass of an entire sun into a city-sized ball. This would be the first event of this type detected with confidence and – after the black holes – the mixtures of black holes and the fusions between two neutron stars – the third variety of collision detected by the gravitational waves. If the current analysis is valid, this event, dubbed S190814bv, will mark the beginning of a new era for astrophysical studies, with implications for researchers' understanding of Einstein's general theory of relativity, death stars and the behavior of extreme matter.
A signal "outside the paintings"
Chad Hanna, LIGO collaborator and astrophysicist at Pennsylvania State University, celebrated his wedding anniversary with his wife when his phone went off. His group specializes in the rapid classification of LIGO events. So he immediately connected to check the details of the wave. "The first thing I knew was that it was extremely important," says Hanna.
The algorithmic pipeline of the LIGO-Virgo collaboration establishes a basic classification based on the shape of a wave, its duration and other factors almost instantaneously (Hanna's team aims for less than 20 seconds) so that astronomers can immediately orient their telescopes in the celestial direction of the wave of.
On Wednesday, the automatic system confidently stated that at least one of the objects that produced S190814bv fell into the "gap", a wasteland, covering three to five solar masses, seemingly devoid of black holes and black holes. neutron stars. All known black holes weigh more than five suns, while all known neutron stars – born from lighter stars that have not become black holes – weigh less than three suns. A mass gap detection would have been a first for LIGO-Virgo – an analysis that would have accentuated the theoretical line separating the heaviest neutron stars from the clearest black holes – but the preliminary label would only last not. "There has been a worldwide transfer," says Jocelyn Read, astrophysicist at C.S.U.F. and a LIGO member, starting with researchers in the United States on the afternoon of August 14 and the calculations continuing in Europe until the next morning.
US scientists woke up Thursday for a new classification. Human analysis established the event as a neutron star-black hole fusion with a confidence greater than 99%. LIGO-Virgo has heard the collisions of more than a dozen pairs of black holes, as well as two pairs of neutron stars, but has never conclusively heard the rumors of any of these. a black hole swallowing a neutron star.
"It's something I've been waiting for for a long time," says James Lattimer, a professor of astronomy at Stony Brook University and a pioneering nuclear astrophysicist, who has shown that fusion of neutron stars and black holes can pulverize heavy elements such as gold and uranium. in the space in his thesis of 1976.
The researchers detected a similar wave in April, but they were not able to confirm that it came from deep space – the models suggest that the signal associated with this potential event had a seven chance of being a false alarm generated by terrestrial sources, which means that a parasitic detection would be expected about once every 20 months. Last week's signal, however, is so clear that a false alarm would be a single event, which would occur once in billions of years. "When it's more than the age of the universe," says Lovelace, "you know it's the real deal."
The deafening signal of S190814bv does not, however, guarantee that the astrophysicists have definitely bagged their first star-black neutron-hole collision. While the current tag clearly places the heavier object on the territory of the black hole (more than five suns), it leaves the lighter partner of the cloudy zone below three solar masses. If a subsequent analysis places this partner between one and two solar masses, it must act as a neutron star. But a closer measure of three suns could break – the heaviest neutron star in the universe or its lightest black hole known.
The future mass estimates will give a clearer picture, but first, LIGO-Virgo will have to compare the wave to our best models, which are too complicated to run overnight. The theoretical tools become fragile as the masses move away from two balanced partners. The researchers are therefore certain not to walk in this unexplored territory. "We always analyze and verify things," says Lovelace. "But this is the most promising case like this one that has been raised so far."
In search of light
The Virgo detector in Italy – with only one of the two LIGO detectors – initially recognized the wave, but the collaboration was able to manually integrate data from the second LIGO detector overnight. Triangulation from this third detection allowed researchers to determine the position of the source in the sky more accurately than any previous wave so soon after detection. "I've opened [the new] Sky map, and I was like, "Oh, they accidentally updated a map of the virgin sky", "she recalls before she noticed the tiny dot that marks the origin of the wave.
The reduced area, which represented 0.06% of the total area of the sky, was a boon for astronomical teams looking for a gamma ray or visible light that could accompany the death of a person. neutron star. "In principle, it's a way to take a few minutes to cover this area," says Marcelle Soares-Santos, a cosmologist at Brandeis University, who coordinated the tracking observations using the dark energy camera on a four-meter telescope in Chile.
The black hole may have shredded the neutron star, leaving behind a ring of glittering wrecks that faded as it fell into the mouth of the hole. Alternatively, the black hole could swallow the neutron star all at once, with little to see. The LIGO-Virgo simulations for S190814bv predict the latter scenario, but no one knows for sure what really happened. For a first observation, even seeing nothing can be informative. "We go with an open mind," says Soares-Santos. "If there is no electromagnetic counterpart, we can establish with sufficient importance that will have a big impact on theories. "
neutronium
And theories about neutron stars abound. Nuclear physicists seek insight into the interior of objects, where matter exists at densities that challenge the best current models. If the pressure dissolves the neutrons in a plasma of fundamental particles, for example, the neutron stars of some mass should appear smaller than they would otherwise be. The fine characteristics of the detected gravitational wave, produced when the star spirals into the black hole, can reveal the size of the star and, therefore, the consistency of the material that fills it. Similarly, whether astronomers see a flash or not, this will also limit the size of the star. Such precise measurements of the size of a neutron star are "a kind of holy grail of nuclear physics," says Ben Margalit, a postdoctoral researcher at the University of California, Berkeley, who is not part of the collaboration that has allowed to observe this event.
A black hole obliterating a neutron star also represents a new arena for testing general relativity. Applying Einstein's theory of gravity to the smooth tissue of space-time around black holes is already quite difficult, explains Lovelace. The addition of magnetized, hot and turbulent neutron star material – an exotic substance sometimes called "neutronium" – takes the challenge to a new, messy level.
Even though Wednesday's waves in space-time reveal none of nature's secrets, researchers have the impression that this is only the first of many to come. "I hope this tells us something about the black hole – neutron star [mergers]Said Lovelace. "But otherwise, it always makes me really optimistic that the gravitational sky is bright."
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