A collision of stars that triggers a scientific tsunami

For the first time, scientists were able to observe the fusion of two neutron stars, one of the best-kept secrets in the universe, a veritable "firework" whose observation began with the detection of gravitational waves. 70 observatories on Earth and in space joined forces to observe this extraordinary event. This set of observations marks the advent of a so-called "multi-messenger" astronomy. A considerable harvest of results has resulted: from a solution to the gamma-ray bursts enigma and the origin of the heaviest chemical elements - such as lead, gold or platinum - to the study of the properties of neutron stars or an independent measurement of the expansion rate of the Universe.
Che wonderful thing is that we saw the whole story unfold: we saw the neutron stars coming closer and closer, spinning faster and faster around each other, we saw the collision, then the matter, the debris being sent everywhere. "Benoît Mours, CNRS research director and scientific manager of the Virgo project for France, told AFP.
This unprecedented live observation provides answers to a host of scientific "mysteries". Let's go back to the story of this fantastic story.

17 August 2017 12:41 GMT: Alert!

It's an extraordinary adventure that started on August 17, 2017 at 2:41 p.m. (Paris time). The two LIGO gravitational wave detectors located in the United States are picking up a strong signal, very different from those previously intercepted. Something new, something big is coming: " That morning, all our dreams came true... "says Alan Weinstein of Caltech (California Institute of Technology). « We immediately turned to Virgo (another gravitational wave detector located in Pisa, Italy, editor's note) to ask if they could have seen him, too. "David Shoemaker, spokesman for the LIGO collaboration, told AFP. Less than an hour later, Virgo confirmed.
American Ligo and European Virgo detectors are sounding the alert. They're picking up a new type of gravitational wave signal. This time, the signal detected is much longer than in the case of black hole fusion (a hundred seconds versus a fraction of a second), a sign that the two objects that end up merging are different from those detected so far.
I was sitting in my dentist's chair when I got the text. ", recalls Benoît Mours. « I hurried to the lab to find out the rest of the story; everyone rushed to our online chat room. ». Patrick Sutton, head of the gravitational physics team at Cardiff University, was on the bus. trying to read the hundreds and hundreds of e-mails that had just come in on his cell phone... ".
Very quickly, the researchers knew which stars were sending them this signal: two neutron stars on the verge of merging, a phenomenon that had never been observed before! « Automatic analysis programs process the data collected by LIGO and Virgo and 6 minutes later we knew it was two neutron stars. ", recalls Benoît Mours.
A little before 18H00 GMT, the researchers were able to tell in which direction the two stars were located. Nearly 90 groups of astronomers were then invited to participate in the treasure hunt.
At 22H00 GMT, researchers are rejoicing: the American Swope telescope in Chile has discovered a bright spot. « As soon as twilight began to fall, telescopes were able to identify the host galaxy and witness a long fireworks display "said David Shoemaker. « I've never seen anything like it. "says Sephen Smartt of the New Technology Telescope at the La Silla Observatory in Chile.
Detailed analysis of the data will indicate that the masses of both objects are between 1.1 and 1.6 times the mass of the Sun, which corresponds to the masses of neutron stars. The two objects are colliding and merging. The event is named GW171817.

The final collision was extremely bright, emitting an intense "fireball" of gamma rays. You can see it in the video below. The large bright spot in the center is galaxy NGC 4993. Just above and to the left, you can see GW170817 turning from intense blue to red.

And in this other video, a representation of the collision of two neutron stars:
(Credit: NASA's Goddard Space Flight Center)
In the hours and days that follow, other "messengers" will arrive from space: gamma-ray bursts, X-rays, ultraviolet and infrared radiation, and radio waves. « It's a first to observe the same cosmic phenomenon with gravitational waves and light... "enthused Benoît Mours, scientific manager of the Virgo collaboration for France.
Almost at the same time and independently, NASA's Fermi satellite records a gamma burst - a flash of high-energy radiation - and immediately triggers an automatic alert. While this type of flash is relatively frequent (it occurs almost every week on average), it has the particularity of being detected about 2 seconds after the end of the gravitational wave signal, indicating a strong link between these two events. Moreover, the analysis of the Fermi data indicates a spatial origin of 1100 square degrees compatible with the localization by the Virgo and LIGO detectors. The gamma-ray burst is also observed by the Integral satellite of the European Space Agency (ESA). These observations confirm that at least some of the short gamma-ray bursts are produced by the fusion of neutron stars.

Race against the clock

In parallel, this source is located in the sky by exploiting the arrival times and the amplitude of the signals measured in the three gravitational wave detectors (the two LIGO detectors in the United States and the Virgo detector in Europe). The area thus determined, which covers about 30 square degrees (or 120 times the size of the full Moon in the sky) in the Southern Hemisphere constellation of the Hydra, is tens of times smaller than that established by Fermi. It is communicated to nearly 90 groups of partner astronomers to point their instruments in that direction. Twelve hours later, the 1M2H group using the American Swope telescope in Chile announced the discovery of a new spot of light in galaxy NGC 4993, located 130 million light years from Earth. Very quickly, this result is confirmed by other telescopes independently. Many other instruments followed, including those of ESO in Chile and the Hubble Space Telescope.
This zone is then scrutinized relentlessly and the first analyses of the light spectra show that it is not a supernova but a type of object never before observed, made of very hot matter that cools down and whose luminosity decreases rapidly - hence the race against time to observe it before it fades away.

Neutron stars are remnants of massive stars. A giant star dies when it explodes, giving birth to a supernova. This extremely luminous phenomenon lasts only a few days to a few weeks: once the explosion is over, only a very dense core composed almost entirely of neutrons remains - a neutron star. It is the size of a city like London, but a teaspoon of its material weighs about a billion tons, the equivalent of 100,000 Eiffel Tower. Neutron stars are the smallest and densest stars known to date. Just like the ordinary stars from which they come, some stars evolve in pairs. They then orbit around each other and slowly approach each other, losing energy in the form of gravitational waves - a process that eventually accelerates to fusion. If this scenario was predicted by models, this is the first time it has been confirmed by observation.

Metal "factories

According to the models, the matter ejected by the fusion of two neutron stars is the site of nuclear reactions leading to the formation of atomic nuclei heavier than iron (such as gold, lead, etc.), thanks to the abundance of neutrons. This very hot and radioactive material then disperses, emitting light in all wavelengths, initially very blue and then reddening as the material cools and disperses. Called kilonova, this phenomenon, so far only predicted by theory, is thus convincingly confirmed. We have thus observed what is probably the main process of formation of the heaviest chemical elements in the Universe!
Neutron stars reach extremely high temperatures, perhaps a million degrees. They're also highly radioactive, their magnetic fields are incredibly intense and would be fatal to anyone who came near them. "said Patrick Sutton, head of the Gravitational Physics team at Cardiff University. « They are probably the most hostile environment in the universe... ".
From their observation, the researchers were able to define a new way to measure the speed of expansion of the universe and were able to confirm that gravitation is indeed propagating at the speed of light as Albert Einstein predicted.

Scientific Harvest

In addition to confirming that neutron star mergers produce short gamma-ray bursts, the first unambiguous detection of a kilonova, and proof that the heavy elements of the Universe are formed during this process, this set of observations also provides a better understanding of the physics of neutron stars and eliminates some extreme theoretical models. It also provides a new way to measure the Hubble constant, which describes the rate of expansion of the Universe. These results, which cover a variety of disciplines (nuclear physics, astrophysics, cosmology, gravitation), illustrate the potential of an emerging astronomy based on several types of cosmic messengers (gravitational waves, electromagnetic waves such as light or gamma rays, and perhaps one day particles such as neutrinos or cosmic rays). They are detailed in a dozen publications, one of which is the work of several thousand researchers grouped in about fifty collaborations. Several simultaneous press conferences were also held in Washington, Paris, London and Berlin...". We're running out of time to get the information out as fast as we can. It's an achievement to be able to complete in such a short period of time the ten studies published on Monday," explains Benoît Mours. « A lot of people haven't slept much in the last two months! ", adds Patrick Sutton.
Neutron star fusion had been predicted by models. But it is the new ability of researchers to detect gravitational waves (since 2015) that has made it possible to identify and locate this phenomenon. Gravitational waves are space-time oscillations caused by cosmic events. They are similar to the way the surface of a body of water deforms when a pebble is thrown into it. Predicted a century ago, they were first observed in 2015 by Ligo, whose creators were honoured two weeks ago with the Nobel Prize in Physics. So far, only four waves - born from the fusion of two massive black holes - have been captured, with the Franco-Italian detector Virgo seeing only the fourth. These detections have opened a new chapter in astronomy.
No less than 1,200 scientists are collaborating on the Ligo and Virgo detectors, and more than 70 observatories on Earth and in space have tracked this fusion. In all, several thousand people worked collectively on this first scientist.

And the adventure isn't over: " we've got enough data to work with for a long time! ", enthuses Benoît Mours. Alain Brillet, the French scientist who developed the Virgo gravitational interferometer, adds in the Journal du CNRS : " These results, which sign the birth certificate of multi-signal astronomy, are proof that we did not work for nothing. And they offer a hundred years of work to our successors! "A feat that allows scientists from all over the world to party, a great party!

Sources: CNRS, AFP

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