It all started on February 11, 2022, at exactly 11:42:40 Paris time. It was still nighttime in California on Palomar Mountain, where several telescopes are located. One of them is equipped with a camera that scans the sky with a wide field of view, looking for what astronomers call transients (comets, asteroids, star explosions, etc.). And, on this winter day, this device called the Zwicky Transient Facility (ZTF) caught a light in the sky that was not there before.
As announced on Wednesday, November 30, by two international studies – one published in Nature and the other in Nature Astronomy – this light was an indication of an event as distant as it is rare. A star unwisely passed close enough to a supermassive black hole, which tore it apart.
When it’s written out like this, it all seems so simple, but before arriving at this conclusion, astrophysicists had to follow a long path that Susanna Vergani, French National Center for Scientific research director at the Paris Observatory and co-author of the study in Nature told us: "When the ZTF scans the sky, it compares it with the previous scan to detect any transient sources. We find many of them every night and the whole point is to figure out the cause of each variation. Since we have developed automatic tools for this, the preliminary analysis is done very quickly and the information is then sent out to an international network of researchers."
When a light source behaves strangely, the astronomical community comes together to make a range of further observations at different wavelengths. In the case of the February 11 event, named AT 2022cmc, a total of 21 telescopes were pointed at the source. "The idea is to then put all the observations together and try to understand what the event is," said Ms. Vergani.
The first hypothesis was a gamma-ray burst, which is triggered when a dying star transforms into a black hole or neutron star, a cataclysmic event that sends a considerable amount of energy into space. But this scenario did not align with the observations.
Accretion disk
"The best interpretation is a phenomenon known as a tidal disruption event (TDE)," explained Ms. Vergani. "A star is ripped apart by the gravity of a supermassive black hole that it got close to." The black hole’s attraction force literally tears matter from the star, just like a shirt sleeve gets torn if it’s pulled too hard. This matter then begins to spin around the black hole in what specialists call an accretion disk.
In very rare cases, which probably involve a rapid rotation of the black hole on itself and the presence of a strong magnetic field, part of the matter is directed toward the poles of the black hole and is ejected along its rotation axis at a tremendous speed, close to the speed of light. These jets, composed of electrically charged particles, then collide with the atoms of the interstellar medium, which causes light emissions at different wavelengths. If Earth happens to be aligned with these lighthouse-like jets, even a TDE occurring billions of light years away suddenly becomes visible, which was the case with AT 2022cmc.
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