By tracking a ghostly particle to a crushed star, scientists have discovered a gigantic cosmic particle accelerator. The subatomic particle, called a neutrino, was launched toward Earth after the doomed star got too close to the supermassive black hole at the center of its home galaxy and was ripped apart by the black hole’s colossal gravity. It is the first particle that can be traced back to a ‘tidal disruption event’ (TDE) and provides evidence that these poorly understood cosmic catastrophes can be powerful natural particle accelerators, as the team led by the DESY scientist reports, Robert Stein, in the magazine. Astronomy of nature. The observations also demonstrate the power of exploring the cosmos through a combination of different ‘messengers’ such as photons (the light particles) and neutrinos, also known as multi-messenger astronomy.
The neutrino began its journey about 700 million years ago, when the first animals developed on Earth. That is the travel time the particle needed to get from the distant unnamed galaxy (cataloged as 2MASX J20570298 + 1412165) in the constellation Delphinus (The Dolphin) to Earth. Scientists estimate that the huge black hole is as massive as 30 million suns. “The force of gravity gets stronger and stronger the closer you get to something. That means that the black hole’s gravity pulls on the near side of the star more strongly than on the far side of the star, which produces an effect stretching, “Stein explains. “This difference is called tidal force, and as the star gets closer, this stretching becomes more extreme. Over time, it rips apart the star and then we call it a tidal disruption event. It’s the same process that drives to ocean tides on Earth, but luckily for us, the moon doesn’t pull hard enough to tear Earth apart. “
About half of the debris from the star was spewed into space, while the other half was placed on a spinning disk around the black hole. This accretion disk is somewhat similar to the vortex of water over a bathtub drain. Before sinking into oblivion, the matter of the accretion disk gets hotter and hotter and shines brightly. This glow was first detected by the Zwicky Transient Facility (ZTF) at Mount Palomar in California on April 9, 2019.
Half a year later, on October 1, 2019, the IceCube neutrino detector at the South Pole recorded an extremely energetic neutrino from the direction of the tidal disruption event. “It crashed into the Antarctic ice with a remarkable energy of more than 100 teraelectronvolts,” says DESY co-author Anna Franckowiak, who is now a professor at Bochum University. “For comparison, that’s at least ten times the maximum particle energy that can be achieved at the world’s most powerful particle accelerator, the Large Hadron Collider at the European particle physics laboratory CERN near Geneva.”
Extremely light neutrinos hardly interact with anything, they can go unnoticed not only through walls, but also through planets or entire stars, and therefore they are often called ghost particles. Therefore, even capturing a single high-energy neutrino is already a remarkable observation. Analysis showed that this particular neutrino had only a one in 500 chance of being a pure coincidence with TDE. The detection sparked further observations of the event with many instruments across the entire electromagnetic spectrum, from radio waves to X-rays.
“This is the first neutrino linked to a tidal disruption event, and it provides us with valuable evidence,” explains Stein. “Tidal disruption events are not well understood. The neutrino detection points to the existence of a powerful central engine near the accretion disk, which spews out fast particles. And the combined analysis of data from radio telescopes, optical and ultraviolet gives us more evidence that TDE acts as a gigantic particle accelerator. “
The observations are best explained by an energetic outflow of fast jets of matter shooting out of the system, produced by the central engine, lasting hundreds of days. This is also what is needed to explain the observational data, as Walter Winter, head of the astroparticle physics theoretical group at DESY, and his theoretical colleague Cecilia Lunardini of Arizona State University, have shown in a theoretical model published in the same number of Astronomy of nature. “The neutrino emerged relatively late, half a year after the Star Festival started. Our model explains this moment naturally,” says Winter.
The cosmic accelerator spews out different types of particles, but apart from neutrinos and photons, these particles are electrically charged and are therefore deflected by intergalactic magnetic fields on their journey. Only electrically neutral neutrinos can travel in a straight line like light from the source towards Earth and thus become valuable messengers of such systems.
“The combined observations demonstrate the power of multi-messenger astronomy,” says co-author Marek Kowalski, director of neutrino astronomy at DESY and professor at Humboldt University in Berlin. “Without detecting the tidal disruption event, the neutrino would be just one of many. And without the neutrino, observing the tidal disruption event would be just one of many. Only through combination could we find the accelerator and learn something new about internal processes. ” The association of the high-energy neutrino and the tidal disruption event was found using a sophisticated software package called AMPEL, developed specifically at DESY to look for correlations between IceCube neutrinos and astrophysical objects detected by the Zwicky transient facility.
The tip of the iceberg?
The Zwicky transient facility was designed to capture hundreds of thousands of stars and galaxies in one shot and can survey the night sky particularly quickly. At its heart is the 1.3 m diameter Samuel-Oschin telescope. Thanks to its large field of view, ZTF can scan the entire sky for three nights, finding more variable and transient objects than any previous optical survey. “Since our inception in 2018, we have detected more than 30 tidal disruption events so far, more than double the known number of such objects,” says Sjoert van Velzen of the Leiden Observatory, a co-author of the study. “When we realized that the second brightest TDE observed by us was the source of a high-energy neutrino recorded by IceCube, we were excited.”
“We may only be seeing the tip of the iceberg here. In the future, we hope to find many more associations between high-energy neutrinos and their sources,” says Francis Halzen, a professor at the University of Wisconsin-Madison and principal investigator for IceCube. , who was not directly involved in the study. “A new generation of telescopes is being built that will provide increased sensitivity to TDEs and other potential neutrino sources. Even more essential is the planned extension of the IceCube neutrino detector, which would increase the number of cosmic neutrino detections by at least tenfold. “. This TDE marks only the second time that a high-energy cosmic neutrino can be traced to its origin. In 2018, a multi-messenger campaign featured an active galaxy, the blazar TXS 0506 + 056, as the first identified source of a high-energy neutrino, recorded by IceCube in 2017.
Researchers detect a galactic source of gamma rays that could produce very high-energy cosmic rays
A tidal disruption event coinciding with a high-energy neutrino, Nature astronomy (2021). DOI: 10.1038 / s41550-020-01295-8, www.nature.com/articles/10.1038/s41550-020-01295-8
Provided by Deutsches Elektronen-Synchrotron
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