Lone high-energy neutrino likely came from a jagged star in a distant galaxy

About 700 million years ago, a tiny subatomic particle was born in a galaxy far, far away and began its journey through the vast expanses of our universe. This neutrino finally reached Earth’s south pole last October, triggering detectors buried deep under the Antarctic ice. A few months earlier, a telescope in California had recorded a brilliant glow emanating from the friction of that same distant galaxy – evidence of a so-called “tidal disturbance event” (TDE), possibly the result of a star being jagged by a a supermassive black hole.

According to two new papers (here and here) published in the journal Nature Astronomy, this solitary neutrino likely originated from TDE, which serves as a cosmic-scale particle accelerator near the center of the distant galaxy, spewing out high-grade subatomics. particle energy because the star’s matter is consumed by the black hole. The discovery also sheds light on the origin of ultra-high-energy cosmic rays, a question that has puzzled astronomers for decades.

“The origin of high-energy cosmic neutrinos is unknown, mainly because they are notoriously difficult to pin down,” said co-author Sjoert van Velzen, post-doctoral fellow at New York University at the time of the discovery. . “This result would only be the second time that high-energy neutrinos have been traced to their source.”

Neutrinos travel very close to the speed of light. John Updike’s 1959 poem “Cosmic Gall” pays homage to two most defining characteristics of neutrinos: They come at no cost, and for decades physicists believed they had no mass ( they actually have a tiny bit of mass). Neutrinos are the most abundant subatomic particle in the universe, but they very rarely interact with any type of matter. We are constantly bombarded every second by millions of these tiny particles, but they pass through us without our even realizing it. This is why Isaac Asimov nicknamed them “ghost particles”.

This low rate of interaction makes neutrinos extremely difficult to detect, but because they are so light, they can escape unhindered (and therefore largely unaffected) through collisions with other particles of matter. This means that they can provide astronomers with valuable clues about distant systems, augmented by what can be learned with telescopes across the electromagnetic spectrum, as well as gravitational waves. Together, these different sources of information have been called “multi-messenger” astronomy.

The majority of neutrinos that reach Earth come from our own Sun, but every now and then neutrino detectors pick up the rare neutrino that comes from further afield. Such is the case with this latest detection: a neutrino that has started its journey in a distant, as yet unknown galaxy of the constellation Delphinus, born from the agony of a jagged star.

As we reported earlier, it’s a popular misconception that black holes behave like cosmic vacuum cleaners, voraciously sucking up any matter in their surroundings. In reality, only things that cross the event horizon – including light – are engulfed and cannot escape, although black holes are messy eaters too. This means that part of an object’s matter is actually ejected in a powerful jet. If that object is a star, the process of being shredded (or “spaghetted”) by the powerful gravitational forces of a black hole occurs outside of the event horizon, and some of the original mass of the star is violently ejected outwards. This in turn can form a rotating ring of matter (aka an accretion disk) around the black hole which emits powerful X-rays and visible light.

Tidal disturbance describes the large forces created when a small body passes very close to a much larger body, such as a star straying too close to a supermassive black hole. “The force of gravity gets stronger and stronger as you get closer to something. This means that the black hole’s gravity pulls the near side of the star more strongly than the opposite side of the star, resulting in a stretching effect, ”said co-author Robert Stein of DESY in Germany. “As the star gets closer, this stretch becomes more extreme. Eventually it tears apart the star, and then we call it a tidal disturbance event. It’s the same process that leads to ocean tides on Earth, but luckily for us the moon isn’t pulling hard enough to tear the Earth apart. “

TDEs are probably quite common in our universe, although only a few have been detected so far. For example, in 2018, astronomers announced the first direct image of the aftermath of a star shredded by a black hole 20 million times more massive than our Sun, in a pair of colliding galaxies called Arp 299 at around 150 million d light years from Earth. And last fall, astronomers recorded the final agony of a star torn apart by a supermassive black hole, publishing the find in Nature Astronomy.

The glow of the latter TDE was first detected on April 9, 2019 by the Zwicky Transient Facility (ZTF) at the Mount Palomar Observatory in California, which has spotted more than 30 such events since it went online. in 2018. Almost five months later, in October On January 1, 2019, the IceCube neutrino observatory at the South Pole recorded the signal of a high-energy neutrino coming from the same direction as the TDE. How energetic was that? DESY co-author Anna Franckowiak fixed the energy at over 100 teraelectronvolts (TEV), 10 times the maximum energy for subatomic particles that can be produced by the Large Hadron Collider.

The probability of detecting this solitary high-energy neutrino was only 1 in 500. “This is the first neutrino linked to a tidal disturbance event, and it gives us valuable evidence,” said Stein. “Tidal disturbance events are not well understood. Detection of the neutrino indicates the existence of a central and powerful motor near the accretion disk, spewing out fast particles. And the combined analysis of the telescope data radio, optical and ultraviolet gives us more evidence that TDE acts as a gigantic particle accelerator. “

This is yet another example of all the new knowledge to be gained by combining multiple data sources to get different perspectives on the same celestial event. “The combined observations demonstrate the power of multi-messenger astronomy,” said co-author Marek Kowalski of DESY and Humboldt University in Berlin. “Without the detection of the tidal disturbance event, the neutrino would be just one of many. And without the neutrino, observing the tidal disturbance event would be just one of many. It was only through the combination that we could find the accelerator and learn something. new about the processes inside. “

As for the future, “we may only see the tip of the iceberg here. In the future, we expect to find many more associations between high energy neutrinos and their sources, ”said Francis Halzen of the University of Wisconsin-Madison. who was not directly involved in the study. “A new generation of telescopes is being built that will provide greater sensitivity to TDEs and other prospective neutrino sources. Even more essential is the planned expansion of the IceCube neutrino detector which would increase the number of detections of at least ten times. cosmic neutrinos. “

DOI: Astronomy of Nature, 2021. 10.1038 / s41550-020-01295-8

DOI: Nature Astronomy, 2021. 10.1038 / s41550-021-01305-3 (About DOIs).