Neutrino linked to black hole that devours star
For only the second time ever, astronomers have linked a neutrino to an object outside our Milky Way. These observations were made using telescopes both on Earth and in space. The researchers, including Leiden astronomer Sjoert van Velzen, were able to trace the neutrino to a black hole that is devouring a star, a so-called tidal disruption event. The results have been published in Nature Astronomy.
The Universe is full of almost elusive neutrinos. These uncharged subatomic elementary particles hardly interact with other matter. Especially neutrinos with an extremely high energy are interesting for astrophysicists. The source of these extreme particles is unknown.
Based on theoretical predictions, it was suspected that so-called tidal disruption events can produce high-energy neutrinos in jets early in their evolution, when they are at their brightest. However, the first high-energy neutrino linked to a tidal disruption event shows some surprising properties. 'The neutrino does not appear to have been produced as we expected,' says first author Robert Stein, a PhD candidate at Humboldt University in Berlin.
The research story begins in April 2019, when a team led by Sjoert van Velzen discovered a new tidal disruption event using the Zwicky Transient Facility, a robotic camera at Palomar Observatory in California, USA. The outburst took place at a distance of 690 million light-years from Earth in the galaxy 2MASX J20570298+1412165, in the constellation Dolphin.
In addition to these optical observations, several ultraviolet and X-ray images were made from space (both with the Swift telescope and the XMM-Newton satellite). Finally, radio telescopes were also used to observe the new event: The Karl G. Jansky Very Large Array in New Mexico, USA, and MeerKAT in South Africa.
The brightness peaked in May 2019, without a jet appearing. Based on theoretical predictions, this new source therefore did not seem like a good neutrino candidate. But then five months later, on 1 October 2019, the IceCube Neutrino Observatory at the Amundsen-Scott South Pole Station in Antarctica detected a high-energy neutrino, called IC191001A, and traced it back to a defined patch in the sky, exactly where the tidal disruption event occurred. The team calculated that there is only a 1 in 500 chance that this association would occur by chance. The question was then how it could have produced neutrinos.
For the answer, observations with radio telescopes were essential. The radio emission remained stable for months, which shows that the acceleration of the particles can also take place after the brightness peak in visible light. After analysing all the data, three possible locations remain for the production of neutrinos in tidal disruption events: in the outer parts of the disc by collisions with UV light, in the inner parts by collisions with X-rays, or in the outflow of particles by collisions with other particles.
Van Velzen prefers the model in which the observed neutrino originates in the outer region of the disc: 'In this part, the density of UV photons is so high that it is very easy to produce neutrinos with the energy we observed.'
The prediction that neutrinos and tidal disruptions could be related was made only a few years ago,' says van Velzen. The fact that we are now able to measure it for the first time is, of course, extremely nice. By detecting just one neutrino, we can learn a lot more about what happens when a star falls into a black hole.
Animation of the event
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Robert Stein, Sjoert van Velzen, Marek Kowalski, Anna Franckowiak, et al. A tidal disruption event coincident with a high-energy neutrino, Nature Astronomy (22 February 2021)
Header image: After the black hole rips apart the star, about half of the star's remnant is hurled back into space, while the other half forms a glowing accretion disk around the black hole. This event has been observed from X-ray to radio wavelengths and with the detection of one neutrino. A central, powerful 'engine' at the centre of the disc could explain the neutrino and radio emission. Photo credit: DESY, Science Communication Lab.
Translation: Bryce Benda