Extremely high-energy neutrinos hit Earth regularly

(ORDO NEWS) — Extremely high energy neutrinos regularly fall to Earth. Physicists suspect that these particles are produced in cosmic processes involving black holes, but exactly which process dominates this production remains unclear.

Astronomers are now reporting the discovery of a high-energy neutrino associated directly with a tidal disruption event (TDE), the violent destruction of a star by the intense gravity of a nearby black hole [1].

This observation is the second strong association of a high-energy neutrino with such a star-breaking event, allowing researchers to make a rough initial estimate of how many neutrinos are produced through this mechanism.

High-energy neutrinos – roughly those in the TeV energy range and above – are giving physicists information about some of the most violent astrophysical events in the universe, many of which are taking place far beyond our galaxy.

Because neutrinos interact so weakly with matter, they move unchanged over vast distances from where they were originally formed.

Theoretical models, supported by observations, have linked them to a wide range of potential sources, including active galactic nuclei, which are supermassive black holes that produce beams of energetic particles that absorb surrounding gas.

TDEs offer another possibility, as large numbers of neutrinos should be generated when a black hole rips apart a nearby orbiting star (see Research News: ”

However, at present, researchers still cannot assess the relative importance of these various processes. For example, active galactic nuclei are much more common than TDEs, but the latter can radiate a very large percentage of their energy as neutrinos.

As a result, “we don’t know where most of the high-energy cosmic neutrinos come from,” says physicist Marek Kowalski of Humboldt University in Germany. Knowing the origin of neutrinos would help researchers understand the extreme astrophysical events that produce some of the most energetic cosmic rays in the universe.”

Last year, Kowalski and colleagues reported the first accidental detection of neutrinos and TDEs [2]. The neutrino was spotted by the IceCube neutrino observatory, an array of detectors buried deep in the ice near the South Pole. The researchers found that the location of neutrinos in the sky is consistent with a long-lived burst of radiation that exhibits TDE signatures in archived astronomical data.

Kowalski and colleagues added to this previous finding and now report the discovery of a second TDE closely related to another neutrino, which was discovered on May 30, 2020 using IceCube.

The researchers discovered the connection by using computers to sort through a database of astronomical observations compiled by California’s Zwicky Transient Facility, which uses a wide-angle optical camera to scan the entire northern sky every two days.

During the search, the team found an event called AT2019fdr from November 2019, which was closely related to the most likely direction of the high-energy neutrino. Using data from other telescopes, they also determined the specific radiation signatures expected for TDE.

According to the researchers, this connection is strong evidence that the neutrino was created during a multi-year radiative burst caused by the interaction of a star with a black hole.

Based on preliminary statistical analysis, they believe that there is only a 0.034% chance that the direction of the neutrino coincidentally coincided with the direction of TDE. But they say further work to localize the direction of the neutrino could change that estimate.

“This is certainly an important result,” says astrophysicist Nicholas Stone of the Rakaha Physics Institute in Israel. He says that the first observed connection suggested that TDEs are sources of high-energy neutrinos, but it was difficult to be sure from just one event. “The second neutrino-TDE association now gives us much more grounds.”

According to team member Simeon Reusch, Kowalski’s graduate student, this second discovery not only strengthens confidence in the earlier discovery. It also allows a rough estimate of the contribution of TDE to the production of high-energy neutrinos.

Comparing these two observations with the full catalog of cosmic neutrinos detected by the IceCube observatory, the researchers concluded that at least 7.8% of the high-energy neutrinos must come from TDE. “Because tidal disruption events are so rare, our results indicate that they are likely to be extremely efficient neutrino factories,” says Kowalski.

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