Sparks fly when a star gets too close to a black hole. And, theoretically, so do neutrinos, which are subatomic particles. When a supermassive black hole rips apart a stray star, it produces a spectacular light show. A high-energy neutrino has been detected for the second time, possibly as a result of one of these “tidal disruption events,” according to a study published in Physical Review Letters.
These light particles, which have no electric charge, whizz through space and can be detected when they arrive on Earth. The sources of such fast neutrinos remain a mystery in physics. Conditions must be ideal in order to dramatically accelerate charged particles, which would subsequently yield neutrinos. Scientists have begun to identify potential cosmic particle accelerator possibilities. In 2020, scientists reported the first neutrino associated with a tidal disruption event. Other neutrinos have been linked to active galactic nuclei, which are luminous spots at the heart of some galaxies.
The tidal disruption event documented in the latest study was discovered in 2019. “It was exceptionally intense; it’s truly one of the brightest transients ever witnessed,” astroparticle physicist Marek Kowalski of the Deutsches Elektronen-Synchrotron in Zeuthen, Germany, says.
It’s obvious how energetic neutrinos may be produced in such a supernova. Protons driven by the shock wave of a supernova could collide with protons in the medium that surrounds the star, producing additional particles that could decay to produce neutrinos.
Kohta Murase
Transients are brief bursts of light in the sky, such as tidal disruption events and exploding stars known as supernovas. Additional studies of the spectacular explosion revealed that it shined in infrared, X-rays, and other light wavelengths.
Roughly a year after the flare’s discovery, the Antarctic neutrino observatory IceCube spotted a high-energy neutrino. By tracing the particle’s path backward, researchers determined that the neutrino came from the flare’s vicinity.
Neutrinos are fundamental particles that outnumber all atoms in the cosmos yet interact with other matter only infrequently. High-energy neutrinos, which have energies up to 1,000 times larger than those produced by the most powerful particle colliders on Earth, are of particular interest to physicists. They believe that the universe’s most extreme events, such as massive galactic outbursts, accelerate particles to nearly the speed of light. When these particles clash with light or other particles, they produce high-energy neutrinos. The first confirmed high-energy neutrino source, disclosed in 2018, was a blazar, a type of active galaxy.
The coincidence between the two incidents is possible. However, when combined with the earlier neutrino linked to a tidal disruption event, the evidence becomes stronger. According to the researchers, the probability of discovering two such connections by chance is only about 0.034 percent.
It is currently unclear how tidal disruption events would generate high-energy neutrinos. In one possibility, a jet of particles ejected from the black hole may accelerate protons, which would then combine with surrounding radiation to produce the fast neutrinos.
‘We need additional data… to say if these are true neutrino sources or not,’ says Penn State University astronomer Kohta Murase, a coauthor of the new study. If the link between neutrinos and tidal disruption events is genuine, he believes researchers will not have to wait too long. “If that’s the case, we’ll see a lot more.”
However, scientists are divided on whether the flare was caused by tidal disruption. Instead, astronomer Irene Tamborra and colleagues speculate in the Astrophysical Journal that it could have been a very luminous form of supernova.
According to Tamborra of the Niels Bohr Institute at the University of Copenhagen, it’s obvious how energetic neutrinos may be produced in such a supernova. Protons driven by the shock wave of a supernova could collide with protons in the medium that surrounds the star, producing additional particles that could decay to produce neutrinos.
Observations of high-energy neutrinos and transients have only lately improved enough to allow scientists to identify potential linkages between the two. Tamborra describes the situation as “exciting.” However, as the argument over the origin of the newly discovered neutrino demonstrates, “at the same time, it’s discovering many things that we don’t know.”
X-ray observations revealed that, while this TDE produced more X-rays than most others, they faded quickly around the time the neutrino was released. Winter and Lunardini hypothesize that this is due to the photon cloud concealing the X-rays while simultaneously providing something for the protons in the jet to smash into in order to generate neutrinos.
“If this is true, then we know that TDEs are a significant source of neutrinos, which is a novel finding,” Lunardini says. “It implies that TDEs that are exceptionally brilliant in X-rays should be of particular interest, and we should perhaps look into them more.”
