When a star gets too close to a black hole, sparks fly. And, potentially, so do subatomic particles called neutrinos.

A dramatic light show results when a supermassive black hole rips apart a wayward star. Now, for the second time, a high-energy neutrino has been spotted that may have come from one of these “tidal disruption events,” researchers report in a study accepted in Physical Review Letters.

These lightweight particles, which have no electric charge, careen across the cosmos and can be detected upon their arrival at Earth. The origins of such zippy neutrinos are a big mystery in physics. To create them, conditions must be just right to drastically accelerate charged particles, which would then produce neutrinos. Scientists have begun lining up likely candidates for cosmic particle accelerators. In 2020, researchers reported the first neutrino linked to a tidal disruption event (SN: 5/26/20). Other neutrinos have been tied to active galactic nuclei, bright regions at the centers of some galaxies (SN: 7/12/18).
Discovered in 2019, the tidal disruption event reported in the new study stood out. “It was extraordinarily bright; it’s really one of the brightest transients ever seen,” says astroparticle physicist Marek Kowalski of Deutsches Elektronen-Synchrotron, or DESY, in Zeuthen, Germany.

Transients are short-lived flares in the sky, such as tidal disruption events and exploding stars called supernovas. Further observations of the brilliant outburst revealed that it shone in infrared, X-rays and other wavelengths of light.

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.

The matchup between the two events could be a coincidence. But when combined with the previous neutrino that was tied to a tidal disruption event, the case gets stronger. The probability of finding two such associations by chance is only about 0.034 percent, the researchers say.

It’s still not clear how tidal disruption events would produce high-energy neutrinos. In one proposed scenario, a jet of particles flung away from the black hole could accelerate protons, which could interact with surrounding radiation to produce the speedy neutrinos.

‘We need more data … in order to say that these are real neutrino sources or not,” says astrophysicist Kohta Murase of Penn State University, a coauthor of the new study. If the link between the neutrinos and tidal disruption events is real, he’s optimistic that researchers won’t have to wait too long. “If this is the case, we will see more.”

But scientists don’t all agree that the flare was a tidal disruption event. Instead, it could have been an especially bright type of supernova, astrophysicist Irene Tamborra and colleagues suggest in the April 20 Astrophysical Journal.

In such a supernova, it’s clear how energetic neutrinos could be produced, says Tamborra, of the Niels Bohr Institute at the University of Copenhagen. Protons accelerated by the supernova’s shock wave could collide with protons in the medium that surrounds the star, producing other particles that could decay to make neutrinos.

It’s only recently that observations of high-energy neutrinos and transients have improved enough to enable scientists to find potential links between the two. “It’s exciting,” Tamborra says. But as the debate over the newly detected neutrino’s origin shows, “at the same time, it’s uncovering many things that we don’t know.

A speck of gold dancing to a pipe organ’s tune has helped solve a long-standing mystery: why certain wind instruments violate a mathematical formula that should describe their sound.

In 1860, physicist Hermann von Helmholtz — famous for his law of the conservation of energy — devised an equation relating the wavelength of a pipe’s fundamental tone (the lowest frequency at which it resonates) to pipe length (SN: 3/31/28). Generally, the longer a pipe is, the lower its fundamental tone will be.

But the equation doesn’t work in practice. A pipe’s fundamental tone always sounds lower than the pipe’s length suggests it should according to Helmholtz’s formula. Fixing this problem requires adding an “end correction” to the equation. In the case of open-ended pipes such as flutes and those of organs, the end correction is 0.6 times the radius of the pipe. Why this was, nobody could figure out.

A break in the case came in 2010. Instrument builder and restorer Bernhardt Edskes of Wohlen, Switzerland was tuning an organ when he spotted a piece of gold that had come loose from a pipe’s gilded lip. Air pumping through the pipe should have carried away the gold. Instead, it seemed to be trapped in a vortex just above the pipe’s upper rim.

Edskes told his friend, physicist Leo van Hemmen of the Technical University of Munich, about the observation. Together with colleagues from Munich and Wageningen University in the Netherlands, they studied how air moves through playing organ pipes using cigarette smoke.

When an organ pipe sounds, a vortex indeed forms over the pipe’s rim, the team reported March 14 in Chicago at a meeting of the American Physical Society. What’s more, this vortex is capped by a hemisphere of resonating air.
This vibrating air cap, van Hemmen says, is the long-sought explanation for the “end correction.” The cap effectively lengthens the organ pipe by the exact amount that must be tacked on to Helmholtz’s formula to explain the pipe’s fundamental tone.