Ultra-High-Energy Neutrino Emission on the Extragalactic Express: A Mystery

Radio image from MeerKAT of a galaxy nicknamed Phaedra, one of three main suspects in a hunt for a neutrino emitter.
Adapted from Filipović et al. 2025

Title: ASKAP and VLASS Search for a Radio-Continuum Counterpart of Ultra-High-Energy Neutrino Event KM3–230213A
Authors: M. D. Filipović et al.
First Author’s Institution: Western Sydney University
Status: Published in ApJL

The Scene of the Crime

On Galentine’s Day this year, an ultra-high-energy neutrino attempted to sneak through the Mediterranean Sea, likely expecting she wouldn’t be caught. The odds were in her favor; neutrinos, ghostly particles with no electric charge and infinitesimal mass, only very rarely interact with matter. However, what she failed to account for was the awaiting undersea neutrino detector, KM3NeT, and the clever lepton within who would finally notice her. She slammed into the lepton, spewing charged particles everywhere at speeds greater than the speed of light in the water. While no particle can outrun a photon in a vacuum, water slows light down, giving us the familiar effect of refraction; similar to supersonic jets creating a boom when they break the sound barrier, these charged particles produced a distinctive blue light, known as Cherenkov light, exposing the neutrino’s position to astronomers and physicists everywhere. Busted.

The Investigation Begins

However, the neutrino was only the messenger; of even more interest is the astrophysical object that produced her. It’s not easy to generate such a high-energy particle, and no one can create a neutrino from thermal emission alone, indicating that wherever she originated, something extreme was going on. To date, only three astrophysical sources have been caught emitting neutrinos at all, and none of them are extragalactic: the Sun, although this is old news (in the 1960s, detections of solar neutrinos showed definitively that the Sun is powered by nuclear fusion, resolving the issue of how the Sun has burned long enough for life to evolve on Earth); the nearest core-collapse supernova to our galaxy in modern times, SN 1987A; and the galactic plane.

Theoretical models predict a much wider variety of objects, including extragalactic sources, to produce neutrinos, usually via cosmic-ray production: supernova remnants, star-forming galaxies, gamma-ray bursts, supermassive black holes (which are found at the centers of most galaxies), active galactic nuclei (a particularly fussy subset of supermassive black holes that are eating their host galaxies), and blazars (an extreme subset of active galactic nuclei that emit jets of radio light directly at Earth). The reason we have not detected their predicted neutrino emission is that neutrino astronomy is a new field, extragalactic sources are super far away, and neutrinos are both difficult to detect and difficult to trace back to their origin.

Rounding Up Suspects

With this in mind, today’s authors embark on a quest to catch the culprit, starting in the radio band. Radio emission, like neutrino emission, is usually an indicator of non-thermal radiative processes, and one such process, synchrotron radiation (emitted by relativistic electrons getting spun around in powerful magnetic fields), can be distinguished from other types of radiation based on its radio characteristics. Conveniently, the region our neutrino hails from is spanned by multiple radio surveys conducted with the Very Large Array (VLA) and the Australian Sub-Kilometer Compact Array Pathfinder (ASKAP), and so our authors use these surveys to round up all the radio riffraff. Unfortunately, the long wavelengths of radio photons and the scarcity of neutrinos result in reduced resolution for both compared to traditional optical telescopes, and our authors find over a thousand radio emitters in the region. Of course, no one can question that many sources, so our authors limit their investigation to objects with at least two radio brightness measurements, which can be used to calculate the brightness as a function of radio wavelength (the spectral energy distribution, which tells us about what type of radiation we see) and/or as a function of time (a light curve, which tells us if our source is variable). Our authors settle on a lineup of 10 likely blazars, any of whom could have emitted our ultra-high-energy neutrino, as well as a shortlist of prime suspects warranting further investigation: Phaedra, a spiral galaxy; Hebe, a radio galaxy; and Narcissus, an unusual compact radio emitter (see Figure 1).

Figure 1: Radio emission detected by ASKAP in the region of the sky in which the neutrino originated. Every yellow dot should be considered suspect, but the three colored squares identify the primary guilty parties: Phaedra (in blue), Hebe (in yellow), and Narcissus (in pink). Credit: Filipović et al. 2025

Phaedra: A Spiral Galaxy with a Secret?

Phaedra (Figure 2), the most radio-luminous in the area, exhibits plenty of behavior typical of a galaxy guilty of neutrino emission. For starters, she has two regions of highly concentrated radio emission, and these regions are offset from her center, making them look suspiciously like active galactic nucleus jets, which are excellent particle accelerators. Furthermore, infrared observations suggest she is a starburst galaxy, churning out stars faster than a bestselling author with a team of ghostwriters churns out books. This intense star formation could have easily been triggered by jet activity. Even more suspiciously, she is closely associated with an X-ray binary, and where there are high-energy photons, there are likely to be other high-energy particles like neutrinos and cosmic rays. Phaedra’s prospects of beating the neutrino emission allegations are not looking good; these high-energy phenomena produce buckets of high-energy particles, and even if they produce only cosmic rays, the cosmic rays are bound to crash into the surrounding dense gas and photons, creating neutrinos anyway.

Figure 2: Radio image of Phaedra, one of our suspects. The east and west components are the likely radio jets, and the third bright blob is the radio counterpart to the X-ray binary, SXPS J062657.7-082939. Adapted from Filipović et al. 2025

Hebe: A Simple Radio Galaxy, or Something More?

Hebe (Figure 3), the nearest extended radio source, isn’t exactly innocent-looking either. She is one of a triplet of galaxies sharing a common envelope, like peas in an extragalactic pod. Galaxies, unlike peas, however, are so massive that they can’t help but interact dynamically in such close quarters, causing a commotion that could totally produce ultra-high-energy neutrinos. She likely also has an active galactic nucleus jet, giving her the same neutrino-wielding powers as Phaedra.

Figure 3: An infrared image of Hebe that clearly shows the common envelope surrounding the triplets. The white contour lines denote levels of polarized intensity, which indicate the presence of a magnetic field. Adapted from Filipović et al. 2025

Narcissus: Double Active Galactic Nucleus?

Our final suspect, Narcissus (Figure 4), consists of not one, but two active galactic nuclei. One appears to exhibit the classic synchrotron spectral energy distribution, and the other is likely a blazar, based on his notable radio variability and infrared observations.

Figure 4: Infrared image of Narcissus, with the purple contours outlining the two radio sources that are likely active galactic nuclei. Adapted from Filipović et al. 2025

Solving the Mystery

So, who really emitted the ultra-high-energy neutrino? For now, our authors can’t jump to any firm conclusions — they’d never risk condemning an innocent galaxy — but they will continue to closely monitor the suspects and gather more evidence. In the meantime, Phaedra, Hebe, and Narcissus should find themselves a good defense attorney experienced in neutrino emission cases.

Original astrobite edited by Sandy Chiu.

Source: American Astronomical Society,SAmerican Astronomical Society/AAS-Nova

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Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

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About the author, Chloe Klare:

I’m a PhD student in astronomy and astrophysics at Penn State (with a physics minor, so I get to use my semester spent in QFT for something!). I study active galactic nuclei (in the radio!), and I’m currently looking for baby synchrotron jets in active galactic nuclei.

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Possible Extragalactic Source of High-Energy Neutrinos

Coincidence of a highly energetic outburst of an active galactic nucleus with a neutrino event at PeV energy

Nearly 10 billion years ago in a galaxy known as PKS B1424-418, a dramatic explosion occurred. Light from this blast began arriving at Earth in 2012. Now, an international team of astronomers, including scientists from the Max Planck Institute for Radio Astronomy in Bonn, have shown that a record-breaking neutrino seen around the same time likely was born in the same event. The results are published in "Nature Physics".

Fermi LAT images showing the gamma-ray sky around the blazar PKS B1424-418. Brighter colors indicate greater numbers of gamma rays. The dashed arc marks part of the source region established by IceCube for the Big Bird neutrino (50-percent confidence level). Left: An average of LAT data centered on July 8, 2011 covering 300 days when the blazar was inactive. Right: An average of 300 active days centered on Feb. 27, 2013, when PKS B1424-418 was the brightest blazar in this part of the sky. © NASA/DOE/LAT-Kollaboration

Neutrinos are the fastest, lightest and most unsociable understood fundamental particles, and scientists are just now capable of detecting high-energy ones arriving from deep space. The present work provides the first plausible association between a single extragalactic object and one of these cosmic neutrinos.

Although neutrinos far outnumber all the atoms in the universe, they rarely interact with matter, which makes detecting them quite a challenge. But this same property lets neutrinos make a fast exit from places where light cannot easily escape -- such as the core of a collapsing star -- and zip across the universe almost completely unimpeded. Neutrinos can provide information about processes and environments that simply aren't available through a study of light alone.

Recently, the IceCube Neutrino Observatory at the South Pole found first evidence for a flux of extraterrestrial neutrinos, which was named the Physics World breakthrough of the year 2013. To date, the science team of IceCube Neutrino has announced about a hundred very high-energy neutrinos and nicknamed the most extreme events after characters on the children's TV series "Sesame Street." On Dec. 4, 2012, IceCube detected an event known as Big Bird, a neutrino with an energy exceeding 2 quadrillion electron volts (PeV). To put that in perspective, it's more than a million million times greater than the energy of a dental X-ray packed into a single particle thought to possess less than a millionth the mass of an electron. Big Bird was the highest-energy neutrino ever detected at the time and still ranks second.

Where did it come from? The best IceCube position only narrowed the source to a patch of the southern sky about 32 degrees across, equivalent to the apparent size of 64 full moons. “It’s like a crime scene investigation”, says lead author Matthias Kadler, a professor of astrophysics at the University of Würzburg in Germany, “The case involves an explosion, a suspect, and various pieces of circumstantial evidence.”

Starting in the summer of 2012, NASA’s Fermi satellite witnessed a dramatic brightening of PKS B1424-418, an active galaxy classified as a gamma-ray blazar. An active galaxy is an otherwise typical galaxy with a compact and unusually bright core. The excess luminosity of the central region is produced by matter falling toward a supermassive black hole weighing millions of times the mass of our sun. As it approaches the black hole, some of the material becomes channeled into particle jets moving outward in opposite directions at nearly the speed of light. In blazars one of these jets happens to point almost directly toward Earth.

During the year-long outburst, PKS B1424-418 shone between 15 and 30 times brighter in gamma rays than its average before the eruption. The blazar is located within the Big Bird source region, but then so are many other active galaxies detected by Fermi.

These radio images from the TANAMI project reveal the 2012-2013 eruption of PKS B1424-418 at a radio frequency of 8.4 GHz. The core of the blazar’s jet brightened by four times, producing the most dramatic blazar outburst TANAMI has observed to date. © TANAMI Collaboration

The scientists searching for the neutrino source then turned to data from a long-term observing program named TANAMI. Since 2007, TANAMI has routinely monitored nearly 100 active galaxies in the southern sky, including many flaring sources detected by Fermi. Three radio observations between 2011 and 2013 cover the period of the Fermi outburst. They reveal that the core of the galaxy's jet had been brightening by about four times. “No other of our galaxies observed by TANAMI over the life of the program has exhibited such a dramatic change”, explains Eduardo Ros, from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany.

Within their jets, blazars are capable of accelerating protons to relativistic energies. Interactions of these protons with light in the central regions of the blazar can create pions. When these pions decay, both gamma rays and neutrinos are produced. "We combed through the field where Big Bird must have originated looking for astrophysical objects capable of producing high-energy particles and light," says coauthor Felicia Krauß, a doctoral student at the University of Erlangen-Nürnberg in Germany. "There was a moment of wonder and awe when we realized that the most dramatic outburst we had ever seen in a blazar happened in just the right place at just the right time."

In a paper published Monday, April 18, in Nature Physics, the team suggests the PKS B1424-418 outburst and Big Bird are linked, calculating only a 5-percent probability the two events occurred by chance alone. Using data from Fermi, NASA’s Swift and WISE satellites, the LBA and other facilities, the researchers determined how the energy of the eruption was distributed across the electromagnetic spectrum and showed that it was sufficiently powerful to produce a neutrino at PeV energies.

"Taking into account all of the observations, the blazar seems to have had means, motive and opportunity to fire off the Big Bird neutrino, which makes it our prime suspect," explains Matthias Kadler.

Francis Halzen, the principal investigator of IceCube at the University of Wisconsin–Madison, and not involved in this study, thinks the result is an exciting hint of things to come. "IceCube is about to send out real-time alerts when it records a neutrino that can be localized to an area a little more than half a degree across, or slightly larger than the apparent size of a full moon," he says. "We're slowly opening a neutrino window onto the cosmos." "This study demonstrates the vital importance of classical astronomical observations in an era when new detection methods like neutrino observatories and gravitational-wave detectors open new but unknown skies", concludes Anton Zensus, director at MPIfR and head of its Radio Astronomy/VLBI research department, also a coauthor of the study.

Source: Max Planck Institute for Radio Astronomy

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Local Contacts

Prof. Dr. Eduardo Ros

Phone:+49 228 525-125

Email: ros@mpifr-bonn.mpg.de  
Max-Planck-Institut für Radioastronomie, Bonn

Prof. Dr. J. Anton Zensus
Director and Head of "Radio Astronomy/VLBI" Research Dept.
Phone: +49 228 525-298 (secretary)
Email: azensus@mpifr-bonn.mpg.de

Max-Planck-Institut für Radioastronomie, Bonn

Dr. Norbert Junkes
Press and Public Outreach Phone:+49 228 525-399
Email: njunkes@mpifr-bonn.mpg.de
Max-Planck-Institut für Radioastronomie, Bonn
 

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Original Paper 

Coincidence of a high-fluence blazar outburst with a PeV-energy neutrino event

 

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Links:

• TANAMI
• IceCube
• Fermi
• Univ. Würzburg
• Radio Astronomy / VLBI

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TANAMI is a multiwavelength monitoring program of active galaxies in the Southern sky. It includes regular radio observations using the Australian Long Baseline Array (LBA) and associated telescopes in Chile, South Africa, New Zealand and Antarctica. When networked together, they operate as a single radio telescope more than 6,000 miles across and provide a unique high-resolution look into the jets of active galaxies.

The IceCube Neutrino Observatory, built into a cubic kilometer of clear glacial ice at the South Pole, detects neutrinos when they interact with atoms in the ice. This triggers a cascade of fast-moving charged particles that emit a faint glow, called Cerenkov light, as they travel, which is picked up by thousands of optical sensors strung throughout IceCube. Scientists determine the energy of an incoming neutrino by the amount of light its particle cascade emits.

NASA's Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership, developed in collaboration with the U.S. Department of Energy and with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.

MPIfR scientists involved in the project are Eduardo Ros and J. Anton Zensus.

Sagittarius A*: NASA X-ray Telescopes Find Black Hole May Be a Neutrino Factory

Sagittarius A*  

Credit: NASA/CXC/Univ. of Wisconsin/Y.Bai. et al.

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The supermassive black hole at the center of the Milky Way, seen in this image from NASA's Chandra X-ray Observatory, may be producing mysterious particles called neutrinos, as described in our latest press release. Neutrinos are tiny particles that have virtually no mass and carry no electric charge. Unlike light or charged particles, neutrinos can emerge from deep within their sources and travel across the Universe without being absorbed by intervening matter or, in the case of charged particles, deflected by magnetic fields.

While the Sun produces neutrinos that constantly bombard the Earth, there are also other neutrinos with much higher energies that are only rarely detected. Scientists have proposed that these higher-energy neutrinos are created in the most powerful events in the Universe like galaxy mergers, material falling onto supermassive black holes, and the winds around dense rotating stars called pulsars.

Using three NASA X-ray telescopes, Chandra, Swift, and NuSTAR, scientists have found evidence for one such cosmic source for high-energy neutrinos: the 4-million-solar-mass black hole at the center of our Galaxy called Sagittarius A* (Sgr A*, for short). After comparing the arrival of high-energy neutrinos at the underground facility in Antarctica, called IceCube, with outbursts from Sgr A*, a team of researchers found a correlation. In particular, a high-energy neutrino was detected by IceCube less than three hours after astronomers witnessed the largest flare ever from Sgr A* using Chandra. Several flares from neutrino detections at IceCube also appeared within a few days of flares from the supermassive black hole that were observed with Swift and NuSTAR.

This Chandra image shows the region around Sgr A* in low, medium, and high-energy X-rays that have been colored red, green, and blue respectively. Sgr A* is located within the white area in the center of the image. The blue and orange plumes around that area may be the remains of outbursts from Sgr A* that occurred millions of years ago. The flares that are possibly associated with the IceCube neutrinos involve just the Sgr A* X-ray source.

This latest result may also contribute to the understanding of another major puzzle in astrophysics: the source of high-energy cosmic rays. Since the charged particles that make up cosmic rays are deflected by magnetic fields in our Galaxy, scientists have been unable to pinpoint their origin. The charged particles accelerated by a shock wave near Sgr A* may be a significant source of very energetic cosmic rays.

The paper describing these results was published in Physical Review D and is also available online. The authors of the study are Yang Bai, Amy Barger, Vernon Barger, R. Lu, Andrea Peterson, J. Salvado, all from the University of Wisconsin, in Madison, Wisconsin.

NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations.

Fast Facts for Sagittarius A*: 


Release Date: November 13, 2014
Scale: Image is about 12 arcmin across (about 91 light years)
Category: Black Holes, Milky Way Galaxy
Coordinates (J2000): RA 17h 45m 40s | Dec -29° 00' 28.00"
Constellation: Sagittarius
Observation Date: 43 pointings from September 21, 1999 to May 18, 2009
Observation Time: 278 hours (11 days 14 hours).
Obs. ID: 242, 1561, 2943, 2951-2954, 3392, 3393, 3549, 3663, 3665, 4683, 4684, 5360, 5950-5954, 6113, 6363, 6639, 6640-6646, 7554-7759, 9169-9174, 10556
Instrument: ACIS
Also Known As: Galactic Center
References: Bai, et al, 2014, Physics Review D, 90, 063012; arXiv:1407.2243
Color Code: Energy: Red (2-3.3 keV), Green (3.3-4.7 keV), Blue (4.7-8 keV)
Distance Estimate: About 26,000 light years

Source: NASA’s Chandra X-ray Observatory

A new neutrino-emission asymmetry in forming neutron stars

Figure 1: Evolution of the neutrino emission asymmetry in a collapsing star of 11.2 solar masses. The ellipses represent the whole surface of the nascent neutron star (analog to world maps as planar projections of the Earth's surface). Red and yellow mean a large excess of electron neutrinos compared to electron antineutrinos normalized to the average, blue means a low excess or even deficit of electron neutrinos. The images show the merging of smaller patches that are present at 0.148 seconds (upper left panel) to a clear hemispheric (dipolar) anisotropy at 0.240 seconds (lower right panel). The dot and cross indicate the emission maximum and minimum, the dark-grey line marks the path described by a slow motion of the dipole direction.

Figure 2: Bubbles of "boiling" gas surrounding the nascent neutron star (invisible at the center). Despite the highly time-variable and dynamical pattern of plumes of hot, rising gas surrounded by inflows of cooler matter, the neutrino emission develops a hemispheric asymmetry that remains stable for periods of time much longer than the life time of individual bubbles.

Figure 3: Neutrino emission asymmetry as observable over a period of 0.1 seconds. Analog to Fig. 1 the ellipses show all possible viewing directions. Observers in the red regions see the highest emission whereas those in the blue areas receive lower emission. While the hemispheric difference of the emission of electron neutrinos plus antineutrinos is only one to two per cent (top), the differences of electron neutrinos (middle) and antineutrinos (bottom) individually amount to up to 20 per cent of the maximum values with extrema on opposite sides. 

The neutron star that is born at the center of a collapsing and exploding massive star radiates huge numbers of neutrinos produced by particle reactions in the extremely hot and dense matter. Three-dimensional supercomputer simulations at the very forefront of current modelling efforts reveal the stunning and unexpected possibility that this neutrino emission can develop a hemispheric (dipolar) asymmetry. If this new neutrino-hydrodynamical instability happens in nature, it will have important consequences for the formation of chemical elements in stellar explosions, for imparting kicks to the neutron star, and for the perspective of detecting neutrinos from a future galactic supernova.

Stars with more than roughly eight times the mass of our sun end their lives in gigantic explosions, so-called supernovae. These spectacular events belong to the most energetic and brightest phenomena in the universe and can outshine a whole galaxy for weeks. Supernovae are not only the cosmic sources of chemical elements like carbon, oxygen, and silicon, which are fused over millions of years of quiescent stellar evolution and disseminated into circumstellar space by the blast wave. Supernovae are also important producers of iron and heavier trans-iron elements, which can be freshly assembled during the explosion. 

While supernovae eject most of the material of the dying star, the stellar core of iron collapses under the influence of its own gravity within fractions of a second to an extraordinarily exotic, ultra-compact remnant, a neutron star. Such an object contains about 1.5 times the mass of our Sun, compressed into a sphere with the diameter of Munich. The central density of a neutron star exceeds that in atomic nuclei, gigantic 300 million tons (the weight of a mountain) in the volume of a sugar cube. 

The matter in newly born neutron stars is extremely hot, up to temperatures of more than 500 billion degrees. At such conditions particle reactions involving neutrons, protons, electrons and positrons (the anti-particles of electrons) create huge numbers of neutrinos. Cooling neutron stars thus radiate a total of 10⁵⁸ of these uncharged, nearly massless elementary particles, which interact extremely rarely with matter on earth. Only one of a billion neutrinos coming from a supernova (or from the sun, which also produces neutrinos in the nuclear fusion "reactor" that burns at its center) hits a particle somewhere inside the earth, all the others cross through the whole of the earth's body without a single collision. 

Neutron stars release neutrinos and their anti-particles in three different flavors, corresponding to the three known families of charged leptons: electron neutrinos, muon neutrinos and tau neutrinos. These neutrinos are expected to be radiated equally in all directions, because neutron stars are nearly perfectly spherical objects due to their extremely strong gravitational fields. Most previous computer models of neutron star formation therefore assumed spherical symmetry. Only recently the first three-dimensional simulations with a detailed treatment of the complex neutrino physics have become possible due to the increased power of modern supercomputers (see here ). 

As expected, the neutrino emission starts out to be basically spherical except for smaller variations over the surface (see Fig. 1, upper left panel). These variations correspond to higher and lower temperatures associated with violent "boiling" of hot matter inside and around the newly formed neutron star, by which bubbles of hot matter rise outward and flows of cooler material move inward (Fig. 2). After a short while and gradually growing, however, the neutrino emission develops clear differences in two hemispheres. The initially small patches merge to larger areas of warmer and cooler medium until the two hemispheres begin to radiate neutrinos unequally. A stable dipolar pattern is established, which means that on one side more neutrinos leave the neutron star than on the other side. Observers in different directions will thus receive different neutrino signals. While the directional variation of the summed emission of all kinds of neutrinos is only some per cent (Fig. 3 top), the individual neutrino types (for example electron neutrinos or electron antineutrinos) show considerable contrast between the two hemispheres with up to about 20 per cent deviations from the average (Figs. 3 middle, 3 bottom). The directional variations are particularly pronounced in the difference between electron neutrino and antineutrino fluxes (Fig. 1, lower right panel), the so-called lepton number emission. 

The possibility of such a global anisotropy in the neutrino emission was not predicted and its finding in the first-ever detailed three-dimensional simulations of dynamical neutron-star formation comes completely unexpectedly. The phenomenon exhibits astonishing properties: In spite of ongoing violent bubbling motions of the "boiling" hot and cooler gas, which lead to rapidly changing structures in the flow around and inside the neutron star (Fig. 2), the dipolar neutrino emission asymmetry establishes itself in a stable state. It thus exists for long periods of time, during which only a slow and moderate drift of its orientation can be observed (cf. the thin, dark-grey line in Fig. 1). The team of astrophysicists named this new phenomenon "LESA" for Lepton-Emission Self-sustained Asymmetry, because the emission dipole seems to stabilize and maintain itself through complicated feedback effects: Interactions with the asymmetric neutrino flow affect the collapse of the stellar core such that a hemispheric asymmetry develops in the matter falling inward to the nascent neutron star. This accretion asymmetry then continues to feed additional anisotropic emission of neutrinos. These findings suggest that the spherical collapse of a stellar core to a neutron star is not stable but the system wants to rearrange itself into a dipolar asymmetry mode. 

If LESA happens in collapsing stellar cores, it will have important consequences for observable phenomena connected to supernova explosions. Neutrinos radiated from the nascent neutron star interact with the innermost material that gets ejected by the supernova blast wave. In doing so, neutrinos determine the ratio of neutrons to protons in the expelled plasma, which is a crucial requisite for the subsequent formation of heavy elements when the outflow cools. A directional variation between electron neutrino and antineutrino emission will thus lead to differences of the chemical element production in different directions. Moreover, a global dipolar anisotropy of the neutrino emission carries away momentum and thus imparts a kick to the nascent neutron star in the opposite direction. Because of the huge number of escaping neutrinos, an emission asymmetry of only one per cent, if lasting for several seconds, could account for a neutron-star recoil of 100 kilometers per second. Also the neutrino signal arriving at Earth from the next supernova event in our Milky Way must be expected to depend on the angle from which we observe the supernova. Detailed predictions of measurements with big underground facilities like the IceCube detector at the South Pole and the SuperKamiokande experiment in Japan therefore need to take into account the directional variations found in the new three-dimensional models. 

However, the stunning neutrino-hydrodynamical instability that manifests itself in the LESA phenomenon is not yet well understood. Much more research is needed to ensure that it is not an artefact produced by the highly complex numerical simulations. If it is physical reality, this novel effect would be a discovery truly based on the use of modern supercomputing possibilities and not anticipated by previous theoretical considerations.

Hans-Thomas Janka

References:

Self-sustained asymmetry of lepton-number emission: A new phenomenon during the supernova shock-accretion phase in three dimensions; Tamborra I., Hanke F., Janka H.-Th., Müller B., Raffelt G.G., Marek, A.; Astrophys. Journal 792, 96 (2014), http://arxiv.org/abs/1402.5418

Neutrino signature of supernova hydrodynamical instabilities in three dimensions; Tamborra I., Hanke F., Müller B., Janka H.-Th., Raffelt G.; Physical Review Letters 111, 121104 (2013), http://arxiv.org/abs/1307.7936

Neutrino emission characteristics and detection opportunities based on three-dimensional supernova simulations; Tamborra I., Raffelt G., Hanke F., Janka H.-Th., Müller B.; Physical Review D 90, 045032 (2014), http://arxiv.org/abs/1406.0006

Source:  Max Planck Institute for Astrophysics