Scientists traced an incoming neutrino back to its galactic birthplace | Science News for Students

Scientists traced an incoming neutrino back to its galactic birthplace

Detectors beneath the South Pole spotted the high-energy particle as it slammed into the ice
Aug 8, 2018 — 6:45 am EST
an illustration of a high-energy neutrino shooting out of a blazar

Scientists traced a high-energy neutrino back to its birthplace: a blazar. That galaxy harbors a supermassive black hole that fuels powerful jets of particles.

Science Communication Lab/DESY

A zippy little subatomic particle has been traced back to its source. This neutrino was born in a flaring galaxy 4 billion light-years away. This discovery solved a longstanding cosmic whodunit.

Scientists had long puzzled where certain high-energy particles from space were born. These bits of matter can batter Earth at energies that outstrip the world’s most advanced particle accelerators. Now, physicists report finding the source of an energetic neutrino. This cosmic voyager came from a type of distant bright galaxy called a blazar.  Owing to a powerful black hole at its core, this type of galaxy flings out particles. They fly across the cosmos blindingly fast — some at nearly the speed of light

Other cosmic sources for high-energy neutrinos also may exist. But on July 12, scientists announced online, in Science, the discovery that blazars create some of those neutrinos.

“This is super exciting news,” says Angela Olinto at the University of Chicago in Illinois. She’s an astrophysicist who was not involved with the new research. The new finding, she says, marks “the beginning of what we call neutrino astronomy.” It uses the lightweight — indeed, nearly massless — neutrinos to unveil secrets of cosmic oddities (such as those blazars).

The new data suggest that blazars also emit cosmic rays. This second type of energetic particle is produced together with neutrinos. Until now, where high-energy cosmic rays come from has been poorly understood. In fact, until now, “nobody has ever been able to pinpoint a source [of] them,” says Francis Halzen. He’s an astrophysicist at the University of Wisconsin–Madison. He’s also a research leader of IceCube, the Antarctic neutrino observatory that detected the particle.

an image of the IceCube Particle detector at the South Pole
The IceCube particle detector at the South Pole uses sensors in the Antarctic ice to spot high-energy neutrinos arriving from sources outside the Milky Way.
The IceCube Collaboration

IceCube is located at the South Pole. It was constructed within a cubic kilometer (a quarter cubic mile) of ice. Thousands of sensors embedded within the ice measure the light produced when neutrinos slam into ice. On September 22, 2017, IceCube detected a neutrino with an energy of nearly 300 trillion electron volts! (For comparison, high energy protons at the Large Hadron Collider in Geneva, Switzerland, reach only some 6.5 trillion electron volts.)

Hunting the neutrino’s birthplace

Physicists traced the neutrino’s path backward. In this way, they zeroed in on a patch of sky. It resides in the direction of the constellation Orion.

Afterward, astronomers leapt into action. They trained telescopes around the world at the same patch of sky. They used these to hunt for light that might reveal the neutrino’s source.

a star map showing the source of a high-energy neutrino just under the arm of the constellation Orion
For the first time, a high-energy neutrino has been traced to a source outside of the Milky Way. It was from a galaxy in the constellation of Orion (location indicated in blue).
The IceCube Collaboration

In that patch of sky, the Fermi Gamma-ray Space Telescope spotted a flare of gamma rays. These rays are a type of high-energy light. Those gamma rays were coming from a blazar. An enormous black hole powers that galaxy, sending radiation blazing toward Earth. Other telescopes then observed the blazar’s flare in wavelengths longer than gamma rays (such as X-rays and radio waves).

High-energy neutrinos with a well-defined incoming path are rare. IceCube sent astronomers only 10 reports of such detections in the 18 months before this neutrino showed up. And this neutrino marked the first time researchers were lucky enough to also spot its source.

“This is really what IceCube was built for — to try to see high-energy neutrinos from these exotic sources,” says Kate Scholberg. She is a neutrino physicist at Duke University in Durham, N.C., but was not involved with the new research.

Previously, scientists had found birthplaces for of neutrinos with much lower energies. One was an exploding star. The other was the sun. But high-energy neutrinos have been more elusive. While there had been hints of high-energy neutrinos coming from blazars, the new detection is the first to solidly link blazars to high-energy neutrinos. 

Researchers then went back to the IceCube data and looked for more neutrinos that might have come from the blazar. “There was something interesting happening there,” says Naoko Kurahashi Neilson. She’s an IceCube researcher at Drexel University in Philadelphia, Pa. Over seven months starting in September 2014, IceCube picked up a neutrino flare — an excess of high-energy neutrinos — from that vicinity. Researchers have now described this flare in a second paper, published July 13 in Science.

Cosmic mystery remains

an illustration of the sensors used to detect neutrinos
Sensors embedded in ice (illustrated) are used in the IceCube experiment to detect light emitted when a neutrino interacts with the ice.
The IceCube Collaboration

Blazars remain poorly understood. One mystery: What types of particles do they blast out? Because high-energy neutrinos must be produced along with protons, finding these neutrinos means that blazars must also create cosmic rays. (Cosmic rays consist of protons and atomic nuclei.)

Ultrahigh energy cosmic rays have been recorded coming in to Earth. Scientists have puzzled over what could rev up particles to such extreme energy levels. “This may be a clue to their origin,” says Floyd Stecker. He’s an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Md. Still, he adds, it’s not clear whether blazars can accelerate protons to the highest energies seen arriving from space.

Those ultra-high-energy cosmic rays are known to come from outside the Milky Way. In general, cosmic rays leave few clues of their source. Why? As they travel through space, their paths can be twisted by magnetic fields that they encounter along the way. Later, their trajectories will no longer point reliably back to their sources.

Neutrinos, however, have no electrical charge. So magnetic fields will not alter their path. That means they will travel in a straight line from their origin. Since high-energy cosmic rays and neutrinos are created together, blazar-born neutrinos can help scientists better understand cosmic rays, too, Olinto says. “What neutrinos gave us is a way through the fog.”

Power Words

(more about Power Words)

accelerator     (in physics) Also known as a particle accelerator, this massive machine revs up the motion of subatomic particles to great speed, and then beams them at targets. Sometimes the beams are used to deliver radiation at a tissue for cancer treatment. Other times, scientists crash the particles into solid targets in hopes of breaking the particles into their building blocks.

astronomy     The area of science that deals with celestial objects, space and the physical universe. People who work in this field are called astronomers.

astrophysics     An area of astronomy that deals with understanding the physical nature of stars and other objects in space. People who work in this field are known as astrophysicists.

atomic     Having to do with atoms, the smallest possible unit that makes up a chemical element.

black hole     A region of space having a gravitational field so intense that no matter or radiation (including light) can escape.

blazar     A bright and distant active galaxy that shoots powerful jets of radiation from its center and directly toward Earth.

collider     (in physics) Sometimes called an “atom smasher,” it is a type of particle accelerator that speeds up charged particles (ions) through an electric field inside a hollow tube or racetrack-shaped structure. Eventually the device will direct the ions to collide with an unmoving target or another beam of moving particles. The ensuing collisions force some particles to interact — and break apart or briefly bind. Some of smashed particle also may recombine, creating new particles. The biggest of these machines are used to hunt for the basic building blocks of all nature.

constellation     Patterns formed by prominent stars that lie close to each other in the night sky. Modern astronomers divide the sky into 88 constellations, 12 of which (known as the zodiac) lie along the sun’s path through the sky over the course of a year. Cancri, the original Greek name for the constellation Cancer, is one of those 12 zodiac constellations.

core     Something — usually round-shaped — in the center of an object.

cosmic     An adjective that refers to the cosmos — the universe and everything within it.

cosmic rays     Very high-energy particles, mostly protons, that bombard Earth from all directions. These particles originate outside our solar system. They are equivalent to the nucleus of an atom. They travel through space at high rates of speed (often close to the speed of light).

cosmos     (adj. cosmic) A term that refers to the universe and everything within it.

data     Facts and/or statistics collected together for analysis but not necessarily organized in a way that gives them meaning. For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.

electron     A negatively charged particle, usually found orbiting the outer regions of an atom; also, the carrier of electricity within solids.

exotic     An adjective to describe something that is highly unusual, strange or foreign (such as exotic plants).

field     (in physics) A region in space where certain physical effects operate, such as magnetism (created by a magnetic field), gravity (by a gravitational field), mass (by a Higgs field) or electricity (by an electrical field).

galaxy     A massive group of stars bound together by gravity. Galaxies, which each typically include between 10 million and 100 trillion stars, also include clouds of gas, dust and the remnants of exploded stars.

gamma rays     High-energy radiation often generated by processes in and around exploding stars. Gamma rays are the most energetic form of light.

hadron     One of a group of particles that are made up of other, smaller particles — quarks — held together by a particular kind of force. The protons and neutrons found in the nucleus of atoms are hadrons.

light-year     The distance light travels in one year, about 9.48 trillion kilometers (almost 6 trillion miles). To get some idea of this length, imagine a rope long enough to wrap around the Earth. It would be a little over 40,000 kilometers (24,900 miles) long. Lay it out straight. Now lay another 236 million more that are the same length, end-to-end, right after the first. The total distance they now span would equal one light-year.

link     A connection between two people or things.

magnetic field     An area of influence created by certain materials, called magnets, or by the movement of electric charges.

matter     Something that occupies space and has mass. Anything on Earth with matter will have a property described as "weight."

Milky Way     The galaxy in which Earth’s solar system resides.

NASA     Short for the National Aeronautics and Space Administration. Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It also has sent research craft to study planets and other celestial objects in our solar system.

neutrino     A subatomic particle with a mass close to zero. Neutrinos rarely react with normal matter. Three kinds of neutrinos are known.

observatory     (in astronomy) The building or structure (such as a satellite) that houses one or more telescopes.

online     (n.) On the internet. (adj.) A term for what can be found or accessed on the internet.

particle     A minute amount of something.

physicist     A scientist who studies the nature and properties of matter and energy.

proton     A subatomic particle that is one of the basic building blocks of the atoms that make up matter. Protons belong to the family of particles known as hadrons.

radiation     (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.

radio waves     Waves in a part of the electromagnetic spectrum. They are a type that people now use for long-distance communication. Longer than the waves of visible light, radio waves are used to transmit radio and television signals. They also are used in radar.

ray     (in mathematics) A line that has a defined endpoint on one side, but the other side continues on forever.

sensor     A device that picks up information on physical or chemical conditions — such as temperature, barometric pressure, salinity, humidity, pH, light intensity or radiation — and stores or broadcasts that information. Scientists and engineers often rely on sensors to inform them of conditions that may change over time or that exist far from where a researcher can measure them directly.

speed of light     A constant often used in physics, corresponding to 1.08 billion kilometers (671 million miles) per hour.

star     The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

subatomic     Anything smaller than an atom, which is the smallest bit of matter that has all the properties of whatever chemical element it is (like hydrogen, iron or calcium).

sun     The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy. Also a term for any sunlike star.

telescope     Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.

trillion     A number representing a million million — or 1,000,000,000,000 — of something.

wave     A disturbance or variation that travels through space and matter in a regular, oscillating fashion.

wavelength     The distance between one peak and the next in a series of waves, or the distance between one trough and the next. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.

X-ray     A type of radiation analogous to gamma rays, but having somewhat lower energy.


Journal:​ M. G. Aartsen et al. Multimessenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A. Science. Published online July 12, 2018. doi: 10.1126/science.aat1378.

Journal: M. G. Aartsen et al. Neutrino emission from the direction of the blazar TXS 0506+056 prior to the IceCube-170922A alert. Science. Vol. 361, July 13, 2018, p. 147. doi: 10.1126/science.aat2890.