For the first time, astronomers have imaged a strange S-shaped jet coming from a neutron star. The strange emission suggests that this dead star resembles the shape of water sprayed from a garden sprinkler.
The “space garden sprinkler” in question is a neutron star located in the binary system Circinus X-1, located more than 30,000 light-years from Earth. It was born when a star at least eight times the size of the Sun died in a supernova explosion, the light of which would have reached Earth nearly 5,000 years ago around the time Stonehenge was being built.
A neutron star feeds on its companion star like a cosmic vampire, causing it to erupt in high-energy jets. These jets get their S-shaped formation from the fact that the dead vampire star oscillates, or “precesses,” like a spinning top as it feeds. The team behind the research hope that this first-of-its-kind observation could help them better understand the extreme physics around neutron stars found nowhere else in the universe.
The S-shaped jet was spotted by astronomers from the University of Oxford using the MeerKAT radio telescope located in South Africa. The data they collected was used to create the most detailed, high-resolution images of Circinus X-1 to date.
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“This image is the first time we have seen strong evidence of a precessing jet from a confirmed neutron star,” team leader Fraser Cowie, an Oxford researcher, said in a statement. “This evidence comes from both the symmetrical S-shape of the radio-emitting plasma in the jets and the fast, broad shock wave that can only be created by changing the direction of the current.
“This will provide valuable information about the extreme physics behind jet ejection, a phenomenon that is still not well understood.”
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Why is this neutron star acting like a garden sprinkler?
Neutron stars are born when massive stars exhaust their hydrogen, the fuel needed for nuclear fusion in their cores. This ends the outward pressure of energy (in the form of radiation pressure) supporting the star against the inward pressure of its own gravity. When this often billion-year-old struggle ends—with gravity emerging as the inevitable victor—the dying star’s outer layers are blown away in a huge supernova explosion.
Meanwhile, the core undergoes a gravitational collapse that crushes the mass equivalent to one to two suns into a width of about 20 kilometers. As a result, this newborn neutron star could fit within the confines of a standard city on Earth, while remaining so dense that a mere spoonful of its material would weigh over 1 billion tons.
Moreover, not all neutron stars are alone. Some exist in binary systems with companion stars. And if these stars are close enough together, then the neutron star can act like a cosmic vampire by removing stellar material from its companion, or “donor” star.
Material removed from the donor star cannot fall directly onto the neutron star because it still has angular momentum. Instead, it creates a flattened cloud around the dead star that gradually feeds it, called an accretion disk.
The incredible density of neutron stars means that when this stolen material hits their surfaces, enormous amounts of energy are released. In just one second, a feeding neutron star can release the same amount of energy as the Sun releases over a million years.
Some of this energy is poured into jets of material that shoot out of the neutron star at a significant fraction of the speed of light.
Observations of Circinus X-1 made in 2007 revealed that the system is particularly bright in X-rays and emits jets usually associated with black hole systems. It was the first time a jet of this kind had been seen emerging from a neutron star system that defined such similarity to black holes.
As a result, Circinus X-1 has become a special system that defies conventional classification. Therefore, it is of great interest to astronomers, who use it to test their knowledge of accretion processes, the jets themselves, and even the interactions of these jets with the surrounding material.
So the new study team trained the recently improved MeerKAT radio telescope on Circinus X-1 in the hope that its sensitivity can provide more information about this fascinating system. They then obtained high-resolution images that show clear evidence of an S-shaped structure in the Circinus X-1 jet.
The team believes that these jets could have a unique shape due to the neutron star “wobbling” like a pinwheel at its source as it begins to slow down. “These shock waves cover a wide angle, which agrees with our model,” Cowie said. “So we have two strong pieces of evidence that tell us that a neutron star jet is going on.”
Cowie and colleagues also detected shock waves called “termination shocks” that ripple through the surrounding material, created when neutron star jets slam into it. This is the first time such an observation has been made for a binary system like this.
But the smoking gun that suggests that shock waves are generated by jets is the fact that they travel at about 10% the speed of light. This extremely high speed could only be caused by such high energy jets. Shock waves can actually act as “cosmic particle accelerators”, generating streams of high-energy particles called “cosmic rays”.
Further investigation of the speed of the shock waves could reveal what the jets that created them are made of.
“Circinus X-1 is one of the brightest objects in the X-ray sky and has been studied for more than half a century,” Cowie said. “Despite this, it remains one of the most mysterious systems we know. Several aspects of its behavior are not well understood, so it is very rewarding to help shed new light on this system and build on 50 years of work by others.”
The next step for the team will be to track the jet to see if it changes over time as predicted, or if the system is in for another surprise. “This will allow us to measure their properties more precisely and continue to learn more about this mysterious object,” Cowie said.
The findings were presented at the Royal Astronomical Society National Astronomers’ Meeting 2024 at the University of Hull on Monday (July 16).