Astronomers have photographed for the first time a strange S-shaped jet emerging from a neutron star. The strange emission suggests that the dead star resembles the shape of water sprayed by a garden sprinkler.
The “cosmic sprinkler” in question is a neutron star in the binary system Circinus X-1, located more than 30,000 light-years from Earth. It was born from the death of a star at least eight times larger than the Sun in a supernova explosion, whose light is thought to have reached Earth nearly 5,000 years ago, around the time Stonehenge was under construction.
The neutron star feeds on a companion star like a cosmic vampire, erupting high-energy jets. These jets take on an S-shape because the dead vampire star wobbles, or “precesses,” like a spinning top as it feeds. The team behind the research hopes this first-of-its-kind observation could help them better understand the extreme physics surrounding neutron stars, which are found nowhere else in the cosmos.
The S-shaped jet was spotted by astronomers at the University of Oxford using the MeerKAT radio telescope in South Africa. The data collected helped construct the most detailed, high-resolution images of Circinus X-1 to date.
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“This image is the first strong evidence for a precessing jet from a confirmed neutron star,” team leader Fraser Cowie, a researcher at Oxford, 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, which can only be produced by a jet changing direction.”
“This will provide valuable insight into the extreme physics behind jet launch, a phenomenon that is not yet well understood.”
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Why does this neutron star act like a garden sprinkler?
Neutron stars are born when massive stars exhaust the hydrogen, the fuel needed for nuclear fusion, in their cores. This ends the outward push of energy (in the form of radiation pressure) that sustains a star against the inward push of its own gravity. When this often billion-year-long struggle ends—gravity emerging as the inevitable victor—the outer layers of the dying star are blown away in a massive supernova explosion.
Meanwhile, the core undergoes a gravitational collapse that crushes a mass equivalent to one or two suns into a width of about 20 kilometers. As a result, this newborn neutron star could fit within the boundaries of a standard city on Earth, while remaining so dense that a single tablespoon of its matter would weigh more than a billion tons.
Furthermore, not all neutron stars are isolated. Some exist in binary systems with companion stars. And if these stars are close enough together, the neutron star can act as a cosmic vampire, stripping stellar material from its companion, or “donor” star.
The material extracted from the donor star, however, cannot fall directly onto the neutron star, because it retains its angular momentum. Instead, it forms 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 surface, a huge amount of energy is released. In just one second, a feeding neutron star can release the same amount of energy that the Sun will release in a million years.
Some of this energy is dumped into jets of matter that shoot out from the neutron star at speeds equal to a significant fraction of the speed of light.
Observations of Circinus X-1 in 2007 revealed that the system is particularly bright in X-rays and emits the type of jets typically associated with black hole systems. This was the first time such a jet had been observed coming from a neutron star system that shows such similarity to black holes.
As a result, Circinus X-1 has become a peculiar system that defies conventional classification. It is therefore 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 surrounding matter.
So the research team turned the recently upgraded MeerKAT radio telescope on Circinus X-1, hoping that its sensitivity could provide more information about this fascinating system. They then obtained high-resolution images that clearly showed the presence of an S-shaped structure in Circinus X-1’s jet.
The team thinks these jets might have a unique shape because the neutron star at its source “wobbles” like a top as it begins to slow down. “These shock waves span a large angle, which is consistent with our model,” Cowie said. “So we have two strong pieces of evidence that tell us that the neutron star jet is precessing.”
Cowie and his colleagues also detected shock waves called “termination shocks” propagating into the surrounding matter, generated when the neutron star’s jets crash into it. This is the first time such an observation has been made for a binary system like this one.
But the irrefutable proof that shock waves are generated by jets is the fact that they travel at about 10% of the speed of light. This extremely high speed can only be achieved by high-energy jets. Shock waves can actually act as “cosmic particle accelerators,” generating streams of high-energy particles called “cosmic rays.”
Further research into the speed of the shock waves could reveal what the jets that created them are made of.
“Circus X-1 is one of the brightest objects in the X-ray sky and has been studied for over half a century,” Cowie said. “But despite this, it remains one of the most enigmatic systems we know of. Many aspects of its behavior are not well explained, so it is very rewarding to help shed new light on this system, building on 50 years of work by other researchers.”
The next step for the team will be to monitor the jet to determine whether it is evolving over time as expected or whether there are other surprises in store for the system. “This will allow us to more precisely measure its properties and continue to learn more about this puzzling object,” Cowie said.
These results were presented at the Royal Astronomical Society’s 2024 National Astronomy Meeting at the University of Hull on Monday 16 July.