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Astronomers have captured for the first time a strange S-shaped jet emanating from a neutron star. The strange emission suggests the dead star resembles the shape of water squirting out of a garden fountain.
The “cosmic garden fountain” in question is a neutron star in the Circinus X-1 binary system, located 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 from this explosion would have reached Earth about 5,000 years ago, when Stonehenge was built.
The neutron star is fed by a companion star, like a cosmic vampire, which causes it to eject high-energy jets. These jets take on an S-shaped formation due to the fact that the vampire dead star wobbles or “wobbles” 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, which are found nowhere else in the universe.
The S-shaped jet was detected by astronomers from the University of Oxford using the MeerKAT radio telescope in South Africa. The data they collected was used to create the most detailed, high-resolution images of Circinus X-1 to date.
Relating to: ‘Vampire’ neutron star explosions associated with jets travelling at near-light speeds
“This image is the first time we have seen strong evidence for precession in a jet from a confirmed neutron star,” team leader Fraser Cowie, a researcher at Oxford, said in a statement. “This evidence comes both from the symmetrical S-shape of the radio-emitting plasma in the jets and from the fast, broad shock wave that can be produced by just a jet changing direction.
“This will provide valuable insight into the extreme physics behind jet propulsion, a phenomenon that is not yet fully understood.”
Why does this neutron star act like a garden sprinkler?
Neutron stars are born when massive stars run out of hydrogen, the fuel needed for nuclear fusion in their cores. This ends the outward push of energy (in the form of radiation pressure) that supports a star against the inward push of its own gravity. When this usually billions-of-years-long struggle is over — with gravity emerging as the inevitable winner — the dying star’s outer layers are blown away in a massive supernova explosion.
Meanwhile, the core undergoes a gravitational collapse that crushes the mass equivalent of one or two suns to a width of about 12 miles (20 kilometers). As a result, this newborn neutron star could fit within the boundaries of a standard city on Earth, but remains so dense that just a tablespoon of its material would weigh more than 1 billion tons.
Also, not all neutron stars are solitary. Some exist in binary systems with companion stars. And if these stars are close enough together, the neutron star can act like a cosmic vampire, stripping stellar material from its companion or “donor” star.
However, material stripped from the donor star cannot fall directly onto the neutron star because it still has 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 surfaces, a tremendous amount of energy is released. In just one second, a nurturing neutron star can release as much energy as the sun would in a million years.
Some of this energy is transferred to jets of matter that blast outward 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 was particularly bright in X-rays and emitted the types of jets usually associated with black hole systems. This was the first time such a jet had emerged from a neutron star system and described such a similarity to black holes.
As a result, Circinus X-1 has become a peculiar system that defies traditional classification and 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 material.
So the team behind the new study pointed the recently upgraded MeerKAT radio telescope at Circinus X-1, hoping its sensitivity could provide more information about this fascinating system. They then obtained high-resolution images that showed clear evidence of an S-shaped structure in Circinus X-1’s jet.
The team thinks these jets may have a unique shape because the source neutron star “wobbles” like a spinning top as it begins to slow down. “The fact that these shock waves are spread out over a wide angle matches our model,” Cowie said. “So we have two strong lines of evidence that the neutron star jet is moving.”
Cowie and his colleagues also detected shock waves, called “termination shocks,” that form when jets from the neutron star hit it and ripple through the surrounding material. This is the first time such an observation has been made for a binary system.
However, the definitive proof that shock waves are produced by jets is that they travel at about 10% of the speed of light. This extremely high speed is only possible with such high-energy jets. Shock waves can actually act as “cosmic particle accelerators” and produce streams of high-energy particles called “cosmic rays”.
Further study of the speed of shock waves may reveal what the jets that create them are made of.
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“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. “Yet despite this, it remains one of the most mysterious systems we know. Several aspects of its behavior are not well explained, so it is very rewarding to help shed new light on this system by building on 50 years of work by others.”
The team’s next step will be to monitor the jet to determine whether it is changing over time as predicted, or whether this system has more surprises in store. “This will allow us to measure its properties more precisely and continue to learn more about this puzzling object,” Cowie said.
These findings were presented at the Royal Astronomical Society’s 2024 National Astronomy Meeting, held at the University of Hull on Monday (July 16).