2024-05-07 20:50:18
This artist’s impression depicts how nuclear explosions in a neutron star feed the jets blasting from its magnetic pole regions. Credit: Daniela Potslaar and Nathalie Dejanard, Anton Pankok Institute, University of Amsterdam
ESA’s Integral Gamma-ray Space Telescope played a crucial role in capturing jets of matter ejected into space at one-third the speed of light. Matter and energy were released when huge explosions occurred on the surface of A
” data-gt-translate-attributes=”({” attribute=”” tabindex=”0″ role=”link”>neutron star. The world’s first observation turned out to be a “perfect experiment” for the study of astrophysical jets of all descriptions.
Jets are produced by many different astronomical objects, but they are difficult to study. These streams of matter are far away and seeing features in them is challenging. This makes it very difficult to track material moving along them to understand how the jet is launched and accelerated.
However, an international team of astronomers including Thomas Russell, National Institute of Astrophysics, INAF, Palermo, Italy, realized that certain types of neutron stars might be open to a new avenue of investigation.
Aside from black holes, neutron stars are among the most confusing objects in the universe. A neutron star forms in the final moments of the life of a very large star (more than eight times the mass of our Sun), when the nuclear fuel in its core eventually runs out. In a sudden and violent end, the outer layers of the star are ejected with monstrous energy in a supernova explosion, leaving behind spectacular clouds of interstellar material rich in dust and heavy metals. In the center of the cloud (nebula), the dense stellar core contracts further to form a neutron star. A black hole can also form when the remaining core mass is greater than about three solar masses. Credit: ESA
Neutron stars are extremely compact stellar bodies. When in orbit with another star, the neutron star’s intense gravitational field can eventually pull material from its companion star. Some of this accreted material is somehow ejected into jets that race away along the neutron star’s spin axis, and the rest of the material rolls down into the neutron star. There it accumulates as a layer on the surface. As more and more material descends on the neutron star, the gravitational field compresses it until a runaway nuclear explosion begins. This creates a catastrophic event known as a type I X-ray burst.
The team reasoned that this sudden release of matter and energy from the surface of the neutron star would affect the jet, and that they could measure this disturbance as it propagated outward. If so, this would provide a powerful new method for studying these violent and energetic events. Today we know of about 125 neutron stars that behave this way.
“It basically gives us a perfect experiment,” says Thomas. “We have a very short burst of extra material shot into the jet that we can follow as it moves down the jet to learn about its speed.”
on the hunt
This is a crucial measurement because once enough accreting neutron stars have been studied, the velocity of the jet can reveal the dominant launch mechanism and show whether the jet is powered by magnetic fields anchored in the accreting material, or in the star itself. The team identified two neutron stars, called 4U 1728-34 and 4U 1636-536 respectively, that showed X-ray burst behavior. However, only 4U 1728-34 proved bright enough at radio wavelengths at that time to perform the experiment in the necessary detail.
Then there was a practical problem. While the explosions were visible in X-rays, the jet emitted only radio waves. So the team had to coordinate radio telescope observations of Earth to occur simultaneously with those of the Integral satellite, which can see in X-rays. But, it was impossible to predict exactly when one of these explosions was going to occur.
“These bursts repeat every two hours, but you can’t predict exactly when they will happen. So, you have to stare at the system for a long time with the telescopes, and hope you catch a few bursts,” says team member Jacob van den Einden,
” data-gt-translate-attributes=”({” attribute=”” tabindex=”0″ role=”link”>University of WarwickUK.
The radio observations were taken over three days with
” data-gt-translate-attributes=”({” attribute=”” tabindex=”0″ role=”link”>CSIROAustralia’s Compact Telescope Array (ATCA), recording a total of about 30 hours of observing time between April 3-5, 2021. Integral viewed from space. It was the only high-energy mission capable of maintaining this long vigil. Its large and elongated orbit meant that it could stare at the celestial bone for many hours at a time. At the end of the observations, Integral captured 14 X-ray bursts from 4U 1728-34, 10 of which occurred when the source was visible to ATCA.
But there was a big surprise. “Based on what we had previously seen in X-ray data, we thought that the explosion would destroy the place where the jet was launched. But we saw just the opposite: strong input to the plane and no disruption,” says team member Nathalie Degner, University of Amsterdam, Netherlands.
Obviously the jet mechanism was more powerful than they thought. The ability to track the extra material injected down the jet at radio wavelengths allowed the team to calculate that the material was launched at an incredible 35-40% of the speed of light.
“We have never been able to observe and observe directly how a certain amount of gas is transferred into a jet and accelerated into space,” says team member Erik Kvalkers, ESA project scientist.
A new method for aircraft research
Having now proven that this is possible, the technique will allow astronomers to study more X-ray bursting neutron stars. This will help them understand and connect the origin of jets to specific properties of neutron stars, such as their rotation rate and the amount of gas falling on their surface. For those who study such phenomena, these are the pressing questions. Answering them will affect studies beyond neutron stars because jets are generated by many astronomical objects.
From newly formed stars to supermassive black holes at the centers of galaxies, jets can also be created by cataclysmic events such as supernova explosions and gamma-ray bursts. They play an important role throughout the universe, from transporting exotic elements synthesized in cosmic explosions into interstellar space, to heating surrounding gas clouds that change how and where new stars can form.
Since all astrophysical jets are thought to be launched in similar ways, namely by the interplay of matter with magnetic fields in rotating celestial bodies, the new results will have wide application in many studies of the cosmos. “This result opens a completely new window to understanding how astrophysical jets are activated, in neutron stars and also in other astronomical objects that produce jets,” says Eric.
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