April 13, 2024

Integral detects giant explosions fueling neutron star jets

Science and Exploration

03/27/2024
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ESA’s Integral gamma-ray space telescope played a decisive role in capturing jets of matter expelled into space at a third of the speed of light. The material and energy were released when huge explosions occurred on the surface of a neutron star. This world-first observation proved to be “a perfect experiment” for exploring astrophysical jets of all types.

Artist’s impression of a neutron star

Jets are produced by many different astronomical objects, but studying them is difficult. These matter streams are distant and seeing their characteristics is a challenge. This makes it extremely difficult to track matter moving along its length to understand how the jet is being launched and accelerated.

However, an international team of astronomers, including Thomas Russell of the National Institute of Astrophysics, INAF, Palermo, Italy, realized that certain types of neutron stars could be open to a new avenue of investigation.

Neutron stars are supercompact stellar corpses. When in orbit with another star, the neutron star’s intense gravitational field can end up pulling matter from its companion star. Some of this accumulated matter is then somehow ejected in jets that run along the neutron star’s rotational axis, and the rest of the matter spirals down toward the neutron star. There, it accumulates as a layer on the surface. As more and more material rains down on the neutron star, the gravitational field compresses it until an uncontrolled nuclear explosion is triggered. This creates a cataclysmic event known as a Type I X-ray burst.

The team concluded that this sudden release of matter and energy from the neutron star’s surface would affect the jet and that they could measure this disturbance as it propagated outward. If so, it would provide a powerful new method for studying these violent and energetic events. We currently know about 125 neutron stars that behave this way.

“This basically gives us a perfect experiment,” says Thomas. “We have a very brief, short-lived burst of extra material that is thrown into the jet that we can track as it moves through the jet to learn about its velocity.”

Artistic animation of how nuclear explosions in a neutron star fire its jets

On the hunt

This is a crucial measurement because, once enough accreting neutron stars are studied, the speed 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 the star itself. The team identified two neutron stars, named 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 the time to perform the experiment in the necessary detail.

Then there was a practical problem. Although the explosions were visible in X-rays, the jet only emitted radio waves. Therefore, the team needed to coordinate the radio telescope’s observations on Earth so that they occurred simultaneously with those from the Integral satellite, which is capable of seeing in X-rays. But it was impossible to predict exactly when one of these explosions would occur.

“These explosions occur again every two hours, but it is not possible to predict exactly when they will happen. So we have to observe the system for a long time with telescopes and hope to catch some explosions,” says team member Jakob van den Eijnden from the University of Warwick in the United Kingdom.

The radio observations were carried out over three days with CSIRO’s Australia Telescope Compact Array (ATCA), recording a total of around 30 hours of observations between 3 and 5 April 2021. Integral observed from space. It was the only high-energy mission capable of maintaining this long vigil. Its large, elongated orbit meant that it could observe the celestial object 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 previously saw in X-ray data, we thought the explosion would destroy the location where the jet was launching. But we saw exactly the opposite: a strong contribution to the jet, rather than a disturbance,” says team member Nathalie Degenaar from the University of Amsterdam in the Netherlands.

Clearly, the jet mechanism was more robust than previously thought. Being able to follow the extra matter injected by the jet at radio wavelengths allowed the team to calculate that the material was being launched at an incredible 35-40% the speed of light.

“Never before have we been able to anticipate and directly observe how a certain amount of gas was funneled into a jet and accelerated into space,” says team member Erik Kuulkers, ESA project scientist.

A new method for studying jets

Having now proven that this is possible, the technique will allow astronomers to study many more X-ray burst neutron stars. This will help them understand and relate the launch of jets to specific characteristics of neutron stars, such as their rotation rate and the amount of gas falling onto their surface. For those who study such phenomena, these are the pressing questions. Answering them will impact studies beyond neutron stars, because jets are created by many astronomical objects.

From newly formed stars to supermassive black holes at the centers of galaxies, jets can also be produced 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 the surrounding gas clouds that will change how and where new stars can form.

Given that all astrophysical jets are thought to be launched in similar ways, namely by the interaction of matter with magnetic fields in rotating celestial objects, the new results will have broad applicability to many studies of the cosmos. “This result opens a completely new window into understanding how astrophysical jets are powered, in neutron stars and also in other jet-producing astronomical objects,” says Erik.

Notes for editors

‘Thermonuclear explosions in neutron stars reveal the speed and power of their jets’ is published in Nature. DOI: 10.1038/s41586-024-07133-5

For more information please contact:
ESA media relations
media@esa.int

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