Neutron Star is Spraying Jets Like a Garden Sprinkler

Radio image from the MeerKAT telescope showing Circinus X-1 in the center, within the spherical remnant of the supernova it was born in. The shock waves caused by the jets are seen above and below Cir X-1, and the S-shape structure in the jets is somewhat obscured by a bright source in the background. Courtesy Fraser Cowie, Attribution CC BY 4.0.

X-ray binaries are some of the oddest ducks in the cosmic zoo and they attract attention across thousands of light-years. Now, astronomers have captured new high-resolution radio images of the first one ever discovered. It’s called Circinus X-1. Their views show a weird kind of jet emanating from the neutron star member of the binary. The jet rotates like an off-axis sprinkler as it spews material out through surrounding space, sending shockwaves through the interstellar medium.

The MeerKAT radio telescope in South African spotted the S-shaped jets emanating from the neutron star. Its images are the first-ever high-resolution views of such jets, according to lead researcher Fraser Cowie. “This image is the first time we have seen strong evidence for a precessing jet from a confirmed neutron star,” he said, referring to the neutron star’s off-axis spin. “This evidence comes from both the symmetric S shape of the radio-emitting plasma in the jets and from the fast, wide shockwave, which can only be produced by a jet changing direction.”

Such an awkward spin gives the jets their peculiar S-like configuration. Since scientists aren’t completely sure what phenomena caused them to launch in the first place, studying the odd behavior gives insight into the extreme physics behind its existence.

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Examining the Neutron Star Jets in Detail

The MeerKAT measurements showed not only the jet but revealed termination shocks moving away from the neutron star. These occur in regions where the jets slam into material in surrounding space. This is the first time astronomers found such shocks around an X-ray binary like Circinus X-1. Those waves are moving fast—at about 10 percent the speed of light and their structure points back to the jet as their source. “The fact that these shockwaves span a wide angle agrees with our model,” Cowie said. “So we have two strong pieces of evidence telling us the neutron star jet is processing.”

A MeerKAT radio image of the S-shape jet precessing in the Circinus X-1 X-ray binary pair system. The jet emanates as a result of the accretion of material around the neutron star. Courtesy: Fraser Cowie, Attribution CC BY 4.0.

The speed of those shockwaves turns them into particle accelerators producing high-energy cosmic rays. The fact that those rays exist tells astronomers the action around the X-ray binary is extremely energetic. That high-energy activity has grabbed astronomers’ attention for half a century. Still, it remains a mysterious system, so as Cowie points out, it’s important to observe the jets and see how their behavior changes over time. “Several aspects of its behavior are not well explained so it’s very rewarding to help shed new light on this system, building on 50 years of work from others,” he said. “The next steps will be to continue to monitor the jets and see if they change over time in the way we expect. This will allow us to more precisely measure their properties and continue to learn more about this puzzling object.”

About Circinus X-1

The Circinus X-1 system contains a neutron star and a companion. The pair lies some 30,000 light-years away in the direction of the southern hemisphere constellation Circinus. It was first spotted in June 1969 by an Aerobee suborbital rocket carrying X-ray-sensitive instruments and has been studied for years by astronomers using optical, X-ray, and radio telescopes.

Composite image of Circinus X-1, which is about 24,000 light-years from Earth in the constellation Circinus. Credit: X-ray: NASA/CXC/Univ. of Wisconsin-Madison/S. Heinz et al; Optical: DSS; Radio: CSIRO/ATNF/ATCA

The system is a pretty young member of the X-ray-binary class of objects. Typically, a binary pair consists of a black hole and a sun-like star, or a neutron star and a sun-like star. The tremendous gravity of the more massive member of the pair continually pulls material from the companion. Eventually, a hot disk of gas forms and spirals down to the surface of the neutron star. This accretion process unleashes huge amounts of energy, and some of it powers the jets. They carry material away from the system at close to the speed of light.

Illustration of Circinus X-1 and its jets.

Circinus X-1 is about 4,600 years old, based on studies of the material around the binary system done using observations by the Chandra X-ray Observatory. It’s one of the brightest objects in the X-ray sky and has been studied ever since its discovery. The neutron star component is extremely dense and is the leftover neutron-rich core of a supermassive star that exploded as a supernova some 4,600 years ago.

Astronomers know of hundreds of X-ray binaries in the Milky Way alone. Studies of Circinus X-1 give them insight into events and processes occurring early in the life of the binary. Interestingly one other X-ray binary shows an s-shape jet structure. It’s called SS 433. However, it may not have a neutron star. Instead, there may be a black hole powering that system. That makes the existence of Circinus X-1 doubly interesting since it contains a neutron star doing much the same thing.