Bizarre 'garden sprinkler-like' jet is spotted shooting out of neutron star

A strange 'garden sprinkler-like' jet coming from a neutron star has been pictured for the first time.

The S-shaped structure is created as the jet changes direction due to the wobbling of the disc of hot gas around the star -- a process called precession, which has been observed with black holes but, until now, never with neutron stars.

This particular object sits in the binary system Circinus X-1 more than 30,000 light-years from Earth and formed from the core of a massive supergiant star that collapsed around the same time Stonehenge was built.

It is so dense that a teaspoon of its material weighs as much as Mount Everest.

Binary systems have two stars that are bound together by gravity. In the case of Circinus X-1, one of these is a neutron star.

Both neutron stars and black holes are cosmological monsters which form when the biggest stars in the Universe die and collapse under their own gravity.

However, the latter are considerably more massive and can only be detected through their gravitational effects, while the former can be observed directly despite their denseness.

They are some of the most extreme objects in the Universe and have interiors almost entirely made of neutrons.

The jet emanating from the neutron star was spotted by a team of astronomers at the University of Oxford, who used MeerKAT -- a radio telescope in South Africa -- to create the most detailed, high-resolution images of Circinus X-1.

The pictures, which were presented at this week's National Astronomy Meeting at the University of Hull, include the first-ever image of an S-shaped jet coming from a confirmed neutron star -- a breakthrough that could help unravel the extreme physics behind the astronomical phenomenon.

Lead researcher Fraser Cowie said there was another system known for its S-shaped jets, called SS433, but recent results suggest that object is likely a black hole.

"This image is the first time we have seen strong evidence for a precessing jet from a confirmed neutron star," he said.

"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.

"This will give valuable information about the extreme physics behind the launching of the jet, a phenomenon which is still not well understood."

The neutron star's huge density creates a strong force of gravity that strips gas from the companion star, forming a disc of hot gas around it that spirals down towards its surface.

This process, called accretion, releases huge amounts of energy per second with more power than a million Suns. Some of this energy powers jets -- narrow beams of outflowing material from the binary system travelling close to the speed of light.

Recent upgrades to the MeerKAT telescope have resulted in excellent sensitivity and higher-resolution images. With these the team saw clear evidence of an S-shaped structure, similar in shape to water spraying from a garden sprinkler, in Circinus X-1's jet.

Not only that, but researchers also discovered moving termination shocks -- the first recorded from an X-ray binary. These are regions where the jet violently rams into the surrounding material, causing a shockwave.

Cowie's team measured the waves moving at roughly 10 per cent of the speed of light, confirming that they were caused by the fast-moving jet and not something slower such as a wind of material from the stars.

"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 precessing."

Measuring the velocity of the shockwaves will also help astronomers understand what the jet causing them is made from.

The shockwaves effectively act as particle accelerators in space -- producing high-energy cosmic rays -- and the maximum energy of particles that can be accelerated depends on their velocity.

"Circinus 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.

"Several aspects of its behaviour 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 added: "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."

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Astronomers witness the birth of a very distant cluster of galaxies from the early Universe

Using the Atacama Large Millimeter/submillimeter Array (ALMA), of which ESO is a partner, astronomers have discovered a large reservoir of hot gas in the still-forming galaxy cluster around the Spiderweb galaxy -- the most distant detection of such hot gas yet. Galaxy clusters are some of the largest objects known in the Universe and this result, published today in Nature, further reveals just how early these structures begin to form.

Galaxy clusters, as the name suggests, host a large number of galaxies -- sometimes even thousands. They also contain a vast "intracluster medium" (ICM) of gas that permeates the space between the galaxies in the cluster. This gas in fact considerably outweighs the galaxies themselves. Much of the physics of galaxy clusters is well understood; however, observations of the earliest phases of formation of the ICM remain scarce.

Previously, the ICM had only been studied in fully-formed nearby galaxy clusters. Detecting the ICM in distant protoclusters -- that is, still-forming galaxy clusters -- would allow astronomers to catch these clusters in the early stages of formation. A team led by Luca Di Mascolo, first author of the study and researcher at the University of Trieste, Italy, were keen to detect the ICM in a protocluster from the early stages of the Universe.

Galaxy clusters are so massive that they can bring together gas that heats up as it falls towards the cluster. "Cosmological simulations have predicted the presence of hot gas in protoclusters for over a decade, but observational confirmations has been missing," explains Elena Rasia, researcher at the Italian National Institute for Astrophysics (INAF) in Trieste, Italy, and co-author of the study. "Pursuing such key observational confirmation led us to carefully select one of the most promising candidate protoclusters." That was the Spiderweb protocluster, located at an epoch when the Universe was only 3 billion years old. Despite being the most intensively studied protocluster, the presence of the ICM has remained elusive. Finding a large reservoir of hot gas in the Spiderweb protocluster would indicate that the system is on its way to becoming a proper, long-lasting galaxy cluster rather than dispersing.

Di Mascolo's team detected the ICM of the Spiderweb protocluster through what's known as the thermal Sunyaev-Zeldovich (SZ) effect. This effect happens when light from the cosmic microwave background -- the relic radiation from the Big Bang -- passes through the ICM. When this light interacts with the fast-moving electrons in the hot gas it gains a bit of energy and its colour, or wavelength, changes slightly. "At the right wavelengths, the SZ effect thus appears as a shadowing effect of a galaxy cluster on the cosmic microwave background," explains Di Mascolo.

By measuring these shadows on the cosmic microwave background, astronomers can therefore infer the existence of the hot gas, estimate its mass and map its shape. "Thanks to its unparalleled resolution and sensitivity, ALMA is the only facility currently capable of performing such a measurement for the distant progenitors of massive clusters," says Di Mascolo.

They determined that the Spiderweb protocluster contains a vast reservoir of hot gas at a temperature of a few tens of millions of degrees Celsius. Previously, cold gas had been detected in this protocluster, but the mass of the hot gas found in this new study outweighs it by thousands of times. This finding shows that the Spiderweb protocluster is indeed expected to turn into a massive galaxy cluster in around 10 billion years, growing its mass by at least a factor of ten.

Tony Mroczkowski, co-author of the paper and researcher at ESO, explains that "this system exhibits huge contrasts. The hot thermal component will destroy much of the cold component as the system evolves, and we are witnessing a delicate transition." He concludes that "it provides observational confirmation of long-standing theoretical predictions about the formation of the largest gravitationally bound objects in the Universe."

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