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.
Examining the Neutron Star Jets in DetailThe 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-1The 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/ATCAThe 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.
For More Information“Garden Sprinkler-like” Jet Seen Shooting out of Neutron Star
X-ray Binary Circinus X-1
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NASA says it intends to discontinue development of its VIPER moon rover, due to cost increases and schedule delays — but the agency is also pointing to other opportunities for robotic exploration of the lunar south polar region.
The Volatiles Investigating Polar Exploration Rover was originally scheduled for launch in late 2023, targeting the western edge of Nobile Crater near the moon’s south pole.
The south polar region is a prime target for exploration because it’s thought to hold deposits of water ice that could sustain future lunar settlements. NASA plans to send astronauts to that region by as early as 2026 for the first crewed lunar landing since 1972.
Unfortunately, the VIPER project ran into a series of delays, due to snags in the testing and development of the rover as well as the Astrobotic Griffin lander that was to deliver the rover to the lunar surface. The readiness date for VIPER and Griffin was most recently pushed back to September 2025.
During an internal review, NASA managers decided that continuing with VIPER’s development would result in cost increases that could lead to the cancellation or disruption of other moon missions in NASA’s Commercial Lunar Payload Services program, or CLPS. NASA notified Congress of its intent to discontinue development.
The budgeted cost for building VIPER was $433.5 million, and the estimated cost of building and launching the Griffin lander is $235.6 million, according to a 2022 report from NASA’s Office of the Inspector General.
NASA said it will continue supporting Astrobotic’s Griffin Mission One, with launch set for no earlier than the fall of 2025. Instead of delivering VIPER, the mission would provide a flight demonstration of the lander and its engines. In January, Astrobotic’s Peregrine lander passed up an opportunity to land on the moon due to a problem with its propulsion system.
NASA said other missions could verify the presence of ice in the moon’s south polar region and determine how such resources could be used to further exploration goals.
“We are committed to studying and exploring the moon for the benefit of humanity through the CLPS program,” Nicola Fox, NASA’s associate administrator for science, said today in a news release. “The agency has an array of missions planned to look for ice and other resources on the moon over the next five years. Our path forward will make maximum use of the technology and work that went into VIPER, while preserving critical funds to support our robust lunar portfolio.”
Late this year, for example, Intuitive Machines is due to deliver an ice-mining experiment called PRIME-1 to the south pole under the terms of the CLPS program. PRIME-1 is designed to drill for water ice and study what happens to the H2O when it’s brought up to the surface.
In league with NASA, the CLPS program and a wide array of other partners, the Canadian Space Agency is planning to send an ice-hunting rover to the lunar south polar region by as early as 2026. The Artemis program’s crewed missions will also study the moon’s ice deposits and how they can be used.
NASA said it plans to disassemble VIPER and arrange for the reuse of the rover’s components and scientific instruments for other missions to the moon. But prior to disassembly, the agency said it would consider expressions of interest from commercial and international partners for use of the existing VIPER rover system at no cost to the federal government. Interested parties can email HQ-CLPS-Payload@mail.nasa.gov anytime between July 18 and Aug. 1.
NASA said the VIPER team would conduct an “orderly close-out” through next spring.
Word of VIPER’s demise was met with disappointment in some quarters of the space community. “In the Artemis era, why is lunar science targeted for cancellation?” Laura Seward Forczyk, founder and executive director of the space consulting firm Astralytical, asked in a posting to the X social-media platform.
Phil Metzger, a planetary physicist at the University of Central Florida, said NASA was making a “bad mistake.”
“This was the premier mission to measure lateral and vertical variations of lunar ice in the soil,” Metzger wrote in a posting to X. “It would have been revolutionary. Other missions don’t replace what is lost here.”
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The Cassini-Huygens mission to Saturn generated so much data that giving it a definitive value is impossible. It’s sufficient to say that the amount is vast and that multiple scientific instruments generated it. One of those instruments was a radar designed to see through Titan’s thick atmosphere and catch a scientific glimpse of the moon’s extraordinary surface.
Scientists are still making new discoveries with all this data.
Though Saturn has almost 150 known moons, Titan attracts almost all of the scientific attention. It’s Saturn’s largest moon and the Solar System’s second largest. But Titan’s surface is what makes it stand out. It’s the only object in the Solar System besides Earth with surface liquids.
Cassini’s radar instrument had two basic modes: active and passive. In active mode, it bounced radio waves off surfaces and measured what was reflected back. In passive mode, it measured waves emitted by Saturn and its moons. Both of these modes are called static modes.
But Cassini had a third mode called bistatic mode that saw more limited use. It was experimental and used its Radio Science Subsystem (RSS) to bounce signals off of Titan’s surface. Instead of travelling back to sensors on the spacecraft, the signals were reflected back to Earth, where they were received at one of NASA’s Deep Space Network (DNS) stations. Critically, after bouncing off of Titan’s surface, the signal was split into two, hence the name bistatic.
A team of researchers has used Cassini’s bistatic data to learn more about Titan’s hydrocarbon seas. Their work, “Surface properties of the seas of Titan as revealed by Cassini mission bistatic radar experiments,” has been published in Nature Communications. Valerio Poggiali, a research associate at the Cornell Center for Astrophysics and Planetary Science, is the lead author.
This schematic shows how Cassini’s bistatic radar experiment worked. The orbiter used its Radio Science Subsystem to send signals to Titan’s surface. The signals then reflected off Titan to Earth, where they were received by one of the DNS receivers at Canberra, Goldstone, or Madrid. The signals are either Right Circularly Polarized (RCP) or Left Circularly Polarized (LCP). Image Credit: Poggiali et al. 2024.The signals that reach the DNS are polarized, which reveals more information about the hydrocarbon seas on Titan. While Cassini’s radar instrument revealed how deep the seas are, the bistatic radar data tells researchers about both their compositions and surface textures.
This image of the hydrocarbon seas on Titan is well-known and was radar-imaged by Cassini. That radar data told us how deep the seas are. New bistatic radar data can reveal more about the composition and surface texture of the seas. Image Credit: [JPL-CALTECH/NASA, ASI, USGS]“The main difference,” Poggiali said, “is that the bistatic information is a more complete dataset, and is sensitive to both the composition of the reflecting surface and to its roughness.”
“It’s like on Earth, when fresh-water rivers flow into and mix with the salty water of the oceans.”
Valerio Poggiali, lead author, Cornell Center for Astrophysics and Planetary ScienceThe experimental bistatic radar required meticulous cooperation.
Philip Nicholson, a professor in the Department of Astronomy at Cornell, is one of the study’s co-authors. “The successful execution of a bistatic radar experiment requires exquisite choreography between the scientists who design it, Cassini mission planners and navigators, and the team who collects the data at the receiving station,” Nicholson said.
These results are based on bistatic radar data from four Cassini flybys from 2014 to 2016. In this work, the researchers focused on three large seas on the surface of Titan’s polar regions: Kraken Mare, Ligeia Mare and Punga Mare.
The bistatic radar data revealed new information about the three seas. Though they’re all hydrocarbon seas, their composition varies based on latitude and their proximity to other features like estuaries and rivers. The bistatic radar measured the dielectric constant of Titan’s seas. The dielectric constant is a material’s capacity to store electrical energy. In practical terms, it’s a measure of a surface’s reflectivity, so it reveals the composition. Earth’s water has a dielectric constant of about 80. Titan’s methane and ethane seas have a dielectric constant of only about 1.7. Kraken Mare’s southernmost region had the highest dielectric constant.
This figure from the study shows Titan’s polar regions with the three large seas labelled. The colour key on the right and the text on the image show the dielectric constants of different regions. The white lines labelled T101, T102, T106, and T124 are the four flybys. Image Credit: Poggiali et al. 2024.Bistatic radar data also showed all three seas had calm surfaces during the four flybys. Waves were no more than 3.3 mm, about 0.13 of an inch. Near estuaries, straits, and coastal areas, the waves were slightly larger: 5.2 mm or 0.2 of an inch. So small they barely merit the name ‘wave.’
This figure from the study is similar to the previous image but shows wave height instead of dielectric constant. Image Credit: Poggiali et al. 2024.The bistatic radar data also revealed the composition of some of the rivers that flow into the seas.
“We also have indications that the rivers feeding the seas are pure methane,” Poggiali said, “until they flow into the open liquid seas, which are more ethane-rich. It’s like on Earth, when fresh-water rivers flow into and mix with the salty water of the oceans.”
These results agree with scientific models of Titan’s hydrocarbon seas and thick atmosphere. Models show that methane rains down from Titan’s atmosphere and then flows into its lakes and seas. They also show that the rain contains only tiny amounts of ethane and other hydrocarbons and almost completely consists of methane.
“This fits nicely with meteorological models for Titan,” Nicholson said, “which predict that the ‘rain’ that falls from its skies is likely to be almost pure methane, but with trace amounts of ethane and other hydrocarbons.”
The Cassini mission is very instructive for future missions. Though it ended its mission when it plunged into Saturn in 2017, scientists are still making new discoveries with its vast trove of data. The same will be true of missions like Juno when they end.
The researchers behind this work say there’s lots left to learn from all of Cassini’s data.
“There is a mine of data that still waits to be fully analyzed in ways that should yield more discoveries,” Poggiali said. “This is only the first step.”
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