Astronomers have found what they call a "smoking gun" for one of the brightest gamma-ray sources ever seen beyond our own galaxy: a strikingly straight trail of heated molecular gas, about 26 light-years long, that points to powerful jets from a microquasar as the engine behind the gamma-ray binary LMC P3, according to a study published in The Astrophysical Journal Letters and reported on October 30, 2026.
LMC P3 sits in the Large Magellanic Cloud, a small satellite galaxy of the Milky Way about 163,000 light-years away. It is the first gamma-ray binary ever discovered in another galaxy, and it remains the most luminous known gamma-ray binary in gamma rays, X-rays, radio waves and visible light. Since its discovery in 2016, its unusually strong gamma-ray glow has puzzled astronomers. The new result suggests the answer was hiding at the opposite end of the spectrum, in faint radio waves from cold gas.
The finding matters because gamma-ray binaries are rare laboratories for extreme physics. They show how nature accelerates particles to energies far beyond anything a laboratory on Earth can reach. Pinning down the mechanism behind LMC P3 — jets from a compact object feeding on a companion star, rather than winds from a spinning pulsar — sharpens the picture of how the universe's most violent particle accelerators work, and it strengthens the case that the system's hidden heart is a black hole.
Illustration: A massive star orbits a compact object whose jets stream into surrounding gas in the Large Magellanic Cloud. This is an AI-generated illustration, not a real photograph.
What Happened
Starting in 2022, a team led by astronomers from Nagoya University and Gifu University in Japan observed the region around LMC P3 with the ALMA observatory in Chile, one of the largest radio telescopes in the world. They used ALMA bands 6 and 7 to measure carbon monoxide emission from the surrounding cold molecular gas, at wavelengths of around 1.3 millimetres and 0.87 millimetres, allowing them to map the gas's structure and motion and to estimate its temperature.
What they found was a trail that stood out immediately. "We discovered a gigantic molecular jet," said Yasuo Fukui, professor emeritus at Nagoya University and lead author of the study. The trail is strikingly straight and narrow: about 26 light-years long and three light-years wide, made of molecular gas heated to between 35 and 60 kelvin, compared with about 15 kelvin for the surrounding gas. Although that is still far colder than room temperature, the contrast marks the trail out clearly against its surroundings.
Fukui said the relatively high temperature observed over a large fraction of the molecular cloud makes it "a very strong case" that the gas is interacting with a high-energy jet coming from the compact object — precisely the kind of jet that can also power LMC P3's gamma rays. The study was co-led by Bhuvana Gadikere Rajendra, a researcher, and Kisetsu Tsuge, a designated assistant professor, both at the Institute for Advanced Study at Gifu University, together with Hidetoshi Sano, an associate professor at Gifu University's Graduate School of Natural Science and Technology.
Background
LMC P3 is a binary system with two very unequal partners. One is a massive star more than 30 times heavier than the Sun. The other is a compact object — either a black hole or a neutron star — and the two take roughly 10 days to orbit each other. The system lies inside DEM L241, the supernova remnant left behind by the explosion that created the compact object. Unlike the Milky Way, where dust toward the galactic centre can hide objects from view, the Large Magellanic Cloud offers relatively unobstructed views of nearby astrophysical phenomena, which is part of why LMC P3 has become such a valuable test case.
Gamma rays sit at the highest-energy end of the electromagnetic spectrum, so a system that shines so brightly in them must be accelerating particles to extreme energies. For years, astronomers debated two competing explanations. In one, the compact object is a pulsar — a rapidly spinning neutron star — whose wind of energetic particles collides with the companion star's own stellar wind, creating a shock wave that accelerates particles and produces gamma rays. In the other, the compact object pulls material from its companion into an accretion disk and fires jets of high-speed particles, a small-scale version of the quasars powered by supermassive black holes at the centres of distant galaxies. That scaled-down version is called a microquasar.
The new trail tips the balance toward the microquasar picture. A jet ploughing through surrounding molecular gas would heat a narrow, straight channel — exactly what ALMA observed — while also providing the high-speed particles needed to generate the gamma rays. The study therefore strengthens support for the microquasar-jet theory of LMC P3's emission.
Illustration: ALMA radio dishes in the Chilean desert observe faint radio signals from cold gas around a distant binary star. This is an AI-generated illustration, not a real photograph.
Why It Matters
First, the result helps settle a decade-long argument about how gamma-ray binaries work. LMC P3 is the most luminous system of its kind, so the mechanism that powers it is a benchmark for the whole class. If jets, rather than pulsar winds, drive its gamma rays, then similar jet systems elsewhere in the universe may be more important gamma-ray producers than previously assumed.
Second, the finding bears on the identity of the compact object itself. Black holes drive more powerful jets than neutron stars do, and LMC P3's gamma-ray emission is particularly intense. Fukui said that intensity "provides very strong support for the possibility that its central object is a black hole" rather than a neutron star. That conclusion is not yet a direct detection — no one has weighed the compact object directly — but it narrows the possibilities and gives future observations a clear target to test.
Third, the study shows the value of looking across the spectrum. The mystery began in gamma rays, the most energetic light there is, but the decisive clue came from cold molecular gas observed in radio waves. The same multi-wavelength approach underpins much of modern astronomy, from the James Webb Space Telescope's infrared views of distant galaxies to radio arrays mapping the cold universe. LMC P3 is a reminder that the faintest, coldest signals can unlock the most violent phenomena.
What Happens Next
Astronomers will now want to confirm and extend the result. Follow-up radio observations could trace the jet trail further, test whether it lines up with the binary's orbit, and look for the same signature in other gamma-ray binaries. Higher-resolution mapping may also reveal whether the jet changes over the system's roughly 10-day orbital cycle, as the compact object moves around its massive companion.
Another open question is the compact object's mass. Direct measurements of the binary's motion could eventually weigh it and settle the black hole versus neutron star question. For now, the jet evidence is described by its authors as a very strong case, not a final proof — a distinction worth keeping as the result is discussed.
Illustration: A microquasar's jets heat a long, narrow trail of molecular gas far hotter than the cloud around it. This is an AI-generated illustration, not a real photograph.
More broadly, the Large Magellanic Cloud will keep rewarding close attention. As one of the nearest galaxies beyond the Milky Way, it offers a clearer view of stellar life cycles — from supernova remnants like DEM L241 to the compact objects they leave behind. Each such system sharpens the understanding of how massive stars live, die and continue to shape their surroundings long after they explode.
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FAQ
What is LMC P3?
LMC P3 is a gamma-ray binary star system in the Large Magellanic Cloud, about 163,000 light-years away. It is the first gamma-ray binary discovered in another galaxy and the most luminous known in gamma rays, X-rays, radio waves and visible light.
What did astronomers just discover about it?
Using the ALMA radio telescope in Chile, researchers found a straight, narrow trail of heated molecular gas about 26 light-years long and three light-years wide near LMC P3, which they interpret as evidence of a high-energy jet from the system's compact object.
What is a microquasar?
A microquasar is a small-scale version of a quasar: a stellar-mass black hole or neutron star that draws material from a companion star into an accretion disk and fires jets of high-speed particles, which can produce gamma rays.
Is the compact object in LMC P3 a black hole?
Not proven, but the new study strengthens the case. Because black holes drive more powerful jets than neutron stars, the system's particularly intense gamma-ray emission provides very strong support for a black hole, according to lead author Yasuo Fukui.
Why did radio observations solve a gamma-ray mystery?
The gamma rays showed that extreme particle acceleration was happening, but not how. The cold molecular gas, seen in radio waves, preserved a visible record of a jet passing through it — a trail heated to 35–60 kelvin against surroundings of about 15 kelvin.
Sources
- Phys.org, "A gamma-ray powerhouse's smoking gun? Gas trail points to microquasar jets," October 30, 2026.
- Yasuo Fukui et al., "ALMA Observations of DEM L241/LMC P3 in the Large Magellanic Cloud: Evidence for the Formation of Cool Molecular Jets Driven by a Microquasar," The Astrophysical Journal Letters (2026), DOI: 10.3847/2041-8213/aea0cb.
- Nagoya University and Gifu University research team, ALMA observations of DEM L241/LMC P3 from 2022, as reported by Phys.org on October 30, 2026.
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