A position on the sky, a set of measurements, and a gap in the catalogue
Near the heart of the Milky Way, something briefly spoke in radio waves six times across nine months in 2020, then went silent — leaving behind no trace in X-ray, infrared, or optical light, only a coordinate and a question. Astronomers working with the ASKAP telescope have catalogued the source, compared it against every known class of celestial object, and found no satisfying match. It may be an exotic variant of something familiar, or it may represent a genuinely new kind of thing the universe makes — a distinction that cannot yet be made, because the object has not spoken again while all instruments were listening at once. In this, it joins a small and growing list of phenomena that wide-field radio astronomy is surfacing faster than existing frameworks can absorb.
- A radio source near the Galactic Centre flickered to life six times in 2020, broadcasting in highly polarised waves before vanishing completely — leaving every other telescope pointed at its location with nothing to show.
- Its behavior defies every tidy explanation: too quiet in X-rays to be a magnetar, too invisible in infrared to be a nearby star, too irregular to be a pulsar, and only loosely related to the small family of Galactic Centre radio transients.
- A 2021 MeerKAT detection added urgency rather than clarity — the source reappeared briefly with up to 80 percent linear polarisation and a rapidly shifting rotation measure, suggesting a turbulent, magnetised environment that may be as mysterious as the object itself.
- A 2024 follow-up study proposed a supersonic neutron star plowing through a changing medium, but framed it as a possibility rather than an answer, underscoring how much the problem has grown beyond the object alone.
- The field of radio astronomy is now surfacing objects that exist in no other catalogue, and ASKAP J173608.2-321635 has become a symbol of that frontier — real, measured, constrained, and still fundamentally unclassified.
- Resolution depends on the source switching on again while radio, X-ray, infrared, and optical observatories watch simultaneously — a coordination that has not yet been achieved, leaving the mystery intact.
In January 2020, something near the center of the Milky Way began broadcasting in radio waves. It appeared six times over nine months through the Australian Square Kilometre Array Pathfinder, then went silent. When astronomers turned their X-ray and infrared instruments toward its position, they found nothing at all. The object left behind only a name — ASKAP J173608.2-321635 — and a set of measurements that didn't fit anywhere in the existing catalogue.
The source was strange in multiple ways: roughly 25 percent circularly polarised, steep-spectrum, wildly variable, and brighter at lower frequencies. Ziteng Wang and colleagues published the discovery in 2021, carefully ruling out the usual suspects. A stellar flare seemed unlikely without any infrared counterpart. A pulsar couldn't explain the absence of regular pulses. A magnetar was tempting until Swift and Chandra found no X-ray emission whatsoever. The closest relatives were Galactic Centre radio transients, but even that comparison was imperfect.
When MeerKAT monitored the field into early 2021, the source reappeared briefly on February 7, peaking at 5.6 millijanskys before fading over roughly a day. This detection added new complexity: up to 80 percent linear polarisation and a rotation measure that shifted significantly over just three days, suggesting the radio waves were passing through turbulent, magnetised material. The environment itself had become part of the mystery.
A 2024 study by Kierra Weatherhead and colleagues, drawing on serendipitous archival observations, detected the source only once more — on April 11, 2020 — and proposed that a highly supersonic neutron star moving through a changing medium might be consistent with the data. It was offered as a possibility, not a conclusion.
What this object ultimately represents remains open. Wide-field radio surveys are increasingly finding things that exist in no optical, X-ray, or infrared archive, and some of those things may require entirely new categories. ASKAP J173608.2-321635 is one of them — a real set of measurements, a real gap in understanding, and a source that needs to switch on again while every available instrument is watching at once. Until that happens, it remains exactly what its coordinate implies: a location in the sky where something happened, and no one yet knows what.
In January 2020, something switched on near the heart of the Milky Way. It broadcast in radio waves six times over the course of nine months, then fell silent. When astronomers pointed their other instruments at the sky where it had been—the X-ray telescopes, the infrared cameras, the usual tools for understanding what the cosmos is doing—they found nothing. The object had left no trace except in radio, and radio alone. It became a coordinate: ASKAP J173608.2-321635, a name that announces its own incompleteness.
The discovery came through the Australian Square Kilometre Array Pathfinder, or ASKAP, a wide-field radio telescope designed to catch exactly this kind of thing—objects that flicker in and out of view on timescales from seconds to years. Between January and September 2020, ASKAP detected the source six times at 888 megahertz, located in the Galactic plane about four degrees away from the Galactic Centre itself. When it appeared, it showed roughly 25 percent circular polarisation, a striking signature that typically points toward either coherent processes or strongly magnetised environments. The source was also steep-spectrum and wildly variable, brighter at lower frequencies and changing dramatically over time. It behaved like nothing steady or predictable.
Ziteng Wang and colleagues published the discovery in The Astrophysical Journal in 2021, but their paper ended with caution rather than certainty. The source did not fit neatly into any existing category. A low-mass star or brown dwarf could produce polarised radio flares, but the complete absence of an infrared counterpart made that unlikely if the object were nearby and stellar. A pulsar could explain the polarised radio emission, yet searches revealed no expected pulses, and the scattering of radio waves in the inner Galaxy complicated any comparison. A magnetar—a neutron star with an extraordinarily strong magnetic field—was tempting, since some magnetars do produce unusual radio behaviour. But magnetars are typically loud in X-rays, and this source had produced nothing when Swift and Chandra looked. The slow, irregular appearance also did not match the regular pulses expected from a rotating neutron star.
The closest family resemblance lay with Galactic Centre radio transients, a small group of radio sources discovered in earlier surveys toward the inner Milky Way. But even that fit was imperfect. ASKAP J173608.2-321635 occupied a different part of the sky, and its particular signature of polarisation and variability set it apart. When the team monitored the field with MeerKAT from November 2020 to February 2021, the source remained absent at first, then appeared on February 7, 2021, reaching a peak flux density of 5.6 millijanskys before fading over about one day. MeerKAT added crucial detail: the source showed up to 80 percent linear polarisation alongside its circular polarisation, and its rotation measure—a quantity related to how magnetised plasma twists radio waves—changed significantly over three days.
In 2024, Kierra Weatherhead and colleagues reported new measurements from serendipitous observations at three epochs in 2020 and 2021, using THOR-GC and VLITE. They detected the source only once, on April 11, 2020, with a flux density of 20.6 millijanskys at 1.23 gigahertz. Their analysis suggested a spectral break below one gigahertz and a rotation-measure range larger than previously reported. They proposed that the data might be consistent with a highly supersonic neutron star interacting with a changing environment—still a possibility, not an identification. This shift in emphasis mattered. The longer the source remained constrained only by intermittent radio detections and non-detections everywhere else, the more the problem became environmental as well as intrinsic. Astronomers needed to understand not only what object was emitting, but what magnetised material the radio waves were passing through.
What ASKAP J173608.2-321635 represents is a broader change in astronomy itself. Wide-field radio surveys are beginning to find objects that do not announce themselves in optical catalogues, X-ray archives, or infrared images. Some may turn out to be unusual versions of known systems. Others may require an entirely new category. For now, the source remains unclassified—a position on the sky, a set of measurements, and a gap in the catalogue. Its six 2020 detections, its February 2021 flare, its strong polarisation, its changing rotation measure, and its missing X-ray and infrared counterparts are all real observational constraints. The label "new kind of object" is still a possibility, not a settled result. The next step is plain: the source needs to switch on again while radio, X-ray, infrared, and perhaps optical facilities are watching simultaneously. Until then, it remains what its name implies—a mystery that radio astronomy has caught, but not yet explained.
Citas Notables
The source may represent part of a new class of objects being discovered through radio imaging surveys— Wang and colleagues, original discovery paper
The data could be consistent with a highly supersonic neutron star interacting with a changing environment— Weatherhead and colleagues, 2024 analysis
La Conversación del Hearth Otra perspectiva de la historia
Why does it matter that this object doesn't show up in X-rays or infrared? Couldn't it just be too faint?
Faintness is part of it, but the real problem is that different types of objects have different signatures. A magnetar should scream in X-rays. A nearby star should glow in infrared. This thing does neither, which means the usual explanations don't work.
So what's the most likely explanation at this point?
Honestly, we don't have one. The 2024 work suggested a supersonic neutron star moving through a changing magnetic environment, but that's still speculative. The polarisation and variability are real, but they don't point clearly to any single object type.
Why did ASKAP find it when other telescopes didn't?
ASKAP is built to search wide fields and catch things that vary over long timescales. It's not looking for a specific object in a specific place—it's watching the whole sky for anything that flickers. That's exactly the kind of survey that catches things that don't fit the usual patterns.
What happens if it never turns on again?
Then it stays a mystery. We'd have a well-documented set of observations but no way to test any hypothesis. That's why the next activation matters so much—simultaneous observations across all wavelengths could finally tell us what we're looking at.
Could this be something genuinely new to astronomy?
It could be. But that's the careful answer. Right now it's an object that doesn't fit existing categories. Whether that means it's a new class or just an unusual version of something we know—we can't say yet.