Astronomers have identified a peculiar radio transient in the Milky Way that has remained active for more than two decades, defying the behavior expected of known classes of galactic radio sources. The paper posted to the arXiv preprint server on Aug. 21 discusses the rare event that may have caused this bizarre radio emission.
Wavering radio waves
Transient radio emission—radio waves from astronomical sources that brighten, fade or change over time—arises from a number of violent cosmic events. These include rapidly spinning neutron stars, shocks produced when powerful stellar winds interact with their surroundings, and mass accretion by white dwarfs from their companion stars in binary systems, among others.
Extragalactic radio transients, particularly fast radio bursts, have been discovered in large numbers in recent years. Within the Milky Way, however, the population of radio transients remains much less explored. While searching for such sources at low galactic latitudes, astronomers discovered VT J1906+0849, the brightest known transient in the Very Large Array Sky Survey (VLASS).
Its first detection with VLASS was recorded on Oct. 27, 2017. The team, led by Jessie M. Miller of the California Institute of Technology, carried out a multiwavelength investigation of VT J1906+0849 to determine what this unusual radio transient actually is. They combined archival observations with new follow-up observations across radio, infrared, optical and X-ray wavelengths.
Researchers estimated that it had brightened by at least a factor of six over roughly 21 years. Archival data revealed an earlier detection in 2005 by the MAGPIS survey, when it was about 35 millijansky (mJy) at 4.86 gigahertz, while its brightness appears to have peaked at more than 200 mJy around 2014. It subsequently faded before undergoing a new rebrightening in late 2025.
No X-ray, only radio
Very Long Baseline Array observations from 2010, 2022 and new observations in 2025 showed that the emitting region is extremely compact. The team estimated the source’s distance to be in the range of 15 kpc (49,000 light-years) to 32 kpc (104,000 light-years). A Swift X-ray observation detected no X-ray counterpart.
They examined the source’s radio spectrum, how it changes, compactness, brightness temperature and multiwavelength properties to determine what could be powering it.
Most transients follow a pattern: They suddenly become bright and then gradually fade. That is not the case for VT J1906+0849. Its properties are unlike any previously identified galactic radio source. It has been observed emitting radio waves for two decades, has not expanded despite spectral lines hinting at gas moving at thousands of kilometers per second—and remains invisible in X-rays.
Events such as flare stars, magnetically active binaries, novae and cataclysmic variables are far too faint to match the source’s estimated luminosity.
Confusing origin
Its brightness temperature is 100 million to 10 billion kelvin. If the light was produced by thermal processes, such as those associated with star formation, the temperature would not be expected to reach such high values. Instead, the high temperature points toward a nonthermal origin, likely linked to energetic activity near a compact object. The most natural mechanism is synchrotron radiation, produced when relativistic electrons spiral through strong magnetic fields and emit radio photons.
A young neutron star could, in principle, provide enough energy. Pulsars lose rotational energy through the “spin-down” process and can power synchrotron-emitting pulsar wind nebulae. However, they evolve over centuries or longer, whereas VT J1906+0849 has changed substantially over just a few years.
Instead, they favor accretion onto a compact object. “These properties favor sustained accretion onto a compact object rather than an impulsive explosion or passively evolving nebula,” the team writes in the paper.
Hidden central engine
A black hole or a neutron star could continuously launch a jet containing relativistic particles and magnetic fields. They say that if the jet were interacting with a dense disk wind, the wind would block some radio-emitting plasma, which would explain why the source remains compact and is not expanding.
Broad near-infrared emission lines provided possible evidence for such a wind, and emission from one component of that wind was blueshifted—meaning that portion is moving toward us. Researchers suggest that it could originate from a fast, asymmetric or lopsided disk wind. Additionally, the absence of X-rays could be due to the central X-ray source being hidden behind a dense inner wind.
“The approaching side of the wind may produce the persistent blueshifted near-infrared line, while a denser inner component suppresses direct X-ray emission and the redshifted component of the disk wind,” the team explains the scenario.
A similar but much more extreme configuration was seen in microquasar SS 433, where a dense wind surrounded an accreting compact object and interacted with its jets.
For now, the nature of VT J1906+0849 remains uncertain. High-resolution infrared spectroscopy could confirm the presence of a disk wind, and deep hard X-ray observations could reveal an obscured central engine. If confirmed, VT J1906+0849 could be the first member of a new population of galactic accreting compact objects that are faint in X-rays but extraordinarily bright in radio.
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Publication details
Jessie M. Miller et al, VLASS Discovery of a Luminous Galactic Radio Transient Evolving on Decade Timescales, arXiv (2026). DOI: 10.48550/arxiv.2608.21320
Journal information:
arXiv
Key concepts
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Bizarre galactic radio source remains active for two decades with no detectable X-rays (2026, September 14)
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