Mysterious radio signal in the Milky Way brightens and fades over decades, baffling astronomers

Object VT J1906+0849, located 49,000 light-years away, shows behavior unlike any known transient, possibly a black hole or neutron star accreting material

By LineZotpaper AI staff

Published22 October 2026

Read Time2 min

A strange radio signal from the outskirts of the Milky Way has astronomers puzzled: the object, designated VT J1906+0849, has been brightening and fading over decades, with its radio spectrum shifting in ways that do not match any known celestial phenomenon. The source, located at least 49,000 light-years from Earth beyond the Norma-Cygnus spiral arm, was first noticed as a bright radio transient in 2017 data from the Very Large Array Sky Survey, but subsequent analysis of older observations revealed it had been active since at least 2005, with no trace in 1996. The findings are published in the Astrophysical Journal.

Most cosmic objects that change in brightness do so on short timescales, from milliseconds to months. But VT J1906+0849 has been evolving over decades in an unusual pattern. After appearing between 1996 and 2005, it brightened steadily, reaching a peak around 200 millijanskies in 2014. It then began to fade, as seen in 2017, 2019, and 2024 observations. Then, late last year, it brightened again.

Adding to the mystery, the object's radio spectrum shows a peak that has moved: from around 1.9 gigahertz in 2019 to 0.8 gigahertz in 2024, then back up to 2.2 gigahertz in 2026. For a typical expanding cloud of synchrotron-emitting material, the peak would shift toward lower frequencies as the cloud spreads. But VT J1906+0849 shows little sign of expansion, despite its dramatic changes in brightness and spectrum.

Astronomers led by Jessie Miller of the California Institute of Technology considered several explanations. A nova explosion is possible, but the ejected material should show expansion, which is not observed. A pulsar-powered nebula is another option, but its spectrum would be more predictable. The third possibility is an accreting compact object, such as a black hole or neutron star, pulling material from a binary companion. This could produce jets and a dense wind that obscures X-rays, explaining why NASA's Swift telescope did not detect the expected X-ray emission.

Infrared observations of VT J1906+0849 revealed gas moving at about 2,000 kilometers per second, consistent with a powerful outflow. This resembles the known object SS 433, a black hole or neutron star system that ejects jets and a dense wind, hiding its X-ray emission. The researchers suggest VT J1906+0849 may be a younger analogue of SS 433, but further observations are needed to confirm.

Analysis

Why This Matters

VT J1906+0849 represents a potential new class of radio transient that operates on decade-long timescales, challenging current models of how objects in the galaxy evolve.
If confirmed as an accreting compact object with obscured X-rays, it would provide insights into how black holes and neutron stars interact with their environments.
The discovery underscores that the Milky Way still holds surprises, and that long-term monitoring of the sky is essential to detect slow-changing phenomena.

Background

Most radio transients detected in the Milky Way change rapidly: pulsars pulse in milliseconds, novae erupt and fade over weeks. Large-scale surveys like the Very Large Array Sky Survey, which began in the 1990s, have helped astronomers track such events. VT J1906+0849 first stood out in 2017 as the brightest radio transient in that survey. Its behavior over the following decade led researchers to dig into archival data, revealing a much longer history. SS 433, a famous object 18,000 light-years away, is known for its jets and dense wind that obscure its central engine, making it a possible template for this new source.

Key Perspectives

Nova hypothesis: A nova could in principle produce brightening and fading, but the lack of expansion and the multi-decade timescale do not fit typical nova behavior. Pulsar-powered nebula hypothesis: A pulsar wind nebula can produce synchrotron emission, but the chaotic shifting of the spectral peak is inconsistent with known examples. Accreting compact object hypothesis (favored by researchers): A black hole or neutron star pulling material from a companion star could launch jets and a wind, explaining the missing X-rays and the lack of expansion. Infrared evidence of fast-moving gas supports this idea.

What to Watch

Future radio observations, especially at multiple frequencies, to track the spectral peak and any signs of expansion.
X-ray observations with sensitive telescopes to try to detect emission that may break through the proposed obscuring wind.
Optical and infrared follow-up to identify a possible binary companion star.
Whether VT J1906+0849 continues to brighten or follows a new pattern.

Sources

A Strange Radio Signal in The Far Reaches of The Milky Way Is Behaving Like Nothing Else in The Galaxy · ScienceAlert

Checked

11 claims in this story checked against 1 source: 9 supported.

Show each claim

SupportedThe radio signal has brightened and faded over decades, puzzling astronomers.ScienceAlert: “The hinky part? Whatever is powering the object seems to operate on a timescale of decades.”
Partly supportedThe summary says VT J1906+0849 is 49,000 light-years away.ScienceAlert: “There, at least 49,000 light-years from Earth, out beyond the galaxy's spiral arm, was the brightest radio transient the survey had discovered.”
SupportedVT J1906+0849 may be a black hole or neutron star accreting material from a companion.ScienceAlert: “Or a black hole or neutron star accreting material from a binary companion, if you want to be all straitlaced about it.”
SupportedThe source was first noticed in 2017 data, was absent in 1996, and had begun appearing by 2005.ScienceAlert: “In 1996… there was absolutely nothing to be seen at the site in data collected as part of the NRAO VLA Sky Survey. But by 2005, that had started to change.”
SupportedThe object peaked at around 200 millijanskies in 2014, faded, then brightened again in late 2025.ScienceAlert: “Over the course of several surveys that observed the object between 2006 and 2013, it showed an overall brightening – reaching a peak of around 200 millijanskies in 2014.”
SupportedIts radio-spectrum peak shifted from about 1.9 GHz in 2019 to 0.8 GHz in 2024, then to 2.2 GHz in 2026.ScienceAlert: “In 2019, it was at around 1.9 gigahertz. By 2024, it had shifted down to around 0.8 gigahertz. But by 2026, it had climbed back up to around 2.2 gigahertz.”
SupportedAn expanding synchrotron-emitting cloud would usually have its spectral peak shift lower as it spreads, but this object shows little evidence of expansion.ScienceAlert: “with the peak shifting toward lower frequencies as the cloud spread out. But VT J1906+0849 doesn’t seem to be spreading out much.”
Partly supportedA nova is possible, but the expected expansion is not observed.ScienceAlert: “One thing that can brighten and then fade over time is an explosion such as a nova – but the ejected material should show signatures of expansion, even if the timescale made sense, which it does not.”
SupportedSwift did not detect the expected X-rays, which a dense wind from an accreting system could obscure.ScienceAlert: “Observations using NASA’s Swift telescope failed to detect a hint of the powerful X-rays expected from such an accreting system.”
SupportedInfrared observations found gas moving at about 2,000 kilometres per second, consistent with a powerful outflow.ScienceAlert: “infrared observations of VT J1906+0849 revealed evidence of gas moving at around 2,000 kilometers per second, consistent with a powerful outflow.”
SupportedResearchers suggest VT J1906+0849 may be a younger analogue of SS 433, but further observations are needed.ScienceAlert: “More observations will be needed to determine whether VT J1906+0849 really is a younger relative of SS 433, or something even stranger.”