For more than twenty years, an unknown source in our galaxy has been emitting a radio signal that changes without following the usual behavior of known objects.
Its name, VT J1906+0849, is not easy to remember. But its story is astonishing. Astronomers spotted it in observations made in 2017, then traced it back through archival data. In 1996, nothing appeared at that location in the sky. By 2005, a radio source was detectable.
Representation of our galaxy, the Milky Way
NASA/JPL-Caltech/R. Hurt (SSC/Caltech) · Public domain
Over the years, its intensity increased until reaching a peak around 2014. Then it decreased for several years. Researchers thought they were witnessing the gradual fading of the phenomenon, before a new surprise emerged: at the end of 2025, the signal began growing stronger again.
The object appears to be located in a distant region of the Milky Way, at least 49,000 light-years from us. It cannot be distinguished like an ordinary star in visible light. Radio telescopes detect its radiation, which is invisible to our eyes.
Astronomers also measure the different frequencies of radio waves, much as we distinguish the low and high notes of a sound. Yet the frequency at which the signal is strongest changes unusually: it decreased, before rising sharply again.
From left to right: VT J1906+0849 at the reference epoch and then at the discovery epoch. All images share the same flux scale.
An explosion could produce such radiation, but it should normally eject matter that moves away and spreads through space. Here, the most detailed observations show no explosion, despite the signal's changes over several years. The explanation of a simple expanding cloud is therefore insufficient.
Researchers are considering whether a very dense object, such as a black hole or a neutron star, could be pulling matter toward itself. It could then emit jets of very fast particles. If these jets remain trapped in dense gas, that might explain why the source does not resemble an explosion.
But this scenario has not been confirmed. The object also displays unusual properties in X-rays and infrared light. No known system yet reproduces all the observations, and researchers still do not know what is actually powering the phenomenon.
New radio measurements will make it possible to track its next change in intensity. This time, astronomers know precisely where to look. Their goal is to understand what can sustain such a powerful and changing signal for several decades, within our own galaxy.