Mysterious Radio Source in the Milky Way Has Puzzled Astronomers for Two Decades

Astronomers have identified a peculiar radio transient in the Milky Way, named VT J1906+0849, that has remained active for more than two decades — far longer than any known class of galactic radio source is expected to last. The discovery, led by Jessie M. Miller of the California Institute of Technology, was posted to the arXiv preprint server on August 21.

The source was first detected in the Very Large Array Sky Survey (VLASS) on October 27, 2017, making it the brightest known transient in that survey. Archival data later revealed an earlier detection by the MAGPIS survey in 2005, and the researchers estimate the source brightened by at least a factor of six over roughly 21 years, peaking around 2014 before fading and then rebrightening in late 2025.

Very Long Baseline Array observations from 2010, 2022 and 2025 show the emitting region is extremely compact, and the team places the source at a distance of roughly 15 to 32 kiloparsecs (about 49,000 to 104,000 light-years). A Swift X-ray observation detected no X-ray counterpart at all, even though the source's estimated brightness temperature — between 100 million and 10 billion kelvin — points to a nonthermal process, most likely synchrotron radiation from relativistic electrons spiraling in a strong magnetic field.

The researchers favor sustained accretion onto a compact object, such as a black hole or neutron star, rather than an exploding star or a passively fading nebula, writing that the pattern is unlike any previously identified galactic radio source. They suggest a dense wind around the object could be blocking its X-ray emission while letting radio waves through, a scenario also seen in the well-studied microquasar SS 433.

If confirmed, the team says VT J1906+0849 could be the first known member of a new population of galactic compact objects that are faint in X-rays but extraordinarily bright in radio.