Loud in radio, invisible in X-rays: the Milky Way source that refuses to fade

Most cosmic outbursts flare and then die away within days or months. One source in our own galaxy, VT J1906+0849, has been blazing in radio waves for about two decades, has barely changed, and shows no X-rays at all, a combination that fits no known type of galactic radio source. A new study led by Jessie M. Miller at Caltech lays out the puzzle, along with a leading suspect: a hidden engine buried in gas. The paper was posted to the arXiv preprint server.

The brightest transient in the sky survey

The source turned up in the Very Large Array Sky Survey, a sweeping radio map of the heavens, and it is in fact the brightest transient that survey has found. When it was first cataloged in October 2017, shining at about 70 millijanskys, an older survey from 21 years earlier had shown nothing at that spot, meaning the source switched on, or brightened enormously, sometime in between. It lies far away, somewhere between roughly 49,000 and 104,000 light-years, out toward the fringes of the Milky Way.

A transient that breaks the rules

What makes it strange is how it behaves, or rather how it refuses to. Transients usually brighten and then gradually fade; this one has kept shining in radio for around twenty years. Sharp radio images taken in 2010, 2022 and 2025 show that the emitting region is extremely compact and has not visibly expanded. A dedicated X-ray look with the Swift satellite found nothing. And yet its spectrum reveals gas streaming outward at some 2,000 kilometers per second. Loud, compact, long-lived, fast-flowing and X-ray silent, no previously known galactic source ticks all of those boxes at once.

A hidden engine

Here is the leading interpretation, offered as a possibility rather than a verdict. The authors suggest that VT J1906+0849 may be a young version of SS 433, the Milky Way’s famous microquasar, a system in which a compact object, either a neutron star or a black hole, feeds off a companion star and drives powerful jets and winds. In this picture, a thick, fast wind from the accretion disk wraps around the central engine, confining and continuously powering the radio emission while smothering the X-rays that such an object would normally give off. That would neatly explain the odd mix: bright and steady in radio, yet hidden in X-rays.

Why it is worth chasing

A rival idea, that the glow comes from a young neutron star gradually spinning down, runs into trouble, because such sources should expand over time and this one has not. Which is precisely why VT J1906+0849 matters. Radio transients inside our own galaxy are far less explored than the flashy fast radio bursts from distant galaxies, and this one may be caught in a rare, fleeting phase, an accreting compact object in the act of switching on, still cocooned in the gas of its birth. Whether it turns out to be a newborn microquasar or something genuinely new, tracking how it changes over the coming years is the surest way to find out. For now, the Milky Way is keeping one of its engines well hidden, and shouting about it in radio.

© 2026 Homer Dávila Gutiérrez, FRAS — SKYCR.ORG. All rights reserved. Publication: 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