Bizarre galactic radio source remains active for two decades with no detectable X-rays

A peculiar radio transient, VT J1906+0849, has remained active for more than two decades without detectable X-rays, defying known galactic radio source behavior. The source, discovered in the Milky Way, was first detected in 2017 by the Very Large Array Sky Survey (VLASS) and has not expanded despite spectral lines suggesting gas movement at thousands of kilometers per second. It emits radio waves with no X-ray counterpart, and its properties are unlike any previously identified galactic radio source. The team estimates its brightness has increased by at least a factor of six over 21 years, and it has not faded significantly. The source is compact and emits at high brightness temperatures, suggesting nonthermal origins, possibly linked to energetic activity near a compact object.

The study highlights the rarity of such a source, as extragalactic radio transients, like fast radio bursts, are more commonly observed. VT J1906+0849 is the brightest known transient in the VLASS survey and has not expanded despite spectral lines hinting at gas movement. The team estimates its distance to be between 15 and 32 kiloparsecs, and its radio spectrum shows no signs of fading. The source's lack of X-ray emission could be due to a hidden central engine, possibly a black hole or neutron star, which is obscured by a dense wind. The source's properties favor sustained accretion onto a compact object rather than an impulsive explosion or passively evolving nebula. The team suggests it 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.

The paper discusses the possibility of a disk wind interacting with a jet, which could explain the source's compactness and lack of expansion. The absence of X-rays may be due to the central X-ray source being hidden behind a dense inner wind. The source's high brightness temperature and radio spectrum suggest a nonthermal origin, possibly synchrotron radiation from relativistic electrons spiraling through strong magnetic fields. The study notes that such a source would be rare, as most transients follow a predictable pattern of brightness changes. The paper emphasizes the importance of high-resolution infrared spectroscopy and deep hard X-ray observations to confirm the presence of a disk wind and 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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