Hidden jet from a ‘missing-link’ black hole lights up the radio sky

Astronomers using the U.S. National Science Foundation Very Large Array (NSF VLA) have detected an extraordinary burst of radio light from a rare cosmic event in which an intermediate-mass black hole tears apart a star, revealing what appears to be the off-axis afterglow of a powerful jet. The research is published in The Astrophysical Journal Letters.

Unusual observations of AT2019ijn

The event, known as AT2019ijn, first appeared as a bright blue flash in optical surveys, rising to peak brightness in just a few days before fading much more slowly than similar transients usually do. When astronomers later examined radio observations, they found something even more unusual: The radio emission kept brightening for nearly two years and reached a luminosity far beyond that seen in typical stellar explosions at similar phases, followed by a slow decay over at least four years.

The researchers concluded that the most likely explanation is a tidal disruption event, which happens when a star strays too close to a black hole and is pulled apart by gravity. In this case, the fast rise in the optical brightness indicates an intermediate-mass black hole, a long-sought class that falls between stellar-mass black holes formed by collapsing stars and the supermassive black holes found at the centers of galaxies. These middleweight black holes have been difficult to find, and astronomers have been eager for new ways to identify them.

AT2019ijn offers one such path: If an intermediate-mass black hole launches a jet that is not aimed directly at Earth, the event may look modest at first, then become dramatically brighter in radio light later as the jet slows and its afterglow emission spreads into view. That delayed brightening is one of the most striking features of this discovery. At 3 gigahertz, the radio signal reached a luminosity more than 100 times brighter than radio emission seen from known fast blue optical transients or supernovae at similar stages.

Multi-facility observations and modeling

To piece together the event, the team combined optical survey data with radio observations from the NSF VLA, including the Very Large Array Sky Survey, and additional measurements from ASKAP in Australia and the upgraded Giant Metrewave Radio Telescope in India. That broad radio coverage let the researchers track how the signal changed over time and test models for an expanding outflow powered by a tidal disruption event.

Their modeling suggests the radio emission came from material moving at a significant fraction of the speed of light. The data are best explained by a narrow relativistic jet viewed from well off to the side, rather than head-on, which naturally accounts for why the radio flare appeared so late.

Scientific significance

This discovery is especially significant because it expands the way astronomers can search for hidden black holes and the extreme jets they launch. It also suggests that some unusual optical transients may be part of a broader family of black-hole-powered events that have been missed because their radio peaks arrive long after the initial flash.

As new sky surveys repeatedly scan the heavens in both visible light and radio waves, astronomers expect to find more events like AT2019ijn. Each new detection could help reveal how intermediate-mass black holes form, how often they tear apart stars and under what conditions they produce powerful jets.

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