Astronomers have uncovered remarkable new evidence of a rare “missing-link” black hole after detecting a powerful burst of radio waves that continued growing brighter for nearly two years.
The unusual discovery provides one of the strongest clues yet that intermediate-mass black holes can launch powerful jets when they tear apart passing stars.
The findings have been accepted for publication in The Astrophysical Journal Letters.
The event, known as AT2019ijn, was first spotted as a bright blue flash in optical sky surveys. It quickly reached its peak brightness within just a few days before fading much more slowly than similar cosmic explosions usually do.
At first, it appeared unusual but not extraordinary.
The real surprise came later when astronomers examined radio observations collected over several years.
Instead of fading away, the radio signal continued to brighten for almost two years before reaching an intensity far greater than normally seen from exploding stars or other similar events. Even after peaking, the radio emission slowly declined over at least four years.
To understand what had happened, researchers combined observations from several radio telescopes, including the U.S. National Science Foundation’s Very Large Array (VLA), Australia’s ASKAP telescope and India’s upgraded Giant Metrewave Radio Telescope.
These long-term observations allowed the team to follow the changing radio signal and compare it with different scientific models.
The researchers concluded that the most likely explanation is a tidal disruption event. This dramatic phenomenon occurs when a star wanders too close to a black hole. The black hole’s immense gravity stretches and tears the star apart, pulling its material into space while releasing enormous amounts of energy.
What makes AT2019ijn particularly exciting is the type of black hole believed to be responsible. The evidence points to an intermediate-mass black hole, a rare class that sits between the much smaller black holes formed when massive stars die and the supermassive black holes found at the centers of galaxies.
Astronomers have spent decades searching for these middle-sized black holes because they are thought to represent an important missing step in the growth of the universe’s largest black holes. However, they have proven extremely difficult to find.
The team believes the black hole also launched a narrow jet of material moving at a significant fraction of the speed of light. Unlike some jets that point directly toward Earth, this one appears to have been aimed off to the side. Because of this angle, the jet’s afterglow was initially hidden from view.
As the jet gradually slowed over time, its radio emission spread outward and eventually became visible from Earth. This delayed appearance naturally explains why the radio signal became brighter long after the original flash had faded. At its peak, the radio emission was more than 100 times brighter than radio signals typically seen from similar fast blue optical transients or supernovae.
The discovery suggests that many unusual cosmic flashes detected in visible light could actually be hidden black hole events whose brightest radio signals appear months or even years later. As new sky surveys repeatedly scan the universe in both visible light and radio waves, astronomers expect to uncover many more examples like AT2019ijn.
Each new discovery will help scientists better understand how intermediate-mass black holes form, how often they destroy passing stars, and why only some of these dramatic encounters produce powerful jets that can eventually light up the radio sky.
