A Black Hole Ate a Star. Then, 641 Days Later, the Radio Sky Lit Up.

Something in a distant dwarf galaxy blazed brilliant blue in 2019, peaked in under six days, and then began to fade. Astronomers catalogued it, classified it as a fast blue optical transient, one of the rarest kinds of cosmic flare, and moved on. Then the radio signal arrived. And kept arriving. And kept brightening, for nearly two years.

Fast blue optical transients are, by definition, fast. They flare, they fade, they're done within a month. AT2019ijn didn't cooperate. Its slow decline looks more like a superluminous supernova or a tidal disruption event, what happens when a star wanders too close to a black hole and gets torn apart. The radio afterglow peaked 641 days after the original optical discovery and came in more than 10 times brighter than any similar event on record.

The leading explanation, from a team led by Hucheng Ding of Anhui Normal University: an off-axis relativistic jet that gradually swept into our line of sight, producing that dramatically delayed peak. The black hole responsible is estimated at around 132,000 solar masses, an intermediate-mass black hole, a class astronomers know must exist but have struggled to catch doing anything. This one was doing quite a lot.

The researchers conclude that AT2019ijn may represent an entirely new class of relativistic optical transient. Which raises an uncomfortable corollary: some events that looked ordinary at first glance may belong to a broader family of black-hole-powered phenomena. The difference, in many cases, may simply be whether anyone kept watching long enough.

Someone kept watching.

Read the full story at Phys.org, July 6, 2026

Hot Take: This cosmic flare thought its crime scene had gone cold two years ago. Turns out the evidence is in because someone left the radio telescope running.