The only other viable explanation for what we see is an extraordinarily rare event in which a star is torn apart by an intermediate-mass black hole: a long-hypothesized, elusive class of black holes that has proven difficult to detect.

Either way, we're watching the slow-motion aftermath of one of the most extreme, rare events the cosmos can produce.

The explosions we usually miss

Gamma-ray bursts are brief but powerful jets of high-energy radiation. Within seconds, they release as much energy as the sun will emit over its entire lifetime. They are caused when massive stars die and form black holes.

While these jets are launched in all directions, we only observe the small fraction whose emission is directed towards us. When it is directed away from us, the initial flash goes unseen, and all we can observe is the slowly fading afterglow.

Although these so-called "orphan afterglows" of gamma-ray bursts have been predicted for decades, finding them has proven extraordinarily difficult. Without a high-energy flash to announce their arrival, astronomers have to search thousands of square degrees of sky.