Last year telescopes around the world recorded the brightest cosmic explosion of all time, and now astrophysicists can explain why it was so dazzling, they report in the journal Science Advances.

The cosmic explosion was recorded on October 22 of last year and was dubbed the Brightest of All Time (BOAT). The event, caused by the collapse of a very massive star and the subsequent birth of a black hole, was witnessed as an immensely bright flash of gamma rays followed by a slow glow of light across the frequencies. Ever since they simultaneously picked up the BOAT signal on their giant telescopes, astrophysicists around the world have struggled to explain the brightness of the gamma-ray burst (GRB) and the curious slowness of its afterglow.

Now an international team including Dr Hendrik Van Eerten, from the Department of Physics at the University of Bath, UK, has come up with an explanation: the initial explosion (known as GRB 221009A) headed straight for Earth and it dragged an unusually large amount of stellar material in its wake. Dr Van Eerten, who co-led the theoretical analysis of the glow, explains that other researchers who have worked on this puzzle have also concluded that the jet was pointed directly at us, like a garden hose angled to spray directly at you, and this “explains to some extent why it was seen with such intensity.” But what remained a puzzle was that the edges of the jet could not be seen at all.

“The slow fading of the afterglow is not characteristic of a narrow gas jet, and knowing this made us suspect that there was an additional reason for the intensity of the explosion, and our mathematical models have confirmed this,” he adds. Our work clearly shows that the GRB had a unique structure, with observations gradually revealing a narrow jet embedded within a broader outflow of gas where an isolated jet would normally be expected.” Van Eerten explains that what they believe made the difference in this case “was the amount of mixing that occurred between the stellar material and the jet, so that the gas heated by the shock continued to appear in our line of sight until the point in which any characteristic signature of the jet would have been lost in the general emission of the glow. “Our model not only helps to understand BOAT, but also previous brightness records that had puzzled astronomers because of their lack of a jet signature,” he continues. These GRBs, like other GRBs, must be directed directly at us when they are produced, since it would not be physical for so much energy to be ejected in all directions at once.”

As he points out, “there seems to be an exceptional class of events that are both extreme and manage to mask the directed nature of their gas flow. Future studies on the magnetic fields that launch the jet and on the massive stars that host them should help reveal why these GRBs are so rare.”

The exceptionally large GRB 221009A is the brightest GRB ever recorded and its afterglow is shattering all records at all wavelengths,” says Dr. Brendan O’Connor, a recent graduate doctoral student at the University of Maryland and George Washington University and lead author of the study.