GRB 221009A: Uncovering the Mystery Behind the Brightest Cosmological Explosion Ever Observed

Friday - June 9 2023, 08:50 UTC - 1 year ago

Gamma ray bursts (GRBs) are the brightest explosions in the universe, typically the result of the cataclysmic birth of a black hole in a distant galaxy. On October 2022, gamma-ray detectors on the orbital satellites Fermi and the Neil Gehrels Swift Observatory noted a burst known as GRB 221009A, dubbed the "Brightest Of All Time", or the "Boat". GRB 221009A originated from a distance of 2.4 billion light years and its energy was estimated to be 1 gigawatt of power into the Earth’s upper atmosphere. The jet launching process of a GRB is still a puzzle, but special relativity theory tells us that light emitted in all directions from the surface of a fast-moving jet will end up focused along its direction of motion. This means that, for GRBs, it should take a long time for the edges to come into view - which is what was observed with GRB 221009A.

First detected accidentally by US military satellites in the late 1960s, cosmic explosions known as gamma ray bursts (GRBs) have come to be understood as the brightest explosions in the universe. Typically, they are the result of the cataclysmic birth of a black hole in a distant galaxy. One way this can happen is through the collapse of a single, massive star. Astronomers such as myself working in the field are well aware of the massive energy scales involved in GRBs. We know they can release as much energy in gamma rays as the sun does throughout its lifetime. But every once in a while, an event is observed that still gives us pause. In October 2022, gamma-ray detectors on the orbital satellites Fermi and the Neil Gehrels Swift Observatory noted a burst known as GRB 221009A (the date of detection). This quickly turned out to be a record-setter. It was dubbed the Brightest Of All Time, or the "Boat," as convenient shorthand among astronomers studying and observing the event. Not only did the Boat start out bright, it refused to fade away like other bursts. We still do not fully know why the burst was so exceptionally bright, but our new study, published in Science Advances, provides an answer for its stubborn persistence.

The burst originated from a distance of 2.4 billion light years—relatively nearby for a GRB. But even when accounting for relative distance, the energy of the event and the radiation produced by its aftermath were off the charts. It is decidedly not normal for a cosmically distant event to deposit about a gigawatt of power into the Earth’s upper atmosphere.

Observing Narrow Cosmic Jets of Gas .

GRBs such as the Boat launch a stream of gas moving at very close to light speed into space. How exactly the jet is launched remains something of a puzzle—but most likely, it involves magnetic fields near where the black hole is being formed. It is the early emission from this jet that we see as the burst. Later, the jet slows down and produces additional radiation, a fading afterglow of light—from radio waves up to (in exceptional cases) gamma rays.

We do not observe jets directly. Instead, like distant stars, we see GRBs as points in the sky. Nevertheless, we have good reason to believe that GRBs do not explode in all directions equally. For GRB 221009A, this would certainly be unreasonable, as it would involve multiplying the amount of energy detected on Earth by all other directions— amounting to much more energy than any star would have available.

Another indication that GRBs come from jets pointing roughly at us is due to special relativity theory. Relativity teaches us that the speed of light is constant, no matter how fast a source moves at us. But that still allows for the direction of light to become distorted. Thanks to this fun-house mirror effect, light emitted in all directions from the surface of a fast-moving jet will end up focused strongly along its direction of motion. That said, the edges of a jet heading in our direction will be very slightly curved away, meaning their light is focused away from our direction. Only later, when the jet slows down, do the edges normally come into view, seen from the side. This means that, for GRBs, it should take a long time for the edges to come into view—and that is exactly what we observed with GRB 221009A.