University of Southampton astronomers have witnessed the largest cosmic explosion ever discovered.

The explosion, known as AT2021lwx, is over 10 times brighter than any known supernova and three times brighter than the brightest tidal disruption event.

It has currently lasted over three years, whereas most supernovae are only visible for a few months. This bang happened nearly eight billion light-years away and took place when the universe was only around six billion years old. A network of telescopes is still detecting it.

Scientists think a supermassive black hole is to blame for the explosion, which they attribute to the violent disruption of a gas cloud thousands of times larger than our sun. Pieces of the cloud would fall into the black hole, sending shockwaves through what was left of the cloud and out into the vast dusty “doughnut” that encircled the black hole. It was extremely uncommon, and never before seen on this scale.

Last year, astronomers witnessed the brightest explosion on record – a gamma-ray burst known as GRB 221009A. While this was brighter than AT2021lwx, it lasted for just a fraction of the time, meaning the overall energy released by the AT2021lwx explosion is far greater.

AT2021lwx was initially detected in 2020 by the Zwicky Transient Facility in California, and then by the Asteroid Terrestrial-impact Last Alert System in Hawaii. These facilities survey the night sky to locate asteroids and comets as well as transient objects whose brightness fluctuates rapidly, indicating cosmic events such as supernovae. The magnitude of the explosion was previously unknown.

An accidental discovery

The discovery was made by chance. A search algorithm flagged it while the team was looking for a type of supernova. Most supernovae and tidal disruption events only last for a couple of months before fading away, making the prolonged brightness of AT2021lwx immediately unusual.

Multiple telescopes were used to investigate the object further: the Neil Gehrels Swift Telescope (a collaboration between NASA, the United Kingdom and Italy), the New Technology Telescope (operated by the European Southern Observatory) in Chile, and the Gran Telescopio Canarias on La Palma, Spain.

The only things in the universe as bright as AT2021lwx are quasars, supermassive black holes with a constant flow of gas falling onto them at high velocity.

However, with a quasar, the brightness flickers up and down over time. Looking back over a decade, there was no detection of AT2021lwx. Then suddenly it just appeared with the unprecedented glow of the brightest thing in the universe.

There are different theories about what could have caused such an explosion. Still, the Southampton-led team believes the most feasible explanation is an extremely large cloud of gas or dust that has come off course from its orbit around the black hole and been sent flying in.

With new facilities, like the Vera Rubin Observatory’s Legacy Survey of Space and Time, coming online in the next few years, the team hopes to discover more events like this and learn more about them. It could be that these events, although extremely rare, are so energetic that they are key processes to how the centers of galaxies change over time.

The study was published in the Monthly Notices of the Royal Astronomical Society.