An explosion, more than ten times brighter than any known supernova (exploding star), and three times brighter than the brightest tidal disruption event, where a star falls into a supermassive black hole, has been measured and is being investigated by scientists at the University of Southampton who came upon it by chance when they were searching for a type of supernova.

AT2021lwx, the explosion, was first detected in 2020 by the Zwicky Transient Facility in California, and subsequently picked up by the Asteroid Terrestrial-impact Last Alert System (ATLAS) based in Hawaii. These facilities survey the night sky to detect transient objects that rapidly change in brightness indicating cosmic events such as supernovae, as well as finding asteroids and comets. Until now the scale of the explosion has been unknown.

By analysing the spectrum of the light, splitting it up into different wavelengths and measuring the different absorption and emission features of the spectrum, the team were able to measure the distance to the object.

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

It is still unknown what caused the explosion but the Southampton team believe the most feasible explanation is an extremely large cloud of gas (mostly hydrogen) or dust that has come off course from its orbit around the black hole and been sent flying in.

Dr Philip Wiseman, University of Southampton said:

“With new facilities, like the Vera Rubin Observatory’s Legacy Survey of Space and Time, coming online in the next few years, we are hoping 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 centres of galaxies change over time.”

Click on this link to access the paper, Multiwavelength observations of the extraordinary accretion event AT2021lwx, published in Monthly Notices of the Royal Astronomical Society.