Three times brighter than the brightest tidal disruption event, the AT2021lwx was the largest known cosmic explosion recorded by the University of Southampton

A cosmic explosion, known as AT2021lwx, has currently lasted over three years and is still being detected by a network of telescopes.

When compared to most supernovae which are only visibly bright for a few months, the explosion took place nearly 8 billion light years away, when the universe was around 6 billion years old, and is possibly thousands of times larger than our sun.

Occurring 8 billion light years away, the cosmic explosion was found in 2020

Fragments of the cloud would be swallowed up by a violent disturbance of a supermassive black hole, inciting huge shockwaves and forming a large dusty ‘doughnut’ surrounding the black hole.

First detected in 2020 by the Zwicky Transient Facility in California, AT2021lwx was later also picked up by the Asteroid Terrestrial-impact Last Alert System (ATLAS) in Hawaii.

The team gaining more data by the day, measuring different wavelengths, including X-rays which could reveal the object’s surface and temperature, and what underlying processes are taking place.

Until now, the scale of the explosion has been unknown

The telescopes which picked up the event continually survey the night sky to detect transient objects that rapidly change in brightness. Able to indicate cosmic events such as supernovae, these facilities are also capable of finding asteroids and comets.

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.

Until now, the scale of the explosion has been unknown. The researchers say that such an event is very rare and nothing on this scale has been witnessed before.

Could a large cloud of hydrogen gas have caused the explosion?

The researchers believe that the cosmic explosion is a result of a vast cloud of gas, mostly hydrogen, or dust that has come off course from its orbit around the black hole and been sent flying in.

The team investigated the object further with several different telescopes:

  • The Neil Gehrels Swift Telescope (a collaboration between NASA, the UK and Italy)
  • The New Technology Telescope (operated by the European Southern Observatory) in Chile
  • The Gran Telescopio Canarias in La Palma, Spain

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.

They are now utilising upgraded computational simulations to test if these match their theory of what caused the explosion.

The only things in the universe that are as bright as AT2021lwx are quasars

Dr Philip Wiseman, Research Fellow at the University of Southampton, said: “We came upon this by chance, as it was flagged by our search algorithm when we were searching for a type of supernova.

“Most supernovae and tidal disruption events only last for a couple of months before fading away. For something to be bright for two plus years was immediately very unusual.”

Professor Sebastian Hönig from the University of Southampton, added: “Once you know the distance to the object and how bright it appears to us, you can calculate the brightness of the object at its source. Once we’d performed those calculations, we realised this is extremely bright.

Professor Mark Sullivan, also of the University of Southampton and another co-author of the paper, explains: “With a quasar, we see the brightness flickering up and down over time.

“But looking back over a decade there was no detection of AT2021lwx, then suddenly it appears with the brightness of the brightest things in the universe, which is unprecedented.”

“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.”

Dr Philip Wiseman added: “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.”


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