The explosion is 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.
The explosion, known as AT2021lwx, has currently lasted over three years, compared to most supernovae which are only visibly bright for a few months. It took place nearly 8 billion light years away, when the universe was around 6 billion years old, and is still being detected by a network of telescopes.
The researchers believe that the explosion is a result of a vast cloud of gas, possibly thousands of times larger than our sun, that has been violently disrupted by a supermassive black hole. Fragments of the cloud would be swallowed up, sending shockwaves through its remnants, as well as into a large dusty ‘doughnut’ surrounding the black hole. Such events are very rare and nothing on this scale has been witnessed before.
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.
The findings of the research have been published today (Friday, 12 May 2023) in Monthly Notices of the Royal Astronomical Society.
Discovery
AT2021lwx 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.
“We came upon this by chance, as it was flagged by our search algorithm when we were searching for a type of supernova,” says Dr Philip Wiseman, Research Fellow at the University of Southampton, who led the research. “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.”
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, and the Gran Telescopio Canarias in La Palma, Spain.
Measuring the explosion
By analyzing 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.
“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 realized this is extremely bright,” says Professor Sebastian Hönig from the University of Southampton, a co-author of the research.
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.
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.”
What caused the explosion?
There are different theories asto what could have caused such an explosion, but the team from Southampton believe the most feasible explanation is an extremely large cloud of gas (mostly hydrogen) or dust that has veered off its course from orbiting the black hole and been propelled outward.
The team is now embarking on collecting additional data on the explosion, including measurements of different wavelengths such as X-rays, which could reveal the object’s surface and temperature, as well as shed light on the underlying processes at play. They will also conduct upgraded computational simulations to test if these align with their theory of what triggered the explosion.
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 hopeful to discover more events like this and gain further insights. It’s possible that these events, though exceedingly rare, are so immensely energetic that they play a pivotal role in the evolution of galactic centers over time.”
The paper titled “Multiwavelength observations of the extraordinary accretion event AT2021lwx” has been published in Monthly Notices of the Royal Astronomical Society and is available for reading online.
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