The largest cosmic explosion ever observed has been discovered by a team of astronomers led by the University of Southampton. This explosion is more than 10 times brighter than any known supernova and three times brighter than the brightest tidal disruption event, which occurs when a star falls into a supermassive black hole. It has been named AT2021lwx and has been visible for over three years, unlike most supernovae, which are only visible for a few months. The explosion occurred almost 8 billion light years away, when the universe was approximately 6 billion years old, and can still be detected by a network of telescopes.
The researchers speculate that the explosion was caused by a massive cloud of gas, potentially thousands of times larger than our sun, which was violently disrupted by a supermassive black hole. Fragments of the cloud would have been absorbed, sending shockwaves through the remaining gas and into a large dusty “doughnut” surrounding the black hole. Such events are extremely rare, and nothing of this magnitude has been observed before.
Last year, astronomers observed the brightest explosion ever recorded, a gamma-ray burst known as GRB 221009A. While this event was brighter than AT2021lwx, it lasted for only a fraction of the time, indicating that the overall energy released by the AT2021lwx explosion is much greater.
The research findings have been published today in the Monthly Notices of the Royal Astronomical Society.
In 2020, the Zwicky Transient Facility in California first detected AT2021lwx, which was later identified by the Asteroid Terrestrial-impact Last Alert System (ATLAS) in Hawaii. These observatories regularly scan the night sky to locate transient objects that change brightness rapidly, indicating celestial events such as supernovae, as well as detecting asteroids and comets. Until now, the magnitude of this explosion had remained unknown.
Dr. Philip Wiseman, a Research Fellow at the University of Southampton who led the research, said, “We discovered this explosion by chance, as it was flagged by our search algorithm while we were searching for a specific type of supernova. Typically, supernovae and tidal disruption events only remain bright for a couple of months before fading away. Therefore, something remaining luminous for over two years was immediately intriguing.”
The team conducted further investigations using several telescopes, including 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
Through the analysis of the spectrum of light, which involved dividing it into different wavelengths and measuring the various absorption and emission features of the spectrum, the research team could determine the distance to the object.
“When you know the distance to the object and its apparent brightness to us, you can calculate the object’s brightness at its origin. Once we had completed those calculations, we realized that this was an extremely luminous event,” says Professor Sebastian Hönig, a co-author of the study from the University of Southampton.
The only objects in the universe that match the brightness of AT2021lwx are quasars, which are supermassive black holes that continuously accrete gas at high speeds.
Professor Mark Sullivan, also from the University of Southampton and a co-author of the paper, explains, “With a quasar, we observe the brightness fluctuating over time. However, there was no detection of AT2021lwx over a decade, and then suddenly, it appeared with the brightness of the brightest objects in the universe, which is unprecedented.”
What caused the explosion?
Various hypotheses have been put forward to explain the origin of such an explosion, but the team led by Southampton considers that the most plausible explanation is an enormous cloud of gas (primarily hydrogen) or dust that has veered off its trajectory around the black hole and been violently ejected.
The team is now embarking on a quest to gather more data on the explosion, including measurements of different wavelengths, such as X-rays that could unveil the object’s surface and temperature, and identify the underlying processes taking place. Additionally, they will carry out advanced computational simulations to test whether they correspond to their theory of what triggered the explosion.
Dr. Philip Wiseman remarked, “With new facilities, such as the Vera Rubin Observatory’s Legacy Survey of Space and Time, set to commence operations in the coming years, we hope to discover more events like this and learn more about them. It could be that these events, while exceedingly rare, are so powerful that they play a critical role in the evolution of galactic centers over time.”
Source: University of Southampton