A team of astronomers led by the University of Southampton has discovered the largest cosmic explosion ever seen, more than ten times brighter than any known supernova and three times brighter than the brightest tidal disruption event, in which a star falls into a supermassive black hole.

The explosion, known as AT2021lwx, has now lasted more than three years, compared to most supernovae, which only shine visibly for a few months. It took place nearly 8 billion light-years away, when the universe was about 000 billion years old, and is still being detected by a network of telescopes.

Researchers believe the explosion is the result of an immense cloud of gas, possibly thousands of times larger than our sun, which has been violently disturbed by a supermassive black hole. Fragments of the cloud would be engulfed up, sending shock waves through its remains, as well as a large dusty "doughnut" surrounding the black hole. Such occurrences are very rare and nothing like it has been observed so far.

Last year, astronomers witnessed the brightest explosion on record: a gamma-ray burst known as GRB 221009A. Although this explosion was brighter than that of AT2021lwx, it only lasted a fraction of the time, meaning that the total energy released by the explosion of AT2021lwx is much higher.

AT2021lwx was first detected in 2020 by California's Zwicky Transient Facility, and subsequently by Hawaii's Asteroid Terrestrial-impact Last Alert System (ATLAS). These facilities monitor the night sky to detect transient objects that rapidly change brightness, indicating the existence of cosmic events such as supernovae, as well as to find asteroids and comets. Until now, the magnitude of the explosion was unknown.

"We discovered it by chance, as our search algorithm pinpointed it when we were looking for a type of supernova," explains Philip Wiseman, a researcher at the University of Southampton who led the research--. Most supernovae and tidal disturbances only last a couple of months before fading away. For something to shine for more than two years was immediately very unusual."

The team continued to investigate the object with several different telescopes: the Neil Gehrels Swift telescope (a collaboration between NASA, the UK and Italy), the New Technologies Telescope (operated by the European Southern Observatory) in Chile, and the Gran Telescopio Canarias in La Palma.

By analyzing the spectrum of light, dividing it into different wavelengths and measuring the different absorption and emission characteristics of the spectrum, the team was able to measure the distance to the object.

"Once you know the distance to the object and how bright it seems to you, you can calculate the brightness of the object at its source. Once those calculations were made, we realised that it is extremely bright," says Professor Sebastian Hönig of the University of Southampton, a co-author of the research.

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

Professor Mark Sullivan, also from the University of Southampton and another of the paper's co-authors, said: "In a quasar, we see brightness waxing and waning over time. But if we look back a decade AT2021lwx had not been detected, and suddenly it appears with the brightness of the brightest things in the universe, which is unprecedented."

There are different theories as to what might have caused such an explosion, but the Southampton-led team believes the most plausible explanation is an extremely large cloud of gas (mostly hydrogen) or dust that has drifted out of its orbit around the black hole and blasted inward.

The team is now preparing to collect more data on the explosion, measuring different wavelengths, including X-rays, which could reveal the surface and temperature of the object, as well as the underlying processes. They will also carry out improved computer simulations to see if they match their theory about the cause of the explosion.

Dr. Philip Wiseman predicts that, "with the commissioning in the coming years of new facilities, such as the Legacy Survey of Space and Time of the Vera Rubin Observatory, we hope to discover more events like this and learn more about the evolution of the Earth."

  • Astronomy

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