Largest Detected Cosmic Explosion 8 Billion Light Years Away

The headline image is of the oldest documented observation of a supernova (related to but not the subject of this story). This image combines data from four space telescopes to create a multi-wavelength view of all that remains of RCW 86, the oldest documented example of a supernova. Chinese astronomers witnessed the event in 185 A.D., documenting a mysterious "guest star" that remained in the sky for eight months. X-ray images from NASA's Chandra X-ray Observatory and the European Space Agency's XMM-Newton Observatory were combined to form the blue and green colors in the image. The X-rays show the interstellar gas that has been heated to millions of degrees by the passage of the shock wave from the supernova. Public Domain, Source: https://www.nasa.gov/multimedia/imagegallery/image_feature_2173.html

The discovery

A team of astronomers has uncovered the largest cosmic explosion ever witnessed. The ongoing event is 8 billion light years away and dates back about 6 billion years (the universe is currently estimated to be 13.7 billion years old).

Led by Southampton University, the researchers identified an explosion more than ten times brighter than any known supernova (exploding star) and three times brighter than the brightest tidal disruption event (TDE), when a star falls into a supermassive black hole.

Tagged AT2021lwx, the event was first detected in 2020 by the Zwicky Transient Facility in California, and later picked up by the Asteroid Terrestrial-impact Last Alert System (ATLAS) based in Hawaii. These systems automatically survey the night sky to detect transient objects that rapidly change in brightness and which may therefore point to cosmic events such as supernovae.

"We came upon this by chance, as it was flagged by our search algorithm when we were searching for a type of supernova," said 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. - phys.org

I've actually been up to that telescope site in the Canaries when I was researching 'Cause of All Causes'. Here's my own take on it (note that we're high up here looking down and the blue background is actually the sea).

Last year, astronomers 'imaged' the brightest explosion on record - a gamma-ray burst (GRB) known as GRB 221009A. That GRB (tagged BOAT - brightest of all time) was brighter than AT2021lwx but lasted for a relatively brief period (about 12 days), meaning the overall energy released by the AT2021lwx explosion is far greater. Note that Chinese astronomers in 185 A.D. observed a supernova for 8 months. But 3 years and counting for AT2021lwx is indicative of a huge event.

What is it?

There is considerable speculation about the underlying mechanism. It had not been visible for the 7 years previous to the first image. The authors of the paper (11 April 2023) believe that the most likely explanation is that is an accretion of a very large molecular cloud by a supermassive black hole of the order of 10⁸-10⁹ solar masses. As such it would represent "an extreme extension of the known scenarios of black hole accretion."

The search is now on for more huge explosions like this, according to Dr Robert Massey who is the Deputy Executive Director of the Royal Astronomical Society.

"We've never seen anything like this before and certainly not on this scale," he told BBC News. "I'd be amazed if this is the only object like this in the Universe".

Source paper: https://academic.oup.com/mnras/article/522/3/3992/7115325#404812207

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