(CN) — In October 2022, astronomers observed the brightest cosmic explosion on record, a gamma-ray burst known as GRB 221009A that lasted several hours. But while the great blast may have been the brightest viewed from Earth, astronomers have since homed in on the largest explosion: AT2021lwx.
In research published Thursday in the journal Monthly Notices of the Royal Astronomical Society, a team of researchers from the University of Southampton in England describe how they happened upon the mysterious explosion that has now lasted over three years.
The team first noticed AT2021lwx in the summer of 2021 while searching for a specific type of supernova or exploding star — a phenomenon lead researcher Philip Wiseman described as an extremely hot, expanding ball of plasma that glows or explodes. The bigger the object that explodes the more material it sends out, Wiseman said in an interview.
The finding came just months after the explosion was first discovered on April 13, 2021, as a transient object in imaging captured by Zwicky Transient Facility in California, a facility that watches the night sky for objects that shift in brightness to determine cosmic events like supernovae, comets and asteroids.
Wiseman and the team did not immediately take to AT2021lwx, though, as their initial inspection of its composition only revealed a hot blue explosion. A year later, the same object appeared for the team’s search algorithm, and, this time, they found the distance of the blast.
After analyzing the explosion’s spectrum of light, researchers determined the explosion took place nearly eight billion light-years away when the universe was about six billion years old.
“We measured the distance and then kind of our eyes popped out because we realized, hey, if it's as bright as it seems on our telescopes, but it's so far away, then the actual kind of intrinsic brightness must be just off the charts,” Wiseman said. “It must be brighter than any known supernova.”
Forced photometry later revealed the object had been brightening since at least June 16, 2020, under the name of ZTF20abrbeie, which is unusual for most supernovae and tidal disruption events because they typically only last for a couple of months before fading away, Wiseman said.
Independent detections occurred elsewhere, too, with the Asteroid Terrestrial-Impact Last Alert System and the Panoramic Survey Telescope and Rapid Response System in Hawaii, and the Public ESO Spectroscopic Survey for Transient Objects.
The only other objects as bright as AT2021lwx are quasars, extremely luminous galactic nuclei, where gas and dust emissions continuously fall into a supermassive black hole at high velocity. However, AT2021lwx bears little resemblance to quasars, Wiseman said, adding that quasars have a characteristic light that flickers up and down for days, months, years and, probably, millions of years.
The team also ruled out two other ideas, the first being that AT2021lwx is too bright to come from a single star exploding and the second being that it’s implausible that it’s a single star that fell into a black hole.
“To get this much energy, you have to have a black hole that is 100 million times the mass of the sun, and that causes a problem because, a star that is like our sun, if you drop it onto a black hole of that scale, it doesn't get shredded apart and cause a light show. It just gets swallowed whole and just dies silently and it just disappears,” Wiseman said.
Researchers suspect the explosion likely stems from a dormant supermassive black hole’s engulfment of a vast cloud of gas, mostly hydrogen, or dust that fell from orbit.
How vast? According to Wiseman, the explosion’s spherical disk is estimated to be about 100 times the size of our solar system.
“And it's kind of cool in that it's only twice the temperature of the surface of the sun, which for things exploding, is not actually that hot, but still it's about 12,000 degrees centigrade (21,632 degrees Fahrenheit),” Wiseman said.
The team also noted the presence of carbon and some magnesium, although there’s no oxygen to be found.
“Normally, when you have a quasar, which are these constant fueling of black holes, you see oxygen and we see no oxygen at all, which is puzzling,” Wiseman said. “So that's something we're going to have to work on in the coming months and years, to try and understand this chemical composition. Why is there no oxygen?”
Additionally, the researchers plan to perform upgraded computational simulations to see whether they fit their theory of the explosion’s origins.
“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,” Wiseman said in a statement accompanying the study. “It could be that these events, although extremely rare, are so energetic that they are key processes to how the centers of galaxies change over time.”
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