Astronomers explain one of the strangest explosions ever seen in our universe

NZ Herald

4 days ago

Before the star could swallow the baby black hole, the black hole's gravitational forces caused the star to explode.
'That black hole has shaped that star's entire life' from its evolution to the type of explosion that ended its life, said Ashley Villar, a study author and professor at Harvard University.
The discovery was published today in the Astrophysical Journal.
It's not unusual to see these objects in close proximity to one another because most massive stars come in pairs or multiples, Villar said.
Presumably, both objects in this scenario were once stars orbiting near each other before one evolved into a small black hole.
Now orbiting around a black hole, the remaining star lost energy and was drawn in closer until it exploded into a new type of supernova never observed before.
Most supernovas follow a typical fate: The star's core quickly collapses and sends a powerful shock wave through the outer layers of the star.
The strong wave of energy causes a brief but bright flash of light.
Meanwhile, the explosion's heat and pressure create radioactive elements, which decay and release energy.
The energy is reabsorbed by surrounding material, causing it to glow and keeps the supernova bright for weeks to months. Over time, this glow dissipates.
But this particular explosion, given the name SN 2023zkd per the International Astronomical Union, was unusual for several reasons.
Firstly, the algorithm caught the supernova as it was dimming, but the light was fading at a much slower rate than expected.
Secondly, scientists looked back at archival data and saw the supernova was actually brightening for about four years before it started to dim, which is an unusually long time for a supernova.
Finally, as they continued to monitor the star, it unexpectedly brightened again.
The team gathered ultraviolet to infrared data of the star from ground and space telescopes, capturing the sequence of both brightening events and running simulations in computer models.
Based on the model scenarios, they determined the first brightening and subsequent decline was probably not powered by a radioactive decay seen in most supernovas.
Instead, the researchers said the light source probably came from a traffic jam around the exploding star.
It started as the distance between the massive star and black hole decreased.
The black hole's strong gravity began unevenly ripping gases and dust from the star - like one side of a bedsheet getting caught in a vacuum cleaner.
When the star started to explode, its blast wave collided with the dislodged gases around it and created the first flash.
The second, later flash was caused by a slower but persistent crash with the thick, disk-like cloud.
'We think that the light source is actually from stuff hitting each other as it's trying to escape,' Villar said.
'That explosion hits that disk, and now we're seeing all this additional light.'
The team considered another possible explanation for the light source, where the black hole tears apart the star before it has the chance to explode on its own. The black hole pulls in the star's debris, which crashes into the surrounding gas to generate a supernova-like flash.
In either scenario, the star collapses to some degree.
Scientists have previously theorised that some supernovas can be influenced by another object tugging on it, even capturing some data of supernovas with double brightening events.
'This is the first time when we have all of those pieces for the same explosion' from beginning to end, said Alex Gagliano, a co-author and researcher at Massachusetts Institute of Technology.
He gives a large credit to their algorithm for catching the event early, detecting subtle anomalies about 100 days before scientists saw more obvious unusual activity.
'This is an exciting paper and I agree with the overall interpretation,' Danny Milisavljevic, an astrophysicist and associate professor at Purdue University who was not involved in the research, said in an email.
He said the extraordinary properties of the light curve and slow changes in the star's spectral data provide compelling evidence that there is a black hole nearby.
Theoretical scientists may continue to analyse these observations with different models for alternative explanations 'as is normal in the field,' said Or Graur, a professor of astrophysics at the University of Portsmouth who was not involved in the research.
That's because the peculiar features of this explosion make it 'hard to easily slot it into one of the more common models' of supernovas.
'There are several weird supernovas that the community is continually observing, precisely because they're interesting and don't easily conform to our more common models,' said Graur, who has written a book about supernovas.
'The question is not whether it's plausible but whether it's probable - whether it's the best model to explain the data.'
This observation is just scratching the cosmic surface of what's out there.
The Vera C. Rubin Observatory - featuring the world's largest digital camera - will soon start a 10-year survey of the entire sky in the Southern Hemisphere. The telescope will capture thousands of supernovas each day, Villar said.
The team plans to use the new AI algorithm, developed with the National Science Foundation's AI Institute for Artificial Intelligence and Fundamental Interactions, to quickly sift through data and detect more supernova events potentially like this one.
'Our dynamic universe is full of surprises. AI is how we keep up with the spectacle,' said Milisavljevic, who said many aspects of astronomy are being revolutionised by the technology. His own research has used AI to discover the most powerful optical explosion ever observed, nicknamed 'Scary Barbie'.
Gagliano said the additional data can help scientists better understand the brightening sequence, which could help them forecast when a supernova may explode.
'We think that statistically we're overdue for a supernova in our own galaxy,' Gagliano said.
'There are many people that are trying to establish early warning systems so that as soon as our telescopes pick up on something unusual, we can all aim our telescopes in that location.'
He added that the AI algorithm 'is somewhat similar in that it's an early warning system to pick up explosions that look somewhat unusual.'
But there's no need to fear any impending supernova eruption in our galaxy, he said. People would be able to see the flash, but they wouldn't be in any danger.
It would simply be a 'really fun light show', Villar said.