A small, bright object that first flared up in June could be the brightest, most powerful supernova ever spotted by astronomers. If it is indeed a supernova, the object ASASSN-15lh pushes the upper limits of what scientists thought exploding stars were capable of. And if it's not a supercharged supernova, it may be something even more mysterious.
The brilliant cosmic explosion – 200 times as powerful as the average supernova, 570 billion times as bright as our sun, and 20 times as bright as all the stars in our Milky Way galaxy combined even though it only spans 10 miles across – is described in a study published Thursday in Science. It was spotted 3.8 billion light-years away using the All Sky Automated Survey for SuperNovae (ASAS-SN), an international collaboration headquartered at Ohio State University that uses telescopes around the world to keep an eye on the entire visible night sky.
A supernova that bright must have a massive star exploding at its center. The researchers who have studied the blast believe it might be fueled by the death of a magnetar, which is a neutron star with an extremely powerful magnetic field that could theoretically magnify the power of an explosion. But ASASSN-15lh's brightness pushes the upper physical limits of that theory, coming dangerously close to upending it entirely.
In fact, during the first four months since the supernova's discovery it's released as much radiation energy as our sun would release in 90 billion years of shining. "It proves very challenging for one of the most popular models for the engine of superluminous supernova – the magnetar model – to explain," lead study author Subo Dong, an astronomer at the Kavli Institute for Astronomy and Astrophysics at Peking University, told The Post.
In addition to being unusually bright, the supernova comes from a neighborhood off the beaten path: Most superluminous supernovae come from galaxies smaller and dimmer than the Milky Way with high rates of star production. But ASASSN-15lh seems to sit in a galaxy more luminous and massive than our own.
Paolo Mazzali, a researcher at the Liverpool John Moores University Astrophysics Research Institute who wasn't involved in the study, agreed that the object, while strange, is most likely a supernova. "Clearly, this must be a very rare event, but I do think it's a supernova," he told The Post. "It would be a challenge to get a magnetar to that level of brightness," he said – the star would have to spin 1,000 times a second and convert that energy into light with almost perfect efficiency, according to the authors of the new study – "but it's not impossible."
Because ASASSN-15lh is so unusual, astronomers will have to continue to study it in the hopes of pinning down its identity for certain. It could turn out to be a totally new phenomenon. Vital clues will emerge as the explosion fades out.
"How ASASSN-15lh fades will reveal much more about this event and many groups of astronomers are intensely observing this object," study author and Ohio State alumnus Ben Shappee, now at the Carnegie Observatories, told The Post just after finishing a round of observations using the Magellan telescope in Chile. "There are many ground-based and space-based observatories following the evolution of ASASSN-15lh at many wavelengths, all the way from the radio through the X-rays."
As the light from ASASSN-15lh fades, Shappee explained, scientists will have a better chance of seeing the object at its core. They'll also be able to figure out where it's positioned within its host galaxy. If it's in the galaxy's center, it might not be a supernova at all, but rather the result of some kind of unusual black-hole activity.
"Supernovae shape the universe we live in and there are still many unanswered questions about these explosions, even for the common ones," Shappee said. "Most of the elements in the world around us that are not hydrogen or helium were either created in supernovae explosions or distributed by them. Supernovae also fundamentally change how galaxies evolve when they explode by ejecting energy and momentum into their surroundings."
So the study of supernovae – especially one pushing at what the laws of physics will allow – can help scientists understand why the universe looks like it does today.
"The good news is that ASASSN-15lh is bright, so it is relatively easy to get high-quality observations," Dong said. "Not only us, but many other groups of astronomers are using some of the most advanced telescopes, ground and space-based, to study ASASSN-15lh. I am sure that in the near future, we will understand much more about it."