It’s the most powerful supernova discovered in all of human history.
ASASSN-15lh, named after the All Sky Automated Survey for SuperNovae (ASAS-SN) telescopic survey that found it, belongs to a very rare class of “superluminous supernovae.” It has 10 times more energy than our sun will produce in the next 10 billion years—and shines 200 times brighter than a typical supernova.
On top of that, ASASSN-15lh is only 3.8 billion light-years away, making it one of the nearest superluminous supernovae ever witnessed. And it came as something of a surprise to the team that discovered it.
Here’s Lee Billings, reporting for Scientific American:
Discovered in June 2015 by ASAS-SN’s twin 14-centimeter telescopes operating in Cerro Tololo, Chile, the supernova just appeared as a transient dot of light in an image, and wasn’t immediately recognized as particularly special. Only after several other telescopes piled on to provide additional observations of the outburst’s fading afterglow did it become clear to Dong and his collaborators that they had seen something record-breaking. The first hint came from a spectrum of the supernova delivered by the 2.5-meter du Pont Telescope in Chile seven days after the initial discovery. “When we saw the spectrum, we were baffled,” Dong recalls. “It didn’t look like any supernova we had seen.”
Dong and his team had to confirm the location of the supernova by analyzing its spectrum multiple times through different telescopes. That was the easy part. Theorizing how the supernova was even physically possible was much more difficult.
One hypothesis pushes the limits of known physics. If ASASSN-15lh’s parent star shed its out layers of gas and then collapsed its inner core to form what’s called a magnetar—a dense, rapidly rotating magnetized core—then magnetized wind emanating from that collapse could have shocked the outwardly flying matter enough produce a massive explosion. The catch is that the magnetar would have to have been spinning at a rate of one revolution per millisecond—that’s 60,000 rpm—which pushes the boundaries of what scientists think is physically conceivable. The magnetar idea may not be correct, but it’s at least plausible. Meanwhile, experts are on the lookout for other explanations.
Looking at the spectrum of the supernova as it fades will be key to acquiring more information about it; as a supernova wanes, it becomes more transparent, allowing scientists to peer into its inner layers. Meanwhile, the ASAS-SN telescope will continue scanning the entire visible sky every two or three nights—and the team has secured a bit of time with the Hubble Space telescope to scrutinize ASASSN-15lh’s characteristics even further. Their work could eventually provide an upper ceiling for the kinds of colossal events that happen in our universe.