A stellar explosion almost 600 billion times brighter than the sun pushes the limits of physics
By Lee Billings
Astronomers have glimpsed the most powerful supernova ever seen, a star in a galaxy billions of light-years away that exploded with such force it briefly shone nearly 600 billion times brighter than our Sun and 20 times brighter than all the stars in the Milky Way combined. The explosion released 10 times more energy than the Sun will radiate in 10 billion years.
If the supernova took place in our own galaxy, it would be easily seen by the naked eye even during the day; if it were 10,000 light-years away, it would appear to us at night as bright as the crescent Moon. If it were only as far away as Sirius, which at a distance of 8.6 light-years is the brightest star in the nighttime sky, it would blaze overhead almost as powerfully as the Sun. If it were as close as Pluto, it would vaporize the Earth and all the other worlds in our solar system.
Called ASASSN-15lh, after the All Sky Automated Survey for SuperNovae (ASAS-SN) telescopic survey that discovered it, the outburst belongs to a class of rare “superluminous supernovae,” which can shine hundreds of times brighter than stellar explosions normally do. But ASASSN-15lh is about three times brighter than the previous brightest record-holder—so luminous that it approaches the limits of what theorists believe is possible for these mighty cosmic outbursts. The findings are published in Science.
“ASASSN-15lh is the most powerful supernova discovered in human history,” said lead author Subo Dong, an astronomer at the Kavli Institute for Astronomy and Astrophysics at Peking University. “It provides a great puzzle—it challenges all our previous theories of explosion mechanisms and power sources of superluminous supernovae.”
The race for spectra
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.”
Working with Jose Prieto at the Universidad Diego Portales in Chile and Kris Stanek, ASAS-SN’s co-principle investigator at Ohio State University, Dong realized the curious spectrum could match that of another superluminous supernovae observed in 2010, but only if the new spectrum had been significantly redshifted—stretched out by the expansion of the universe as it traversed vast cosmic distances. A high redshift would suggest the supernova had taken place very far away, and was thus very, very bright.
Confirming the hunch required going to larger telescopes to obtain better spectra. After more than a week of delays due to bad weather and instrument problems at multiple observatories, the crucial spectrum at last came from the 10-meter South African Large Telescope, confirming Dong’s hunch and revealing that the outburst had occurred some 3.8 billion light-years away. Dong received the news at 2 a.m. in Beijing; realizing he had probably just found the most powerful supernova ever seen, he became too excited to sleep the rest of the night.
Bright beyond belief
Closer inspection of the spectra revealed more details about the event, some of which pointed to a possible explanation for its extreme brightness. ASASSN-15lh wasn’t just far brighter but also far hotter than other supernovae. Unlike most superluminous supernovae, which tend to occur in dimmer, smaller galaxies roiled by intense bursts of star formation, ASASSN-15lh appeared to be located in a galaxy even bigger and brighter than the Milky Way. Most tellingly, it appeared to be hydrogen-poor, probably a sign that its progenitor star had somehow shed its outer shell of thick gas before exploding. To Todd Thompson, an ASAS-SN collaborator at Ohio State, that suggested its intense power came from a magnetar, the ultra-dense, rapidly spinning, highly magnetized collapsed core of a giant star.
In this scenario, the star would first need to blow off its outer layers of gas, followed by the collapse of its core to form the magnetar and the resulting supernova. If the newly formed magnetar was spinning fast enough to complete a revolution once every millisecond—a rate most theorists believe is just barely possible—as it slowed, it would release a huge amount of energy via a magnetized wind. If the wind was closely coupled with the overlying matter being ejected by the explosive force of the star’s collapse, it could shock the material enough to produce ASASSN-15lh’s enormous burst of light.
“The pros of the magnetar model are basically that it works,” Thompson says. “The cons are that the spin period and coupling efficiency have to be dialed to a maximum, and the magnetic field has to be very high, but not out side of observational bounds. That is, it just barely works.” The implication, Thompson says, is that if the magnetar model is valid, then ASASSN-15lh is not only the most energetic supernova ever seen—it is representative of the most energetic supernovae that can ever be seen.
Looking for more
If indeed ASASSN-15lh is the most luminous supernova the universe can make, Thompson says, it and other superluminous supernovae could be useful as calibrating beacons, tic marks on the cosmic ruler astronomers use to measure great distances. The key would be to look for more. “Even with a rate 1,000 times smaller than the normal massive star supernova rate, there should still be one happening every 10 minutes or so in the visible universe,” Thompson says. “We should find them.”
As compelling as the magnetar model might be, Dong suspects that its tight constraints suggest the need for alternative explanations. It could be, he says, that ASASSN-15lh is simply the sort of supernova produced by the deaths of the universe’s biggest stars, poorly understood objects that could be hundreds of times as massive as our Sun. Such stars could be so rare that we have simply never seen one die. If ASASSN-15lh was produced this way, Dong says, then astronomers should be able to trace a telltale tweak in the light of its fading afterglow caused by the gradual decay of 30 solar masses’ worth of radioactive nickel.
The team has already obtained observing time on the Hubble Space Telescope to keep examining the outburst. A better understanding of just what caused it may soon be in hand—or, Dong warns, solving this mystery may require decades, even centuries.
In the meantime, the ASAS-SN team is hoping this discovery will help boost the project to its next phase: adding another small telescope to effectively double its coverage of the entire visible sky to seek out more supernovae. The project currently scans the entire visible sky every 2 or 3 nights. “ASAS-SN is a very inexpensive project,” Stanek says. “We have spent at most about $1 million for this unprecedented capability. This is an example of ‘if you build it, they will come.’ We have built a unique discovery machine, for relatively little money. Discoveries followed, and we will keep making them.”