In 2015 ASASSN-15lh gained fame as the most luminous supernova ever discovered. Almost a year later and against all odds, the supernova has rebrightened.

Remember the most luminous supernova ever? Well, it just got a little brighter.

Subo Dong (Peking University, China) and colleagues discovered the powerful stellar explosion known as ASASSN-15lh on June 14, 2015, using the All-Sky Automated Survey for Supernovae (ASASSN). Four robotic 14-centimeter telescopes, collectively known as Cassius and stationed at Cerro Tololo, Chile, were staring at the entire visible sky when they spotted the flash. Since then, the supernova has baffled astronomers.

The typical supernova goes something like this: a star ages, burning up its core’s hydrogen, then helium, and so on up the element chain until it reaches iron, when fusion stops. At that point, the core can no longer support itself against the inward crush of gravity. The star’s outer layers rush inward too, but they bounce off the collapsed core and energy from the collapse throws them back out in a brilliant flash.

The light from ASASSN-15lh took almost 3 billion years to arrive at Earth, its extreme distance muting the visible brilliance to a mere 17th magnitude. Nevertheless, its peak power was more than twice that of any previously known stellar explosion. Usually, much of a supernova’s initial glow actually comes from radioactive nickel, created in abundance near the core. What’s weird about ASASSN-15lh (and a few others like it) is that they’re so bright, they’d need an awful lot of nickel to explain their glow.

Bright Again

Of course, ASASSN-15lh faded, as supernovae are wont to do. But this particular supernova held a surprise for researchers. Roughly three months after it began dimming, the supernova changed course. For 40-some days, its ultraviolet radiation charged up, increasing fivefold before plateauing for another couple of months and finally dropping away again. Radiation at visible wavelengths ignored this transformation and continued to fade unabated.

A born-again supernova isn’t unheard of. But typically when that happens, the blast has run into nearby gas that the star threw out before it exploded. ASASSN-15lh doesn’t display any of the emission lines you’d expect in its spectra if this were the case.

“Typically, a supernova interacting with its own ejecta should produce very strong emissions lines,” explains Krzysztof Stanek (Ohio State University), coauthor on the paper that will appear in the Monthly Notices of the Royal Astronomical Society. “We simply do not see any evidence for it. . . . I basically think that we can rule it out.”

But that means the reason for the resurgence remains, as coauthor Todd Thompson (Ohio State University) puts it, “difficult to explain.”

What is ASASSN-15lh?

Until now, the most successful explanation of ASASSN-15lh’s oddities has been the magnetar. In this scenario, the core of an aging, massive star collapsed to form a spinning stellar remnant that’s like a neutron star but with a magnetic field at least 100 billion times the strength of the Sun’s strongest fields. Rather than radioactive elements, the supernova’s power would come from the magnetar’s gyrating magnetic field.

But as magnetars go, this one has to be pretty weird: to produce the radiation initially seen from ASASSN-15lh, the magnetar would have had to convert almost all its magnetic and rotational energy to radiation.

Now, factoring in the extra energy recently emitted in the ultraviolet makes the demands on the magnetar model even more stringent. “The magnetar model is safe, but barely,” says Thompson. “The magnetar [scenario] would be in danger if we saw roughly two times more energy. It would begin to push what we think is possible.

“Of course,” Thompson adds, “without another viable alternative, we might still try to make the magnetar model work.”

Not everybody agrees: Peter Brown (Texas A&M University) just posted another paper on the topic on the arXiv's astrophysics preprint server. Brown and colleagues argue that while the magnetar scenario might provide a good explanation of the initial ASASSN-15lh observations, it doesn't suffice to explain the object's resurgence at ultraviolet wavelengths.

Whatever ASASSN-15lh may be, one thing's for sure — the exceptional object isn't lending itself to any easy explanations.