Astronomers still don't know quite what they're looking at with ASASSN-15lh. But whatever it is, it's the brightest supernova ever recorded. And maybe one of the weirdest.

As reported in a paper published in Science, the supernova is located about 3.8 billion light years away. The event was recorded by the All-Sky Automated Survey for SuperNovae (ASASSN, because astronomers love acronyms.) It's two to three times more luminous than any previously recorded supernova event, booming bright enough to be seen even from its perch in a distant galaxy.

Just how bright is it? If it were within 1,000 light years, the light would affect the ozone layer. If it were 3,000 light years away, it would be brighter than the full moon in the night sky. If it were on the other side of the galaxy entirely but not obscured by dust, it would still be faintly visible in daylight, according to co-author Todd Thompson, a professor in the Department of Astronomy at Ohio State University.

While researchers scratch their heads trying to figure out what happened, they are looking toward a giant star with a massive magnetic field as the culprit. The magnetic field would spin (or pulse) once per millisecond, and the whole field would be 10-100 trillion times the magnetic field of the Earth. In other words, this thing is powerful.

It could be a star massive enough that its core had already collapsed into a magnetar, a super-powerful kind of neutron star that radiates an intense magnetic field. From there it expanded the outer gases outward. This star would be very young, between 3 to 10 million years but not much older.

"Imagine a star sitting there and all the sudden its core collapses and forms a rapidly rotating highly magnetic neutron star," Thompson says. "This emission from this magnetar is basically blasting the outer envelope outwards at high velocities and with high energies."

One of the weirder qualities of the supernova is that there doesn't appear to be much hydrogen or helium in the cloud of exploding gas. At this point the researchers say there is some evidence for oxygen in the cloud, but no conclusive proof. This spectral signature makes up just a fraction of previously spotted superluminous supernova, Thompson said.

"When we looked at this thing, it didn't seem to have any hydrogen and helium, which is a little bit strange," co-author Krzysztof Stanek, one of Thompson's colleagues at OSU, said. "Then, as we realized it both had that and was extremely bright, it seemed it had to be a member of this class of weird ultra-luminous supernova."

One other possibility is that a supermassive black hole in the center of the parent galaxy came in contact with a very large star and ripped it apart, causing the bright galactic explosion. Thompson says this possibility has a few issues.

"This guess has several problems: ASASSN-15lh does not look like any previously seen tidal disruption event. It shows no evidence of Hydrogen or Helium, which would be expected for a tidal disruption event," he said. "If it is supermassive black hole activity, it is a new type, the event is unprecedented, and it happens to look very much like superluminous supernovae of type I."

To determine what it might be, they'll monitor the brightness and spectra of the event. If the brightness doesn't eventually dim, "then we're going to start to get more worried about the supernova interpretation of this event," Thompson says, adding that they don't quite know what else it might be, but "We'll get increasingly twitchy if the thing continues to stay bright."

"If it turns out to be wrong, in some sense it's even cooler," Stanek says. "Now it's a really, really luminous event, which no one has ever seen before, instead of the most luminous example of something someone has seen before. "

The ASASSN telescopes are relatively small, something on the order of a larger amateur telescope at 400 mm, meaning that the researchers had to rely on a number of amateurs to aide in follow-up observations. In addition, they've scheduled time on the Hubble Space Telescope, which will help pinpoint the location of the object. If it's in the center of its galaxy, a tidal disruption event becomes a more real possibility that can be explored with better spectral evidence. But otherwise, magnetar seems to be the most likely possibility.

"That's one idea for how to do it, but we have never seen anything this luminous before," Thompson says. "All ideas should be taken with a grain of salt."

OSU / ASASSN

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