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takyon writes:

Scientists on new supernova: WTF have we been looking at?

Today in Nature, a team of researchers is announcing observations of a supernova that it simply can't explain. In some ways, the event looks like a prosaic stellar explosion. Except it's stayed bright over six times longer than it should and experienced five periods of enhanced brightness that we can't explain. Different features of the supernova appear to be arising from physically distinct locations in space. And even the best model for what triggered this—something that involves a type of explosion we haven't definitively observed previously—doesn't account for all the observations.

[...] In September of 2014, the survey covered an area of sky that it had not imaged in 100 days, and it found a telltale brightening. By January, follow up observations of the event (termed iPTF14hls) showed that its luminosity was similar to that at its first discovery and dominated by hydrogen emissions. This led to its classification as a Type-IIp supernova. A Type-IIp's steady production of light, which typically lasts 100 days, is caused by ionized hydrogen cooling off enough to recombine with electrons, emitting light at a specific wavelength in the process. The critical temperature is typically reached at a set distance from the site of the explosion, meaning there's a steady flow of debris through this point that keeps things lit for 100 days.

Before too long, however, it became clear that this wasn't what was happening with iPTF14hls, which remained bright well past the 100-day mark. In fact, by the time a general dimming was apparent, it was 600 days after the supernova was first spotted. Obviously, that's hard to explain by a steady flow of debris spreading out and cooling off.

The other problem here is that there's no sign of an actual flow of debris. Obviously, material that gets far enough away from the site of the explosion to cool off later is moving more slowly than the material that manages that process quickly. As such, the velocity of the material that's emitting light should drop over time. (Another way of looking at this is that, as the fast moving material stops emitting light, it lets us see the slower moving material underneath it.) iPTF14hls again is refusing to cooperate here, as there is little sign of slower moving materials in the debris. Over the entire 600 days, hydrogen-rich material seems to only be slowing by 25 percent; iron rich material doesn't slow at all.

Also at Carnegie Science, Sky & Telescope, Air & Space Magazine, BBC, and CNN.

Energetic eruptions leading to a peculiar hydrogen-rich explosion of a massive star (DOI: 10.1038/nature24030) (DX)

Original Submission