Astrophile: Super-bright supernova hints at dark lens

Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

Object: An extraordinarily bright supernova
Distance: About 9 billion light years away

Call it the case of the invisible magnifying glass. A bizarrely bright supernova, snapped by the world's largest digital camera, may really have been a normal stellar death ? intensified by a "dark" gravitational lens.

For more than two years cosmic detectives have been combing over the facts surrounding PS1-10afx. At first glance this exploding star had all the features of a type Ia supernova, which happens when a small, dense white dwarf star steals material from an orbiting companion and then explodes.

But PS1-10afx was 10 to 20 times brighter than any known type Ia. "It had a luminosity that was extraordinary," says Alicia Soderberg of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

Soderberg and colleagues considered the notion that a gravitational lens was making the supernova look bigger and brighter. The gravity of a galaxy cluster between Earth and PS1-10afx could warp space-time and, in effect, focus the supernova's light towards Earth. The only problem was that the team found no such cluster anywhere along the line of sight.

"We put together all the pieces of the puzzle," says Soderberg. "It didn't meet any of the criteria that would let it fall it into a typical [supernova] class."

Smashing into itself

Instead the researchers thought they had found a whole new type of star explosion. It's possible the supernova slammed into debris that had been ejected during its death throes, energising the material and causing it to glow. In a sense, the star smashed into itself, bumping up its brightness. "At the end of the day, we decided that the theorists have to do more homework and explain events like this," says Soderberg.

But Robert Quimby of the University of Tokyo and colleagues are not convinced. Except for the intense luminosity, many aspects of PS1-10afx are just too similar to a standard type Ia.

Quimby's team gathered the supernova's spectra ? holding clues to its composition ? recorded by the PS1 telescope in Hawaii. They then removed any contaminating light from the host galaxy. As the supernova evolved over four days, the best fits to its spectra were always the spectra of regular type Ia supernovae.

The team argues that something we cannot see must have magnified PS1-10afx. The most likely candidate is a blob of dark matter, the invisible stuff thought to make up roughly 80 per cent of the matter in the universe.

Dark dwarfs

Theorists have long argued that each galaxy is embedded in a halo of dark matter. A large galaxy should be surrounded by numerous faint dwarf galaxies which get most of their mass from dark matter. One of these "dark dwarfs" could be between us and PS1-10afx, placed so that its gravity magnified the supernova.

If so, large-scale supernova surveys could turn up more of these invisible lenses, helping astronomers find and put limits on the number of dark-matter dwarfs in the universe, Quimby and colleagues conclude.

The new work means the case is far from closed, and PS1-10afx will continue to intrigue astronomers. "This was an interesting object and it'll be discussed for many years," says Soderberg.

Journal reference: arxiv.org/abs/1302.2785v1

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