June 21, 2018
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Astronomers discover new type of supernova

By Amina Khan, Los Angeles Times

LOS ANGELES — A team led by astronomers at the California Institute of Technology has discovered a new type of supernova that may burn 100 times brighter than typical exploding stars — and they’re trying to figure out exactly how this new type works.

The study, which identified four newly discovered supernovae as part of this unknown class, also solves the mystery behind two previously unexplained events: one that had been thought to be an extremely luminous type II supernova, and another whose nature had scientists completely baffled.

“It was an amazing night when I put this all together,” said Robert Quimby, the Caltech astronomer who led the study published online Wednesday in the journal Nature.

Supernovae are massive explosions of stars that flash in the sky for weeks before going dim. They can outshine all the other stars in their galaxies, combined.

Scientists look for these transient pinpricks of light in a fairly simple way — by taking a picture of the sky with a telescope and comparing that picture to an image taken on a previous night. If a new bright spot appears, it could be a supernova.

But a telescope’s field of vision is limited — a mere 0.25 square degrees, out of the 40,000 square degrees in the entire sky. So astronomers usually focus on bright, densely packed galaxies to maximize their chances of spotting one of those dying stars.

Instead, Quimby used a telescope at the Palomar Observatory in northern San Diego County that could snap 7-square-degree sections of the sky at a time, allowing it to survey a much larger area in one night. Rather than focus on brighter galaxies, Quimby looked at the sky without prejudice, which is how he came across these incredibly bright objects in unlikely places — small, dim galaxies about a tenth of the size of the Milky Way.

Quimby and his colleagues from around the world found four strange, bright objects that seemed to defy explanation. When they look to identify a supernova, scientists try to figure out whether its light is due to typical causes like the heat of radioactive decay or by the interaction between the star’s debris and a surrounding hydrogen envelope. They conduct a spectral analysis, identifying fingerprints of light associated with certain chemicals in the supernova.

But the light given off by these bodies didn’t match any of the typical supernova profiles. There was no hydrogen signature, and the ultraviolet light was fading three times too fast to be explained by radioactive decay.

Quimby and colleagues concluded that these strange objects must be in a class of their own.

“It was a remarkable synthesis of some bits and pieces we understood about these objects,” said J. Craig Wheeler, an astronomer at the University of Texas at Austin who has worked closely with Quimby but was not involved in this study.

Though they’re not sure what could be illuminating these exploding stars, two models might explain their brightness, Quimby said. Perhaps a very massive star is ejecting material in several separate pulses, and all that material is rubbing together. Another possibility is that a magnetar — a neutron star with a powerful magnetic field — is causing the glow.

“I’d like to see a lot more of these [supernovae], and I’d like to see them observed for a long time,” said Stanford Woosley, an astrophysicist at the University of California, Santa Cruz, who was not involved in the study. “It is a truly unusual supernova.”

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