Supernova Legacy Survey Astronomers Reveal New Class Of Supernovae
December 19, 2013

Looking Back In Time For Supernova ‘Dinosaurs’

April Flowers for - Your Universe Online

Two of the brightest and most distant supernovae ever recorded have been discovered by astronomers affiliated with the Supernova Legacy Survey (SNLS). The findings, published in The Astrophysical Journal, reveal that the supernovae are 10 billion light-years away and a hundred times more luminous than a normal supernova.

The extreme luminosity of these supernovae cannot be explained by the mechanism that powers most supernovae — namely the collapse of a giant star to a black hole or normal neutron star. This makes the new supernovae, discovered in 2006 and 2007, puzzling for scientists who were initially unable to figure out what they were or to even determine their distances from Earth.

"At first, we had no idea what these things were, even whether they were supernovae or whether they were in our galaxy or a distant one," said D. Andrew Howell, a staff scientist at Las Cumbres Observatory Global Telescope Network (LCOGT) and adjunct faculty at UC Santa Barbara. "I showed the observations at a conference, and everyone was baffled. Nobody guessed they were distant supernovae because it would have made the energies mind-bogglingly large. We thought it was impossible."

SNLS-064eu, one of the newly discovered supernovae, is the most distant and possibly the most luminous member of an emerging class of stellar explosions called superluminous supernovae, which belong to a special subclass of superluminous supernovae that have no hydrogen.

The creation of a magnetar, an extraordinarily magnetized neutron star spinning hundreds of times per second, probably powers the supernovae, according to the researchers. Magnetars pack the mass of our sun into a star the size of a good sized city and have magnetic fields a hundred trillion times that of the Earth. Since they were first announced in 2009, a handful of superluminous supernovae have been seen, and prior research had postulated the creation of a magnetar as a possible energy source. This study, however, is the first to match detailed observations to models of what such an explosion might look like.

Models of the supernova that explained the data as the explosion of a star only a few times the size of the sun and rich in carbon and oxygen were created by collaborator Daniel Kasen from UC Berkeley and Lawrence Berkeley National Lab. He said that the star was likely much larger initially, but apparently shed its outer layers long before exploding, leaving only a smallish, naked core.

"What may have made this star special was an extremely rapid rotation," Kasen said. "When it ultimately died, the collapsing core could have spun up a magnetar like a giant top. That enormous spin energy would then be unleashed in a magnetic fury."

The SNLS is a five-year program based on observations at the Canada-France-Hawaii Telescope, the Very Large Telescope (VLT) and the Gemini and Keck telescopes to study thousands of supernovae. The two newly identified supernovae were discovered by the SNLS team, who were unable, initially, to properly identify nor pinpoint the exact locations of the explosions. Subsequent observations of the faint host galaxy, using the VLT in Chile, were necessary for the astronomers to determine the distance and energy of the explosions. These observations were followed by years of theoretical work to figure out how such an astounding energy could be produced.

Because the supernovae are so far away, the ultraviolet (UV) light emitted in the explosion was stretched out by the expansion of the universe, causing it to redshift, or increase in wavelength, into the part of the spectrum our eyes and telescopes on Earth can see. This extreme shift explains the early confusion of the astronomers, they had never seen a supernova so far into the UV before — giving them a rare glimpse into the inner workings of these supernovae. Although superluminous supernovae are so hot that the peak of their light output is in the UV part of the spectrum, the Earth's atmosphere blocks UV light—meaning that this phenomenon has never been fully observed before.

The universe was only 4 billion years old when these supernovae exploded. "This happened before the sun even existed," Howell explained. "There was another star here that died and whose gas cloud formed the sun and Earth. Life evolved, the dinosaurs evolved and humans evolved and invented telescopes, which we were lucky to be pointing in the right place when the photons hit Earth after their 10-billion-year journey."

Only one in perhaps 10,000 supernovae is superluminous, making them rare. They seem to prefer to explode in more primitive galaxies — those with smaller quantities of elements heavier than hydrogen or helium — which were more common in the early universe.

"These are the dinosaurs of supernovae," Howell said. "They are all but extinct today, but they were more common in the early universe. Luckily we can use our telescopes to look back in time and study their fossil light. We hope to find many more of these kinds of supernovae with ongoing and future surveys."