The supernova known as DES13S2cmm plays by its own rules.
Cerro Tololo Inter-American Observatory in northern Chile. Credit: Andreas Papadopoulos.
Regular supernovas are so passé. There's a new class of stellar deaths in town, and they can outshine even the brightest normal supernova by 100 times.
Today, researchers with the Royal Astronomical Society's Dark Energy Survey announced that they have discovered one of these ultra-rare “superluminous supernovas” in a galaxy 7.8 billion light years away. The massive explosion is called DES13S2cmm, and is a rule-breaker even in its own mysterious sub-group.
“Fewer than forty such supernovae have ever been found and I never expected to find one in the first DES images!” said postgraduate student Andreas Papadopoulos in the RAS's statement. Papadopoulos discovered DES13S2cmm in images taken by the Dark Energy Camera, a wide-field imaging lens attached to the Blanco Telescope, part of the Cerro Tololo Inter-American Observatory in the Chilean Andes.
“As they are rare, each new discovery brings the potential for greater understanding—or more surprises,” he said.
These extraordinarily bright events have been recognized as a separate class of supernova for about five years, and their origins continue to perplex cosmologists.
“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 supernova specialist D. Andrew Howell in a December 2013 statement from Las Cumbres Observatory. Howell was among the first cosmologists to study these supernovas.
“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," he said.
In a recent study published in The Astrophysical Journal, Howell and his team at the Supernova Legacy Survey suggested that highly-magnetized neutron stars might be the secret ingredient behind these super-radiant events. The study speculated that when these primordial stars collapsed, they forged rapidly spinning magnetars in their cores.
Magnetars are a special class of neutron stars, equipped with the strongest magnetic fields in the known universe. They also spin about 300 times a second. This rapid rotation might transfer spin energy to these giant stars, which in turn could power the extraordinary brightness of superluminous supernovas.
But DES13S2cmm is an iconoclast even for its mysterious subclass, and it doesn't seem to play by the same rules as the events observed by Howell's team.
Before (left) and after (center) images of the region where DES13S2cmm was discovered. On the right is the supernova. Credit: Dark Energy Survey
The most obvious difference is the supernova's “light curve,” or change in brightness over time. DES13S2cmm is dimming at a snail's pace compared to other superluminous supernovas, and was visible in the DES images for six months straight.
"We have tried to explain the supernova as a result of the decay of the radioactive isotope nickel-56," said co-author Chris D'Andrea in the RAS statement. "But to match the peak brightness, the explosion would need to produce more than three times the mass of our Sun of the element. And even then the behavior of the light curve doesn’t match up."
While DES13S2cmm may be powered by a magnetar, D'Andrea was skeptical on that front too. “Neither model is a particularly compelling match to the data," he said.
The Dark Energy Survey will start its second season in August, so hopefully, sharper insights into these bizarre, primordial supernovas are on the horizon.