Zero Genie

An international team of astronomers has detected the brightest and most energetic known supernova: SN 2016aps.

An artist’s illustration of a supernova. Image credit: NASA / CXC / M. Weiss / University of California, Berkeley / N. Smith et al / Lick Observatory / J. Bloom & C. Hansen.

SN 2016aps was discovered by the Panoramic Survey Telescope and Rapid Response System (Pan- STARRS) Survey for Transients on February 22, 2016 with an apparent magnitude of 18.

Also known as PS16aqy, the explosion occurred in a low-mass galaxy some 3.1 billion light-years from Earth.

University of Birmingham’s Dr. Matt Nicholl and colleagues believe SN 2016aps could be an example of an extremely rare ‘pulsational pair-instability’ supernova, possibly formed from two massive stars that merged before the explosion. Such an event so far only exists in theory and has never been confirmed through astronomical observations.

“We can measure supernovae using two scales — the total energy of the explosion, and the amount of that energy that is emitted as observable light, or radiation,” Dr. Nicholl said.

“In a typical supernova, the radiation is less than 1% of the total energy. But in SN 2016aps, we found the radiation was five times the explosion energy of a normal-sized supernova. This is the most light we have ever seen emitted by a supernova.”

In order to become this bright, SN 2016aps must have been much more energetic than usual.

By examining the light spectrum, the astronomers were able to show that the explosion was powered by a collision between the supernova and a massive shell of gas, shed by the star in the years before it exploded.

“While many supernovae are discovered every night, most are in massive galaxies,” said co-author Dr. Peter Blanchard, an astronomer at Northwestern University.

“This one immediately stood out for further observations because it seemed to be in the middle of nowhere. We weren’t able to see the galaxy where this star was born until after the supernova light had faded.”

Ground-based and Hubble images of SN 2016aps and its host galaxy. Image credit: Nicholl et al / MMT Observatory / NASA / ESA / Hubble.

The researchers observed SN 2016aps for two years, until it faded to 1% of its peak brightness.

Using these measurements, they calculated the mass of the supernova was between 50 to 100 solar masses. Typically supernovae have masses of between 8 and 15 solar masses.

“Stars with extremely large mass undergo violent pulsations before they die, shaking off a giant gas shell. This can be powered by a process called the pair instability, which has been a topic of speculation for physicists for the last 50 years,” Dr. Nicholl said.

“If the supernova gets the timing right, it can catch up to this shell and release a huge amount of energy in the collision. We think this is one of the most compelling candidates for this process yet observed, and probably the most massive.”

“SN 2016aps also contained another puzzle. The gas we detected was mostly hydrogen — but such a massive star would usually have lost all of its hydrogen via stellar winds long before it started pulsating,” he said.

“One explanation is that two slightly less massive stars of around, say 60 solar masses, had merged before the explosion. The lower mass stars hold onto their hydrogen for longer, while their combined mass is high enough to trigger the pair instability.”

“Finding this extraordinary supernova couldn’t have come at a better time,” said co-author Professor Edo Berger, an astronomer at Harvard University.

“Now that we know such energetic explosions occur in nature, NASA’s James Webb Space Telescope will be able to see similar events so far away that we can look back in time to the deaths of the very first stars in the Universe.”

The discovery is reported in a paper in the journal Nature Astronomy.

_____

M. Nicholl et al. An extremely energetic supernova from a very massive star in a dense medium. Nat Astron, published online April 13, 2020; doi: 10.1038/s41550-020-1066-7