Irene Klotz
Discovery News

A network of small, ground-based telescopes hunting the night-time skies for transient supernovas fished out a whopper — a one-of-a-kind cosmic explosion that at its peak blasted out more light than 50 times all the stars in the Milky Way galaxy.

The object, which takes its name, ASASSN-15lh from the All-Sky Automated Survey for SuperNovae, was first spotted on June 14, 2015. A week later, astronomer Subo Dong, with the Kavli Institute for Astronomy and Astrophysics at Peking University, was looking at the object’s spectra, a chemical breakdown of its light, and realized something strange was going on.

The measurements were so different, the survey’s automated software didn’t even recognize it as a supernova, Dong wrote in an email to Discovery News.

“We thought about various non-supernova, exotic scenarios, but none seemed to work,” Dong said.

Working with colleagues at observatories worldwide, Dong started thinking the object may belong to a rare class of so-called superluminous supernovae, a suspicion bolstered by follow-up measurements taken by the 10-meter South African Large Telescope .

“Seeing the SALT spectrum was the moment we knew for sue that we were on to something big,” Dong said.

ASASSN-15lh is located about 3.8 billion light-years away, but is so bright that if were as close as Sirius, a bright star 8.6 light-years away from Earth, the supernova would appear to be almost as bright as the sun.

It is 200 times more powerful than the average supernova and more than twice as bright as any supernova previously found.

Scientists are virtually in the dark about what triggered the blast. One theory is that a dense, rapidly spinning neutron star, known as a magenetar, is powering the supernova. The star would have to be spinning at least 1,000 times a second, a speed that challenges the laws of physics. It also would have to be nearly 100 percent efficient at converting its spin energy into light, another theoretical limit.

Another option is that ASASSN-15lh is powered by some nuclear reactions associated with a supermassive black hole at the center of its host galaxy, but scientists don’t know of any such phenomenon, nor have they pinpointed the object’s precise location in its galaxy. Additional information is expected this year from Hubble Space Telescope observations.

“The Hubble observations are meant to show whether or not the event happened right at the nucleus of the host galaxy, or whether it is offset from the center. If it is offset, this would rule out the interpretation of this event as somehow related to the host galaxy’s central supermassive black hole — assuming it has one, as most massive galaxies do,” Ohio State astronomer Todd Thompson wrote in an email to Discovery News.

“What kind of event from a supermassive black hole could produce such an event? We don’t know since we’ve never seen something like it, but it has been suggested that it might be a so-called ‘tidal disruption event’ — a bright explosion arising when a star gets torn apart and then eaten by a supermassive black hole. The problem is that we’ve seen these events before, and they don’t look like ASASSN-15lh. In particular, most stars are composed of mostly hydrogen and helium, and yet the spectra of ASASSN-15lh show no signs of either,” Thompson said.

The supernova’s host galaxy itself presents another puzzle. Most superluminous supernovae are found in small, dwarf galaxy where lots of stars are forming. ASASSN-15lh, in contrast, appears to be in a large, but relatively quiescent galaxy about three times more massive than the Milky Way.

Scientists have eliminated the possibility that the supernova appears brighter than it actually is due to any magnifying effects of intervening galaxies, a phenomena known as gravitational lensing.

“The good news is that ASASSN-15lh is bright, so it is relatively easy to get high-quality observations. Many groups of astronomers are using some of the most advanced telescopes, ground- and space-based, to study ASASSN-15lh. I am sure that in the near future, we will understand it much better,” Dong said.

The research appears in this week’s issue of Science.