The Universe is so huge that it’s estimated that a star explodes as a supernova once every second.

Astronomers capture a small fraction of these catastrophic events because they are comparatively short-lived, like fireflies flickering on a summer evening. After skyrocketing to a sudden peak in brightness, a supernova can take weeks to slowly fade away.

For the past decade scientists have been befuddled by more curious cosmic explosions that pop up and then disappear in just a few days, not weeks — FELTs.

Only a few FELTs have been seen in telescopic sky surveys because they are so brief.

The FELT event spotted by Kepler — dubbed KSN 2015K — had a rise time of only 2.2 days and a time above half-maximum of only 6.8 days.

“FELTs appear to be a new kind of supernova that gets a brief turbo boost in brightness from its surroundings,” said Space Telescope Science Institute researcher Armin Rest and co-authors.

“The source of the flash is from a star after it collapses to explode as a supernova.”

“The big difference is that the star is cocooned inside one or more shells of gas and dust. When the tsunami of explosive energy from the blast slams into the shell, most of the kinetic energy is immediately converted to light. The burst of radiation lasts for only a few days — one-tenth the duration of a typical supernova explosion.”

This illustration shows a proposed model for a Fast-Evolving Luminous Transient. In the left panel, an aging red giant star loses mass via a stellar wind. This balloons into a huge gaseous shell around the star. In the center panel, the massive star’s core implodes to trigger a supernova explosion. In the right panel, the supernova shockwave plows into the outer shell, converting the kinetic energy from the explosion into a brilliant burst of light. Image credit: NASA / ESA / A. Field, STScI.

A guide to supernovae. Image credit: NASA.

Unlike Kepler, which collects data on a patch of sky every 30 minutes, most other telescopes look every few days. Therefore they often slip through undetected or with only one or two measurements, making understanding the physics of these explosions tricky.

In the absence of more data, there have been a variety of theories to explain FELTs: the afterglow of a gamma-ray burst, a supernova boosted by a magnetar (neutron star with a powerful magnetic field), or a failed Type Ia supernova.

Then along came Kepler with its precise, continuous measurements that allowed astronomers to record more details of the FELT event.

“We collected the awesome light curve of KSN 2015K. We were able to constrain the mechanism and the properties of the blast,” Dr. Rest said.

“We could exclude alternate theories and arrive at the dense-shell model explanation. This is a new way for massive stars to die and distribute material back into space.”

“We show that, unlike Type Ia supernovae, the light curve of KSN 2015K was not powered by the decay of radioactive elements,” the researchers said.

“We further argue that it is unlikely that it was powered by continuing energy deposition from a central remnant (a magnetar or black hole).”

“Using numerical radiation hydrodynamical models, we show that KSN 2015K’s light curve is well fitted by a model where the supernova runs into external material presumably expelled in a pre-supernova mass-loss episode.”

“The rapid rise of KSN 2015K therefore probes the venting of photons when a hypersonic shock wave breaks out of a dense extended medium.”

Source: Sci News