Flash-in-the-pan supernovas explained

March 26, 2018
By: Robert Sanders

Most exploding stars flare brightly and then slowly fade over weeks to months, but an unusual group of supernovas noticed only in the last 10 years flare up and disappear within days.

Thanks to the ability of NASA’s Kepler Space Telescope to precisely measure starlight over long periods of time, astronomers now have a pretty good idea what these flash-in-the-pan supernovas are: exploding stars probably too dim to be detectable until the stellar matter ejected during the explosion collides with a shell of material puffed off years earlier by the star.

The collision generates a shock wave that heats up the shell of gas, but then rapidly fades. While supernovas continue to glow because of the radioactive material generated in the explosion, these shocked clouds contain little radioactive material and have no residual glow.

This interpretation for what have been called fast-evolving luminous transients, or FELTs, comes from models created by University of California, Berkeley astrophysicists that match closely the observed light from a recently recorded supernova dubbed KSN2015K.

“The fact that Kepler completely captured the rapid evolution of this supernova really constrains the exotic ways in which stars die. The wealth of data allowed us to disentangle the physical properties of the phantom blast, such as how much material the star expelled at the end of its life and the hypersonic speed of the explosion,” said UC Berkeley graduate student David Khatami. “This is the first time that we can test FELT models to a high degree of accuracy and really connect theory to observations.”

Diagram of fast-evolving luminous transient in 3 stages
A fast-evolving luminous transient is a supernova unlike any other. I occurs when a star has sloughed off a shell of material (red in 1) and then later explodes (2). When the debris from the star slams into the shell (3), it heats it up and makes it glow. NASA/JPL-Caltech images.

Though observations of other FELTs are needed to confirm this conclusion, the unusually rapid rise and fall in brightness appears to be a signature of stars that belch out shells of matter in mini-eruptions before exploding entirely.

Khatami and his colleagues, including Daniel Kasen, an associate professor of physics and of astronomy at UC Berkeley and scientist at Berkeley Lab, published their findings today in the journal Nature Astronomy.

The Kepler telescope was designed to hunt for planets in our galaxy, the Milky Way, by detecting faint changes in a star’s brightness over months and years as a planet moved across its face.

This sensitivity enabled detection of KSN2015K and the faint changes in brightness over a little more than a week in February 2016, though it wasn’t discovered in the Kepler data until two years later. It occurred in the spiral arm of a galaxy 1.3 billion light years from Earth, which means that the light has been traveling towards us for one-tenth the age of the universe.

The star appears to have exploded 2.2 days before the peak brightness, which means that the star shed an envelope of gas about a year earlier. The brightness then faded to half the peak over about eight days. The peak was about one-tenth the duration of a typical Type Ia supernova, which are the standard candle for measuring distance in the universe.

Since only a few FELTs have actually been observed, a variety of theories have emerged to explain them: 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.

Khatami tried several scenarios to explain the rise and fall in brightness, such as the explosion of a white dwarf star, or a binary merger followed by a supernova, but none could reproduce the observations except the collision of ejecta with an envelope of gas shed earlier by the star.

“We collected an awesome light curve,” said Armin Rest of the Space Telescope Science Institute in Baltimore, Maryland. “We were able to constrain the mechanism and the properties of the blast. 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.

See NASA story for more about the menagerie of supernovas astronomers have uncovered.