Rare Supernova
© Northwestern University
The blue dot marks the approximate location of the supernova event.
Astrophysicists have spotted a spectacular flash of ultraviolet light accompanying a white dwarf explosion.

It's only the second time such a rare type of supernova has been seen, they say, and may help explain how white dwarfs explode.

The flash "is telling us something very specific", says Adam Miller from Northwestern University, US, the lead author of a paper in in the Astrophysical Journal.

"As time passes, the exploded material moves farther away from the source. As that material thins, we can see deeper and deeper.

"After a year, the material will be so thin that we will see all the way into the centre of the explosion."

Using the Zwicky Transient Facility in California, the researchers spotted the event, named SN2019yvq, just a day after it occurred last December in a galaxy about 140 million light-years from Earth and very close to the tail of the dragon-shaped Draco constellation.

They quickly studied it in ultraviolet and X-ray wavelengths using NASA's Neil Gehrels Swift Observatory and classified it as a type Ia supernova.

The flash, which lasted for a couple days, indicates that something inside or nearby the white dwarf was incredibly hot, which was puzzling, the researchers say, because white dwarfs become cooler as they age.

"The simplest way to create UV light is to have something that's very, very hot," says Miller. "We need something that is much hotter than our Sun: a factor of three or four times hotter.

"Most supernovae are not that hot, so you don't get the very intense UV radiation. Something unusual happened with this supernova to create a very hot phenomenon."

There are multiple competing hypotheses as to why white dwarfs explode. Miller says he is particularly interested in exploring four, which match his team's data analysis from SN2019yvq.

1. A white dwarf becomes so large and unstable after consuming its companion star that it explodes. Its materials and the star's collide, causing a flash of UV emission.

2. Extremely hot radioactive material in the white dwarf's core mixes with its outer layers, causing the outer shell to reach higher temperatures than usual.

3. An outer layer of helium ignites carbon within the white dwarf, causing an extremely hot double explosion and a UV flash.

4. Two white dwarfs merge, triggering an explosion with colliding ejecta that emit UV radiation.

"Within a year, we'll be able to figure out which one of these four is the most likely explanation," Miller says.