Incredibly Rare UV Flash Seen When White Dwarf Went Supernova

A previously snapped image of the nebula left behind by a type Ia supernova. NASA/CXC/U.Texas

If a star is not massive enough to become a supernova, it will eventually lose its outer layers and its exposed core will become known as a white dwarf. Some of these white dwarfs, however, are still committed to an explosive ending.

These kinds of supernovae, known as Type Ia (pronounced One-A), are well studied and yet they continue to surprise researchers. The latest example is the unexpected emission of ultraviolet (UV) light from a Type Ia event that took place on December 28, 2019. Reported in The Astrophysical Journal, the UV flash lasted for about a day.

This is only the second time that a UV flash has been recorded from a Type Ia supernova. Usually, these events don’t get hot enough to produce UV flashes, so the researchers think the white dwarf experienced something unusual to do so this time.

“The UV flash is telling us something very specific about how this white dwarf exploded,” lead author Adam Miller, from Northwestern University, said in a statement. “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 center of the explosion.”

Blue dot indicates the location of SN2019yvq in a relatively nearby galaxy 140 million light-years from Earth. Northwestern University

The team is working with four scenarios while they wait for more observations. The first scenario has the white dwarf consuming its companion so quickly that the material thrown out by the explosion interacted with the material coming from the companion. A second possibility suggests the white dwarf's core, rich in radioactive material, interacted with the degenerate star's outer layers, making it reach higher temperatures. The third possibility is the white dwarf had a layer of helium and an inner layer of carbon. The material from its companion led to the ignition of the helium and produced the first explosion. As this was happening, the carbon also ignited, producing a second explosion and the UV flash. Finally, this might be the result of a white dwarf merger.

“Within a year,” Miller said, “we’ll be able to figure out which one of these four is the most likely explanation.”

Type Ia supernovae explode with a characteristic luminosity, which makes them ideal “standard candles” to measure the distance of faraway galaxies. These explosions also create most of the iron in the universe, so understanding how they work is key to finding out where our planet came from. 

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