There’s a flash of ultraviolet just as a white dwarf is exploding as a supernova

Astronomers recently spotted a rare type of supernova explosion that was accompanied by a massive flare of ultraviolet radiation. Untangling the mystery of the UV flash could help unravel the mysterious nature of these supernova explosions, and even help us understand the age of the universe.

Enjoy Some Ultraviolet

It doesn’t have a pretty name, but the supernova dubbed SN2019yvq is rather spectacular. First spotted at the Zwicky Transient Facility in California in December 2019, teams using NASA’s Neil Gehrels Swift Observatory followed up on the blast within only a few hours – and they got a telescope full of ultraviolet radiation, as reported in a recent paper published in the Astrophysical Journal.

That UV blast is rare for supernovae. In fact, it’s only the second time that astronomers have seen such an intense burst accompanying the explosion.

SN2019yvq is a particular kind of supernova called Type 1a, where a white dwarf pulls enough material off of a companion star to trigger a runaway thermonuclear event (a.k.a. a supernova), and in the process completely destroy the white dwarf. It’s such an intense blast that astronomers can see such explosions from across the universe, but SN2019yvq was only 140 million light-years away.

But the vast majority of Type 1a supernovae don’t get to boast a surge of UV radiation. Despite the intense energies, supernovae aren’t all that hot temperature-wise – most of the energy goes to shock waves, neutrinos, and radioactive fusion.

In a Type Ia supernova, a white dwarf (left) draws matter from a companion star until its mass hits a limit which leads to collapse and then explosion. Credit: NASA

What made SN2019yvq so special?

Astronomers aren’t sure. It could be that before the supernova, material from the companion star collided with the white dwarf, raising temperatures and releasing a flood of ultraviolet light. Or maybe extremely hot material in the white dwarf’s core somehow mixed with the surface material. Or maybe there was a double-flash: the outer layers ignited before the rest of the white dwarf obliterated itself. And maybe even this particular explosion was caused by two white dwarfs colliding.

Within about a year, as the situation around SN2019yvq cools off, astronomers can examine the area for any potential leftovers, hopefully picking up a clue.

A Clue in the Dark

Astronomers are very keen to understand the nature of Type 1a supernova, as they serve as a “standard candle” for measuring the expansion history of the universe. Since white dwarfs tend to explode in roughly the same way, we know how bright they are. We can compare that to how bright they look to us, and calculate a distance.

But with its surge of ultraviolet light, SN2019yvq bucks the trend. Because it didn’t explode in the exact same way, any supernovae like it could contaminate the standard candle picture, making it harder to use Type 1a events as a cosmological probe.

And so: more staring in the dark, waiting for stars to die.