Astronomers witnessed a flash of ultraviolet light after a white dwarf star exploded in a supernova, according to a new study.
This is only the second time such an event has ever been observed.
A white dwarf is the dense remnant of red giant stars once they explode. But white dwarfs can explode, too. Scientists are still trying to figure out why, and this detection may help them find that answer.
The unusual supernova was first spotted by astronomers who were using the Zwicky Transient Facility in California in December 2019. Using the extremely wide field camera, they were able to observe the supernova, and the ultraviolet flash that followed, just one day after the explosion.
“Early detection was essential for finding the early (ultraviolet) flash,” said Adam Miller, study author, astrophysicist and fellow at Northwestern University’s Center for Interdisciplinary Exploration and Research in Astrophysics, in an email. “If we had found this supernova just two days later, we never would have been able to catch the UV flash.”
Surveys like Zwicky map the entire visible sky every night, which allows scientists to find supernovae quickly after they happen, Miller said.
The event was called SN2019yvq and traced back to a location close to the tail of the Draco constellation, 140 million light-years away from Earth. Astrophysicists at NASA’s Neil Gehrels Swift Observatory quickly followed up by observing the event through ultraviolet and X-ray wavelengths.
The event was dubbed a “Type Ia” supernova (pronounced one-A), which is common when a white dwarf explodes. But the ultraviolet flash was unexpected.
Only one other event like this has been observed before. The previous white dwarf explosion associated with an ultraviolet flash was published in a 2015 study.
But what intrigues the researchers the most is the fact that these two events aren’t exactly alike.
“The other Type Ia supernova with a detected UV flash is called iPTF 14atg,” said Miller, who is the director of the Legacy Survey of Space and Time Corporation Data Science Fellowship Program. “One aspect of iPTF 14atg and SN 2019yvq that is a bit puzzling, beyond the initial ultraviolet flash, is that each is considered a ‘peculiar’ supernova compared to normal Type Ia supernovae.”
While observing a second event that coincides with an ultraviolet flash told scientists that the occurrence could be more common than previously believed, it also has shown that the ultraviolet flashes can accompany different kinds of supernovae.
The two events were “each peculiar in unique ways, so aside from the UV flash they are not similar explosions,” Miller said. It could suggest that white dwarfs can explode without reaching what’s called the Chandrasekhar mass, which is a long-held belief in the astronomy community, he said.
It was previously believed that a white dwarf below the Chandrasekhar mass or limit, which is equivalent to 1.4 times the mass of the sun, would stay a white dwarf forever.
The study published Thursday in the Astrophysical Journal.
What’s in an ultraviolet flash?
The ultraviolet flash only lasted for a couple of days, but it was enough to provide intriguing insight.
Previously, astronomers thought the only way an ultraviolet flash like this could occur is if the material exploded by the star collided with a large, nearby companion star, which would rapidly heat the material enough to emit ultraviolet light. Ultraviolet emissions indicate that a strong heat source is either inside or close to the white dwarf, but white dwarfs cool as they age.
“The simplest way to create UV light is to have something that’s very, very hot,” Miller said. “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 four potential hypotheses for the ultraviolet flash seen in this event.
The white dwarf could have devoured a companion star, growing so large and unstable that it exploded and the collision of the material from both stars caused an ultraviolet flash. Material from the white dwarf’s core could have mixed with the outer layers and heated the outer shell.
The outer helium layer of the white dwarf could have actually ignited the carbon inside the star to cause a super-heated explosion and flash. Or the flash is the result of two white dwarfs merging, causing an explosion of material that collides and creates a flash.
Miller and his team are continuing to observe the supernova and as the exploded material, called ejecta, expands out, it will become thinner.
“As the ejecta thin, we are able to see deeper and deeper inside the material that was ejected and that allows us to understand what elements (and how much) was expelled from the explosion,” Miller said.
A year after the explosion took place, they should be able to see all the way to its center and learn which hypothesis is the most likely. And studying white dwarfs can help scientists understand more about the universe.
“Exploding white dwarfs are responsible for the creation of most of the iron-group elements (iron, nickel, cobalt, titanium, chromium, etc) in the Universe,” Miller said. “If iron otherwise was not created in these explosions it would be difficult to build planetary cores, and eventually planets.”
The Type Ia supernovae associated with white dwarfs also act as standard candles for researchers. This means that their brightness, which is thought to be the same for each explosion, can be used to measure where these explosions take place in reference to Earth.
The measurements of brightness can also help astronomers to determine the acceleration of the universe and study dark energy, a hidden energy that affects much of the universe.
“Dark energy remains a mystery,” Miller said. “But these supernovae are the best way to probe dark energy and understand what it is.”
