Astronomers have detected a dazzling flash of ultraviolet light from an exploding white dwarf for the second time, and could give researchers important clues as to what stimulates the disappearance of these ancient worn-out stars.
The researchers noticed this unusual supernova, called SN2019yvq, last December, just one day after the explosion. Within hours, the scientists classified the event as a Type Ia supernova, not an unusual stellar event, usually at least, except that this time it was accompanied by the extremely rare flash of ultraviolet light.
“These are some of the most common explosions in the Universe,” says astrophysicist Adam Miller of Northwestern University.
“But what is special is this UV flash. Astronomers have searched for it for years and have never found it. As far as we know, this is actually the second time that a UV flash has been seen with a Type Ia supernova.”
Composite image of SN2019yvq (blue dot). (ZTF / Northwest / Caltech)
SN2019yvq took place in a galaxy approximately 140 million light years from Earth, with the rare UV flare seen for a couple of days before emission ceased.
As for what was behind the strange light, the team says they are still unsure, because white dwarfs (depleted stellar debris that are generally relatively cold) don’t usually get hot enough to generate the heat required for this type of UV light. .
“Most supernovae are not that hot, so you don’t get very intense UV radiation,” says Miller.
“Something unusual happened with this supernova to create a very hot phenomenon.”
While ongoing observations over the next year or so are expected to more or less narrow down the exact mechanism responsible for the SN2019yvq flare, there are currently four possible culprits that could have triggered these events.
Potentially, the researchers think, the white dwarf could have become unstable after consuming stellar material from a companion star in a binary system, with a collision of material between the two causing the UV flare.
Alternatively, the flare could have resulted from a dramatic increase in heat due to mixing between the white dwarf’s inner core and the outer layers, or from the helium that ignites the carbon in the star.
Lastly, it is possible for two white dwarfs to merge, with SN2019yvq’s UV flash representing an explosion when material ejected from both stars came into contact.
While we still can’t be sure which explanation is the best match for what happened on SN2019yvq, as the dust from this incredible stellar event begins to settle, the image is expected to become clearer.
“As time passes, the exploited material moves further away from the source,” says Miller. “As the material thins, we can see deeper and deeper. After a year, the material will be so thin that we will see to the center of the explosion.”
At the time, the researchers say we should finally be able to understand what made this white dwarf explode so dramatically, and the answers could teach us about how the iron generated in supernovae is distributed across space (helping to seed the formation of planets).
It can also help us unravel some of the unknowns around dark energy, which is believed to affect how quickly the Universe is expanding.
“If there is a type Ia supernova in a distant galaxy, we can use it to measure a combination of distance and speed that allows us to determine the acceleration of the Universe,” says Miller.
“Dark energy remains a mystery. But these supernovae are the best way to explore dark energy and understand what it is.”
The findings are reported in The Astrophysical Journal.
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