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Credit: NASA

Grab your sunscreen, because an intense UV flash from an exploded star could light up a cosmic mystery

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Jul 28, 2020

White dwarf stars have been doing some weird things lately. There was that one which got flung across the Milky Way half-burnt, and now a white dwarf that went supernova was found to have emitted a monstrous and mysterious UV flash.

This is not your typical supernova. Whatever it is, since it doesn't yet have a name, the phenomenon of a white dwarf exploding into UV light is so rare that this is just the second time it has ever been observed. Nobody knows exactly how or why it happens yet. Finding out what sets off such a flash could help reveal even more dark secrets, from how the universe creates heavy metals to the cosmic acceleration believed to be caused by dark energy. Astrophysicist Adam Miller of Northwestern University and his team of researchers might be able to find out.

"I would say the biggest mystery right now is –– how does the white dwarf explode? A white dwarf sitting alone in space will just slowly cool over time, and nothing of note will ever happen," Miller, who recently published a study in The Astrophysical Journal, told SYFY WIRE. "All of these explosions must be coming from binary (or even triple) star systems, but we do not know which types of binaries actually lead to explosion, and whether or not there are several different pathways towards triggering the explosion itself."

There is definitely something abnormal about this cosmic outburst (now known as SN2019yvq). Type Ia (one-A) supernovae are the typical way binary white dwarf systems enter their death throes, though they don’t always finish what they started. Many are also superluminous and the brightest supernovae known to occur. White dwarfs made of carbon and oxygen keep accreting star stuff until they reach a limit at which they end up exploding — the Chandrasekhar Limit of 1.4 solar masses. It is thought to be the maximum mass a white dwarf can reach without collapsing into a neutron star or black hole. Something that deviates from the standard Type Ia could even tell us more about dark energy.

"Type Ia supernovae (the result of a total explosion from a white dwarf star) are standardizable candles. This means we can measure precise distances with these events (a task that otherwise is nearly impossible for just about any other object outside the Milky Way)," Miller said. "Dark energy was actually discovered using type Ia supernovae, when it was realized that the most [distant] explosions are in galaxies that are accelerating away from us. We now have a few different ways to study the properties of dark energy, but supernovae remain one of the best, and if we manage to do a better job calibrating our distance measurements we believe that will help us better understand dark energy."

Human eyes first caught sight of the strange supernova the day after it exploded. Using the Zwicky Transient Facility in California, researchers were able to tell it happened right next to the Draco constellation, and astrophysicists then took a closer look at the X-ray and UV wavelengths at NASA’s Neil Gehrels Swift Observatory. SN2019yvq was initially classified as a type Ia. It almost passed for one, but the intense UV emission could not be ignored. This wasn’t a flash that was gone in a blink. It lasted several days, meaning something unfathomably hot must have been in or at least close to the dying star. Except white dwarfs cool down as they decline.

UV light this intense needs something at least three or four times hotter than the sun to generate it. It is invisible to us because UV wavelengths are too short for our eyes to process, and X-rays have even shorter wavelengths. Previously, another team of researchers had found ways to determine characteristics of superluminous supernovae using this kind of light. Computer simulations that showed the event at UV wavelengths is how the explosion mechanism behind it was determined, and that method could be used to identify explosion mechanisms in other supernovae. This goes way beyond that.

“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,” Miller said.

Miller has four hypotheses on how this could have happened. The dying white dwarf might have accreted gas and dust the Ia supernova way, exploding when it exceeded the limit, and the exploded star colliding with the other star in its binary system caused the UV flash. Maybe there was superhot radioactive material in its core that heated up its outer shell past the point of no return. There is a chance helium was what drastically raised the star’s temperature so high that the double explosion which ensued also released the UV flash. Finally, it is possible the immense amount of UV radiation lit up the cosmos when the two white dwarfs in the star system merged and the remnants of the explosion collided.

So out of these hypotheses, which is most likely to be the real reason for such an outburst?

"I don't think any of the four truly stands out relative to the others," Miller said. "This might not be [an] exciting thing to say, but I think the scenario featuring a lot of mixing of radioactive Nickel to the outer layers of the supernova ejecta is the least likely. This is part of why this event will be so exciting to study over the next year. Each scenario makes specific predictions for what we will see this coming winter, so I think at that time we will have a much better idea about what actually happened."

What actually caused these real-life movie special effects could reveal itself in about a year, according to Miller. Ejecta will travel further and further from the source until the core of the explosion is exposed. 

"We think that a spectrum obtained in December 2020 or January 2021 will provide a definitive answer. If the ejecta collided with a companion star we will see hydrogen emission, if there was a double explosion due to helium on the surface of the white dwarf we will see very strong calcium emission, and if it was due to the merger of two white dwarf stars we will see strong oxygen emission," he explained. "Each of these scenarios is unique to the specific model for what happened."

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