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Spectacular ultraviolet flash may finally explain how white dwarfs explode

Event also could give insight into dark energy

July 23, 2020

By Linda Vu

SN2019yvq RGB

Zwicky Transient Facility composite image of SN2019yvq (blue dot in the center of the image) in the host galaxy NGC 4441 (large yellow galaxy in the center of the image), which is nearly 140 million light-years away from Earth. SN 2019yvq exhibited a rarely observed ultraviolet flash in the days after the star exploded. (Credit: ZTF, Adam Miller, Northwestern and Daniel Goldstein, Caltech)

For just the second time ever, astrophysicists have spotted a spectacular flash of ultraviolet (UV) light in a supernova, an extremely rare event following a white dwarf explosion. This event is poised to offer insights into several long-standing mysteries, including what causes white dwarfs to explode and how dark energy is accelerating the universe’s expansion.

“The UV flash is telling us something very specific about how this white dwarf exploded,” said Northwestern University astrophysicist Adam Miller, who led the research. “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.”

At that point, Miller and his team, which includes Lawrence Berkeley National Laboratory (Berkeley Lab) scientist Peter Nugent and graduate student researcher Abigail Polin, will know more about what caused this white dwarf, and all others, to explode. White dwarfs are the remnants of dead stars. 

There are currently four competing theories about what caused this rare event; one of them was modeled by Polin using Berkeley Lab’s CASTRO code and supercomputing resources at the National Energy Research Scientific Computing Center (NERSC).  A paper describing this work was published today (July 23) in Astrophysical Journal. 

Common Event with a Rare Twist

Using the Zwicky Transient Facility in California, researchers first spotted the peculiar supernova in December 2019 — just a day after it exploded. The event, dubbed SN2019yvq, occurred in a relatively nearby galaxy located 140 million light-years from Earth.

Within hours, astrophysicists used NASA’s Neil Gehrels Swift Observatory to study the phenomenon in ultraviolet and X-ray wavelengths. They immediately classified SN2019yvq as a type Ia (pronounced “one-A”) supernova, a fairly frequent event that occurs when a white dwarf explodes.

“These are some of the most common explosions in the universe,” Miller said. “But what’s special about this is the UV flash. Astronomers have searched for this for years and never found it. To our knowledge, this is actually only the second time a UV flash has been seen with a type Ia supernova.”

The rare flash, which lasted for a couple days, indicates that something inside or nearby the white dwarf was incredibly hot. Because white dwarfs become cooler and cooler as they age, the influx of heat puzzled astronomers.

2Dseries Abi Polin1

These images show a time series of a double detonation explosion. (Image by Abigail Polin, Berkeley Lab)

The researchers believe that this is an important clue to understanding why white dwarfs explode, which has been a long-standing mystery in the field. Currently, there are multiple competing hypotheses. One of them, called the double-detonation, has been modeled by Polin.

Her theory predicts that the white dwarf orbits a companion that has some helium on it. As the white dwarf siphons helium gas from its neighbor, a shell of helium forms on its surface. Instabilities created during this accretion process ignite a helium detonation in the shell, which sends a shock front toward the white dwarf’s core. This shock increases the density and temperature within the white dwarf, which compresses its core and detonates a supernova explosion. The shock from this explosion then hits helium ash on its way out into space, which explains the UV flash.

“This is shockingly similar to how the original atomic bomb was designed,” said Polin. “I was modeling behavior for what would happen if we had a double detonation triggered by a thick helium shell on supercomputers at NERSC. We predicted what it would look like, but I doubted that we would ever find one because they are extremely rare. The smoking gun was SN2018byg, a peculiar type Ia supernova with an early optical bump, the observations matched our model perfectly. Now that we know they’re out there, finding this recent one is very exciting.”

In addition to the double detonation theory, the researchers are also interested in three other potential scenarios that could cause a white dwarf to explode with a UV flash:

  1. A white dwarf consumes its companion star and becomes so large and unstable that it explodes. The materials from the white dwarf and the companion star collide, causing a flash of UV emission;
  2. Extremely hot radioactive material in the white dwarf’s core mixes with its outer layers, causing the outer shell to reach higher temperatures than usual;
  3. Two white dwarfs merge, triggering an explosion with colliding ejecta that emit UV radiation.

Each of these scenarios leads to different predictions for what SN2019yvq will look like at later times. “Within a year,” Miller said, “we’ll be able to figure out which one of these four is the most likely explanation.”

Illuminating Dark Energy 

White dwarfs already play an enormous role in physicists’ current understanding of dark energy. Physicists predict that white dwarfs all have the same brightness when they explode. So type Ia supernovae are considered “standard candles,” allowing astronomers to calculate exactly how far the explosions lie from Earth. Using supernovae to measure distances led to the discovery of dark energy, a finding recognized with the 2011 Nobel Prize in Physics.

“We don’t have a direct way to measure the distance to other galaxies,” Miller explained. “Most galaxies are actually moving away from us. If there is a type Ia supernova in a distant galaxy, we can use it to measure a combination of distance and velocity that allows us to determine the acceleration of the universe. Dark energy remains a mystery. But these supernovae are the best way to probe dark energy and understand what it is.”

And by better understanding white dwarfs, Miller believes we potentially could better understand dark energy and how fast it causes the universe to accelerate.

“At the moment, when measuring distances, we treat all of these explosions as the same, yet we have good reason to believe that there are multiple explosion mechanisms,” he said. “If we can determine the exact explosion mechanism, we think we can better separate the supernovae and make more precise distance measurements.”

The paper, “The spectacular ultraviolet flash from the peculiar type Ia supernova 2019yvq,” was partially supported by the Large Synoptic Survey Telescope Corporation, the Brinson Foundation, and the Moore Foundation.

Read more on the Northwestern University news center.

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