Spectacular UV flash may explain how white dwarfs explode

For only the second time ever, astrophysicists have spotted a spectacular flash of ultraviolet (UV) light accompanying a white dwarf explosion.

An extremely rare type of supernova, the event is poised to offer insights into several long-standing mysteries, including what causes white dwarfs to explode, how dark energy accelerates the cosmos and how the universe creates heavy metals, such as iron.

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, very close to tail of the dragon-shaped Draco constellation.

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

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.

“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.”

Miller and his team believe 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. Miller is particularly interested in exploring four different hypotheses, which match his team’s data analysis from SN2019yvq.

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 white dwarf’s and companion star’s materials 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. An outer layer of helium ignites carbon within the white dwarf, causing an extremely hot double explosion and a UV flash;
  4. Two white dwarfs merge, triggering an explosion with colliding ejecta that emit UV radiation.

“Within a year,” Miller said, “we’ll be able to figure out which one of these four is the most likely explanation.”

Source: “Spectacular UV flash may explain how white dwarfs explode” PhysOrg, 23 July 2020

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