The night sky is filled with bursts and flashes, constantly changing sea of lights. Some of these changes come from terrestrial events, such as the flight of airplanes overhead, but some from distant sources in space. Astronomers prey on these fleeting phenomena known as astronomical transitions, regularly observing the sky and looking for differences that appear.
Researchers recently found a transition that obscures all others like it– a supernova known as AT2020mrf. They described their discovery in an article posted on the preprint server arXiv.org last December and sent to Astrophysical Journal. They also presented it at a virtual press conference Meeting of the American Astronomical Society 2022 in January. This supernova was one of the brightest and most energetic star deaths ever seen, and it can give a rare glimpse into how massive stars give birth to some of the most amazing objects in the universe: black holes and neutron stars.
The biggest stars are dying spectacularly. They explode with the energy of a million (i.e. “1” with 30 zeros behind) atomic bombs in the supernova, shining so brightly that we can sometimes even see them in the night sky with the naked eye. The AT2020mrf was 10,000 times brighter in X-rays than a typical supernova. This happened after several other super-bright events that have been observed in recent years – astronomers call them supernovae like “cows”., or “cows,” after their first type was found: AT2018cow. Unlike traditional supernovae, cows shine brightly in X-rays and high-energy radiation (most supernovae glow brightest in visible light). But the AT2020mrf was the brightest cow in the group – 20 times brighter than the original.
Astronomers Johan Yao and Sri Kulkarni of the California Institute of Technology, as well as Anna Ho of the University of California, Berkeley and Daniel Perley from the University of Liverpool John Moore in England, first noticed the explosion in June 2020 in visible light images from the Nails Transient Facility (ZTF), an automated telescope at the Palamar Observatory in California. The bright spot they saw initially seemed inconspicuous, and astronomers ignored the event almost a year later.
In April 2021, Russian scientists who collaborated with the Yao group observed the same event while reviewing data from their X-ray observatory Spectrum-Rentgen-Gamma (SRG). Their images from July 2020 showed X-rays in the same place as the bright spot in the ZTF data. Hearing the news, Yao’s mind switched to mysterious events similar to a cow because it is the only type of supernova that is known to emit so many X-rays. Despite the fact that a year had passed, the initial explosion of the AT2020mrf was so incredibly bright that Yao suspected that she would still be able to see it through the Chandra X-ray Observatory. Her calculations were correct, and observations showed this clearly, glowing 200 times brighter than the original cow a year after the AT2020mrf explosion.
“It was really really nice when SRG saw this source because we thought maybe [cow-like events] can be detected first on an X-ray, ”says Yao. “This is the first time it has really been discovered on an X-ray.”
Astronomers are still puzzled over what makes these supernovae special. The current theory is that cows have a very active “central engine” – what is left after the star’s core, such as a black hole that absorbs matter, or a neutron star that rotates rapidly, providing supernova energy. They also have less material that envelops this central region than most exploding stars, providing a view of their interesting center.
The stars, which create events similar to cows, erupt material as they approach death, making the environment around them denser and the stars themselves a little smaller. When they explode, less gas remains in the immediate area around the star’s core, allowing X-rays to exit the central engine. The shock wave that the supernova sends through the surrounding space also heats up the new dense medium, creating radio radiation that astronomers observe. Yao believes that perhaps the AT2020mrf was even brighter than most cows because it gained even more mass, which allowed the central engine to shine so brightly in X-rays.
“AT2020mrf is a really exciting event because it confirms the growing class [cow-like objects] and what it tells us about the variety of these mysterious stellar explosions, ”says Brian Metzger, an astrophysicist at Columbia University and the Center for Computational Astrophysics.
With just four previously known cow-like events, the AT2020mrf more firmly strengthens this group as a new explosion category. The recent explosion also stands out among its classmates, featuring an exciting variety of stellar deaths. Although scientists understand how massive stars are dying, the details are still unclear. This is especially true for some stages of a star’s end of life, such as the silicon burning stage, the last stage of synthesis that a large star can complete when it merges silicon together to create iron. This period lasts only about 18 days from the life of the star from a million to a billion years. Cows can provide a window at this time that is difficult to observe, and improve our understanding of how black holes and neutron stars are born in supernovae.
The AT2020mrf and other cows are also simply captivating scientists. “I like the excitement when I just see how one source is different from all the others,” Yao says. “You know, maybe one day in your life you’ll find one of those events and you need to act. You have to be aware of everything to tell the story of the object. ” Future observations with the Vera K. Rubin Observatory, a future power plant for surveying the sky, and other telescopes will hopefully give astronomers even more information to work on.
The record supernova is part of a new class of facilities
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