Supersoft X-ray emissions in the universe can come from nuclear fusion as well as accretion, claims a new study led by scientists from Texas Tech University.
Supersoft X-rays are the weakest type of X-rays - characterised by very low energy. For years, scientists have believed that supersoft X-ray emission result from nuclear fusion occurring on the surface of a white dwarf star.
The little star pulls volatile material from its companion stars, and lets the material accumulate on its surface. Eventually, the material becomes so hot that a nuclear fusion explosion occurs on the surface of the star.
Recently, a group of astronomers reported detecting supersoft emissions that were not powered by nuclear fusion, indicating that there is more to the story. The event, called ASASSN16-oh, was first spotted by the All-Sky Automated Survey for Supernovae (ASASSN), a collection of optical telescopes distributed across the globe to survey the sky for transient events, like supernovas.
ASASSN16-oh event occurred in the Small Magellanic Cloud, a nearby galaxy located about 200,000 light years from Earth, and was verified by NASA's Swift Observatory and the Chandra X-ray Observatory.
Tom Maccarone, a professor in the Department of Physics and Astronomy at Texas Tech, and the lead author of the study, reveals that the source of the supersoft emissions in ASASSN16-oh was found to be a region on the white dwarf, but much smaller than its entire surface.
Had supersoft X-rays emission resulted from a nuclear fusion, it should start with an explosion and come from the entire surface of the white dwarf rather than a small region on it.
"We have strong arguments against any kind of explosion having taken place on the white dwarf," said Maccarone.
"Specifically, there are no broad emission lines in the X-ray or optical spectra, so there cannot have been any kind of strong wind generated. In some cases, nuclear fusion can be steady on the surface of a white dwarf, but it cannot start immediately as steady fusion. There must be an explosion of some kind when the fusion starts."
Scientists state ASASSN16-oh supersoft emissions resulted from accretion (accumulation of matter) happening in a system of a red giant star and a white dwarf with a massive disk around it.
The white dwarf is pulling gas away from the red giant onto a large disk surrounding the white dwarf. As the disk spirals towards the white dwarf, it becomes hotter. Eventually, the gas falls onto the white dwarf and produces X-rays along a belt.
The system shows an unstable rate of inflow of matter through the disk, and its brightness increases as the material starts flowing more quickly.
"I am excited by this result," Maccarone said.
"It was a totally new phenomenon, and any time one finds one of those, it's exciting."
The findings of the study are published in the journal Nature Astronomy.
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