Astronomers discover a new X-ray production mechanism in the stars
An artist’s impression of the ASASSN-16oh system. Source: NASA/CXC/M.Weiss
Researchers studying the brightening of a binary system pointed X-ray observatories Swift and Chandra at the phenomenon. As a result, they discovered a new mechanism of X-ray production in the so-called cataclysmic variables. The discovery was made with the participation of astronomers from the Astronomical Observatory of the University of Warsaw.
The article describing the discovery was published in the journal Nature Astronomy.
Representatives of the Astronomical Observatory of the University of Warsaw explain in the press release sent to PAP the Sun and other stars in the Milky Way emit radiation in the range visible to our eyes. But astronomers know many objects that emit a significant amount of energy in the form of X-rays - this is what the new discovery concerns.
The object of interest of scientists was the star located in the Small Magellanic Cloud, in the neighbouring galaxy approx. 200,000 light years from the Milky Way. This area of the sky has been regularly studied for over a dozen years by astronomers from the OGLE team from the Astronomical Observatory of the University of Warsaw. The studied object is in fact a binary system - a red giant and a white dwarf (a cataclysmic system). It is called ASASSN-16oh, and its burst was observed in December 2016. Photometric observations showed that the star brightened several dozen times.
To determine what caused the ASASSN-16oh brightening, astronomers pointed orbital X-ray observatories Swift and Chandra at this object. They noticed that the star emitted large amounts of X-rays, which must have been generated in matter at a temperature of nearly a million degrees.
Supersoft X-ray emission - lowest energy X-rays generated in very hot matter, with a temperature of several hundred thousand degrees - has long been considered a result of nuclear fusion on the surface of a white dwarf, a small, burnt star that attracts fresh fuel (hydrogen gas) from its neighbours. When enough gas is accumulated and it becomes hot enough, a thermonuclear reaction chain begins in it and the entire surface of the star explodes.
But that is inconsistent with observations of astronomers: they exclude such a violent explosion. X-ray emission comes from a small fragment of the white dwarf`s surface.
Scientists believe that X-rays were generated as a result of accretion - the process of accumulation of matter on the surface of the white dwarf. ASASSN-16oh is actually a binary system - a red giant and a white dwarf. Because the flow of matter from one star to another is not stable, when the gas starts flowing more quickly, the brightness of the whole system dramatically increases.
According to the representatives of the Astronomical Observatory of the University of Warsaw, the discovery shows that in nature there are two types of supersoft X-ray sources: thermonuclear fusion reactions and accretion.
This is important information not only for astronomers studying x-ray sources in space; it can also contribute to a better understanding of the expansion of the Universe. Type Ia supernovae are used in measurements of the expansion of the Universe. Their radiation power is believed to be constant: by measuring the observed supernova brightness, distance to the parent galaxy can be measured.
Scientists believe that type Ia supernovae are formed as a result of white dwarf explosions, but the mechanism and origin of explosion are not well understood and it is not known whether the supernovae formed in the oldest galaxies look the same as the more recent ones. The discovery published in Nature Astronomy shows that the white dwarf-red giant systems similar to ASASSN-16oh may become type Ia supernovae in the future.
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