Star Anah White Unexpectedly Emitting X-rays Supersoft


The discovery of a white dwarf star emitting supersoft X-rays questions the conventional wisdom about how X-rays are produced by the death of stars.

When stars run out of fuel after millions of years, they shrink to a smaller size and become much weaker in a phase known as a white dwarf. The mass of the star remains the same, but the size becomes much smaller, so these dwarves have a very strong gravity. White dwarfs are often found in pairs, called binary systems, and their strong gravity means they can pull matter away from their fellow stars.

One such system was identified by NASA's Chandra X-Ray Observatory project, which found x-ray emissions outside the binary system containing a white dwarf named ASASSN-16oh. Normally, a star is expected to produce low energy X-rays, called soft X-rays. But the white dwarf ASASSN-16oh is emitting X-rays that are much brighter than expected, which earned it the unusual rating of being a supersoft X-ray source.

Supersoft X-rays suggest that the white dwarf should have a temperature of several hundred thousand degrees, which is warmer than normal stars. However, this high temperature is only found at certain points on the surface of the star. Astronomers believe this may be because ASASSN-16oh is pulling material from its red giant at a fast pace, and this issue gets hotter and hotter as it spirals toward the white dwarf. Hot gas is installed on the accretion disk (the disk of matter spinning around the star), where it eventually merges with the surface of the star and produces X-rays. However, as this process happens at a variable rate, there are some times when matter flows faster and the star temporarily produces brighter X-rays.

chandra white dwarf supersoft xray
An illustration of a drawing matter of the white dwarf star of a red giant. NASA

It was previously believed that supersoft X-rays were only observed when nuclear fusion was occurring in white dwarfs. But in the case of ASASSN-16oh, the uneven distribution of X-rays, in addition to the optical light emitted as being too weak, indicates that fusion is not happening in this star. "Our result contradicts a decades-old consensus on how supersoft X-ray emission from white dwarfs is produced," Thomas Nelson of the University of Pittsburgh told NASA. "We now know that the emission of X-rays can be done in two different ways: by nuclear fusion or by the accumulation of matter of a companion."


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