In this artist’s impression, a white dwarf, left, draws material in from a companion red giant. Image: NASA/CXC/M. Weiss
Astronomers have spotted an X-ray outburst from a white dwarf in the Small Magellanic Cloud some 200,000 light years away that indicates the compact star is pulling in material from a companion red giant at such a high rate that it may be the fastest growing white dwarf ever observed.
The X-ray emissions also suggest an alternative to the long-held idea that such radiation is generated by fusion reactions in a hot, dense layer of hydrogen and helium packed onto a white dwarf’s surface. That line of reasoning holds that when the weight of accumulated material pulled in from the companion star reaches a critical point, a fusion explosion goes off across the white dwarf’s visible surface.
But data collected by the Chandra X-ray Observatory and the Neil Gehrels Swift Observatory indicate another mechanism is at work in the white dwarf binary system known as ASASSN-16oh. The observed low-energy “supersoft” X-rays indicate temperatures of several hundred thousand degrees, hotter than what is seen in the atmospheres of normal stars but cooler than the tens of millions of degrees associated with high-energy events.
If fusion reactions were responsible for the supersoft X-rays detected from ASASSN-16oh, the emissions should have come from an explosion across the white dwarf’s entire visible surface. But data collected by the Chandra X-ray Observatory show the emissions came from a smaller region. In addition, optical observations show the source is 100 times fainter than visible light from white dwarfs known to be experiencing fusion on their surfaces.
Given no signs of nuclear fusion are present with ASASSN-16oh, the authors of tahe new study in Nature Astronomy offer an alternative explanation. As the white dwarf pulls material away from the companion red giant in a process known as accretion, the captured gas gets hotter and hotter as it spirals inward. The gas then falls onto the white dwarf in a belt where that accretion disk merges with the star’s surface. The rate of accretion varies, and outbursts can occur when material flows in more quickly.
“Our result contradicts a decades-long consensus about how supersoft X-ray emission from white dwarfs is produced,” said co-author Thomas Nelson from the University of Pittsburgh. “We now know that the X-ray emission can be made in two different ways: by nuclear fusion or by the accretion of matter from a companion.”
ASASSN-160h is gaining mass faster than any other known white dwarf. When it eventually grows to the point where it can no longer support its own weight, the star likely will collapse and explode as a Type 1a supernova. Based on the new observations, the white dwarf already is unusually massive and may blow up relatively soon, astronomically speaking.
“The transfer of mass is happening at a higher rate than in any system we’ve caught in the past,” said lead author Tom Maccarone, a professor in the Texas Tech Department of Physics and Astronomy.