Colliding White Dwarves Behind Astronomical Standard Candles

So-called type 1a supernovae are widely used in astronomy for measuring distances due to their consistency in brightness, but until recently scientists were not sure how these "standard candles" came into being. It was known that SN 1a came about when a white dwarf – an old star with a density 50-100 times that of the Sun – manages to take hold of extra matter and collapses under the extra weight, which causes a thermonuclear explosion. However, the source of that extra matter can vary.

The most popular scenario suggests that in an accreting binary star the white dwarf slowly steals mass from its younger companion. A less popular explanation involves two white dwarves, which spiral onto each other to finally merge and go supernova. A study by Marat Gilfanov of Russia's Institute for Space Research and Akos Bogdan of the Max Planck Institute for Astrophysics in Germany says the second scenario is much more probable.

The scientists analyzed data from the orbiting Chandra X-Ray Observatory. They reasoned that the accretion scenario must produce large amounts of X-rays millions of years before the actual explosion happens, while in the merger scenario a burst of X-rays would only start shortly before the event.

The actual data from six elliptical galaxies showed 30-50 times less X-ray flux than that predicted in the accretion scenario. In their paper published in Nature magazine the researchers conclude that no more than 5% of type 1a supernovae are results of accretion – at least in the early-type galaxies which they studied.

The finding goes contrary to the prevailing thinking in astronomy. The popularity of accretion scenario among scientists was probably due to the difficulty of finding paired white dwarves, which turn out to be more numerous than previously thought.

Source: Russian Federal Space Agency