White Dwarfs Can Mimic Black Holes, Astronomers Claim

According to an international team of astronomers, bright X-ray flares in nearby galaxies, once assumed to indicate the presence of black holes, can in fact be produced by white dwarf stars.

This picture is an artist’s impression showing how the binary star system of Sirius A and its diminutive blue companion, Sirius B, might appear to an interstellar visitor. Sirius B is one of the more massive white dwarfs known (NASA / ESA / G. Bacon / STScI)

A short-lived X-ray flare, labeled XRF111111 and MAXI J0158-744, was detected in 2011 with the Monitor of All-sky X-ray Image (MAXI) mission on the International Space Station.

Using optical telescopes in South Africa and Chile, the astronomers showed that MAXI J0158-744 was located in the Small Magellanic Cloud. The flare was so luminous that astronomers initially thought it was likely to be a black hole producing X-rays but further study by the team revealed that its X-ray temperature was so low that it had to be a white dwarf instead.

White dwarfs are very common, burnt-out cinders of normal stars like the Sun that are typically about one solar mass but are contained in a volume no bigger than the Earth.

However, white dwarfs were not considered capable of producing such a huge X-ray flash but the optical observations showed that the white dwarf was orbiting a hot B star – a normal star about 10 times the mass of our Sun that is much hotter and brighter. This was something that had only been seen twice previously and both times with much lower X-ray luminosities.

The astronomers revealed that material was probably collecting on the surface of the white dwarf from the B star and eventually underwent runaway thermonuclear burning that was seen on Earth as a nova explosion.

“Our observations show that the thermonuclear burning probably caused a shell of matter to be ejected from around the white dwarf and when the shell hit the hot wind of the B star it produced a huge shock leading to the X-ray flash that was seen on the International Space Station,” said Prof Phil Charles of the University of Southampton, co-author of the study published in the Astrophysical Journal (arXiv.org version).

“We think that this incredible X-ray flash was not due to accretion onto a black hole but was instead due to a nova explosion on a white dwarf that took place close to a hot massive star. This was something that we, as astronomers, have never seen before.”

“This surprising result shows that, in the right circumstances, white dwarfs are capable of mimicking black holes, the most luminous objects we know of,” Prof Charles concluded.

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Bibliographic information: K. L. Li et al. 2012. A Luminous Be+White Dwarf Supersoft Source in the Wing of the SMC: MAXI J0158-744. ApJ 761, 99; doi: 10.1088/0004-637X/761/2/99