Astrophile: Black hole exposed as a dwarf in disguise

Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

Object: A sneaky white dwarf
Behaviour: Short, ultra-bright X-ray flares

The white dwarf thought it was sending all the right signals. Embedded near a small satellite galaxy of the Milky Way, the star was emitting short, bright X-ray flashes that made it look like a feeding black hole. But after a multi-agency stakeout, cosmic detectives have blown the dwarf's cover.

Behind the black hole fa?ade, the white dwarf was stealing mass from a much larger companion, a process that occasionally causes a titanic thermonuclear blast. The discovery marks the first such binary system known, and hints that similar pairings may be hiding in plain sight across the universe.

White dwarfs had never before produced such enormous flashes, according to Phil Charles, an astronomer at the University of Southampton, UK. "That's why they weren't thought of as the natural explanation," he says. "These findings indicate that binary stars comprising a massive hot star and a much less massive white dwarf are indeed possible, and also show why it has been hard to find them in the past."

Playing it cool

The white dwarf mimic, aka MAXI J0158-744, made its revealing slip in November 2011. An X-ray camera aboard the International Space Station detected a super-bright flare lasting 92 minutes, in the vicinity of the Small Magellanic Cloud.

At first glance, the flare matched the behaviour of other ultra-luminous X-ray sources thought to be black holes firing off radiation bursts. But one piece of evidence did not ring true: despite its brightness, the flare's observed temperature was much lower than that of other black hole outbursts.

Luckily, within 24 hours of the blast astronomers had NASA's Swift space telescope and ground-based observatories in Chile and South Africa swinging into action to monitor the mysterious source. After combing through the signatures in its light, astronomers matched the flare to computer models of a white dwarf ? the dense, burned-out core of a sun-like star ? orbiting a much hotter and brighter companion star.

As the white dwarf orbits, it pulls matter from the larger star. When enough hydrogen builds up on its surface, the overload triggers a runaway nuclear reaction that ignites a flash of radiation. Astronomers have seen other white dwarf binaries flare before, but MAXI J0158-744 is the first to create an ultra-luminous eruption, giving the impression of a black hole drawing material from a companion.

Black-hole rethink

"Its X-ray luminosity is so similar to many black hole binary systems. Such a discovery will lead us to rethink other black-hole candidates in nearby galaxies," says Albert Kong of the National Tsing Hua University in Hsinchu, Taiwan. "They may in fact belong to a whole new class of white dwarf binaries."

Charles agrees that MAXI J0158-744 may be just the tip of the iceberg. "I actually think there are more out there, but we have been missing them because you need a particular combination of ground and space-based telescopes in order to pick them up," he says.

Journal reference: Astrophysical Journal, doi.org/j3c

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