The truth about variable black holes
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: October 15, 2008

By studying the flickering light in the surroundings of two black holes, astronomers have discovered that magnetic fields play a crucial role in the way these galactic monsters consume matter.

Like a bonfire night sparkler, light coming from the surroundings of a black hole is not constant — it flares, sputters and sparkles. Observations of  the flickering light from two black holes, GX 339-4 and SWIFT J1753.5-0127, the remnants of massive dead stars, were made simultaneously in X-rays and visible light using NASA’s space-based Rossi X-ray Timing Explorer Satellite and ESO’s Very Large Telescope on the ground, respectively. The visible light was collected with the high speed camera ULTRACAM, recording up to 20 images a second, among the fastest observations of a black hole ever obtained with a large optical telescope.

"The rapid flickering of light from a black hole is most commonly observed at X-ray wavelengths," says Poshak Gandhi, who led the international team that reports these results. "This new study is one of only a handful to date that also explore the fast variations in visible light, and, most importantly how these fluctuations relate to those in X-rays."

Artist impression of the black holes observed in the study. Both systems contain a black hole and a normal star separated by just a few million kilometres, allowing matter to spill from the star toward the black hole. Intense magnetic fields in the disc of matter surrounding the black hole accelerate some of this hot gas into tight jets that flow in opposite directions away from the black hole. Image: ESO/L. Calcada.

The two black holes are embedded in separate binary stellar systems, where the black hole is bound to a normal star that is losing matter to its dark companion. Both black holes have masses of around ten times that of our Sun, yet the size of their orbits is only a few million kilometres, much more compact than the orbit of Mercury around our Sun.

The observations turned up some surprising revelations. First, the brightness fluctuations in the visible light were even more rapid than those seen in X-rays. And second, the visible-light and X-ray variations were found to occur separately, following a repeated pattern that sees an initial dimming in visible light just prior to an X-ray flare, and then a bright flash in visible light for a tiny fraction of a second before rapidly decreasing again.

Furthermore, the observations revealed that the radiation was emanating from the intense energy flows of electrically charged matter in the black hole’s vicinity, rather than from the heart of the black hole itself. The environment of a black hole is extremely chaotic, constantly being pushed and pulled by the competing forces of gravity, magnetism and explosive pressure. As a result, light emitted by the hot flows of matter varies in brightness in a muddled and haphazard way. "But the pattern found in this new study possesses a stable structure that stands out amidst an otherwise chaotic variability, and so, it can yield vital clues about the dominant underlying physical processes in action," says team member Andy Fabian.

Until now, the visible-light emission from the locale of black holes was widely thought to be a secondary effect, with a primary X-ray outburst illuminating the surrounding gas that subsequently shone in the visible range. But this behaviour should mean that any visible-light variations will lag behind the X-ray outburst, and would be much slower to peak and fade away.

"The rapid visible-light flickering now discovered immediately rules out this scenario for both systems studied," says Gandhi. "Instead the variations in the X-ray and visible light output must have some common origin, and one very close to the black hole itself."

Gandhi and colleagues propose that strong magnetic fields are responsible for the observations. Acting as a reservoir, they can soak up the energy released close to the black hole, storing it until it can be discharged either as hot multi-million degree X-ray emitting plasma, or as streams of charged particles travelling at close to the speed of light. The division of energy into these two components can result in the characteristic pattern of X-ray and visible-light variability.

The results of the observations of GX 339-4 are described in a paper appearing in the October 2008 edition of the Monthly Notices of the Royal Astronomical Society Letters, while the observations of SWIFT J1753.5-0127 are described in the July 2008 edition of The Astrophysical Journal.

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