October 15, 2008
By mapping out how well the variations in visible light match those in Xrays on very short timescales, astronomers have shown that magnetic fields must play a crucial role in the way black holes swallow matter.
Like the flame from a candle, light coming from the surroundings of a black hole is not constant — it flares, sputters and sparkles. "The rapid flickering of light from a black hole is most commonly observed at X-ray wavelengths," Poshak Gandhi said, leader of the international team that reports these results. "This new study is one of only a handful to date that also explores the fast variations in visible light, and, most importantly how these fluctuations relate to those in X-rays."
The observations tracked the shimmering of the black holes simultaneously using two different instruments, one in space and one on the ground. NASA's Rossi X-ray Timing Explorer satellite captured the X-ray data. The high-speed camera ULTRACAM, a visiting instrument at European Southern Observatory's (ESO) Very Large Telescope (VLT) collected the visible light by recording up to 20 images a second. Vik Dhillon and Tom Marsh developed ULTRACAM. "These are among the fastest observations of a black hole ever obtained with a large optical telescope," Dhillon said.
To their surprise, astronomers discovered the brightness fluctuations in the visible light were even more rapid than those seen in X-rays. In addition, the visible-light and X-ray variations were found not to be simultaneous, but to follow a repeated and remarkable pattern. Just before an X-ray flare, the visible light dims, and then surges to a bright flash for a tiny fraction of a second before rapidly decreasing again.
None of this radiation emerges directly from the black hole, but from the intense energy flows of electrically charged matter in its vicinity. A riotous mêlée of strong and competing forces such as gravity, magnetism, and explosive pressure constantly reshape a black hole's environment. As a result, light emitted by the hot flows of matter varies in brightness in a muddled and haphazard way. "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," team member Andy Fabian said.
The visible-light emission from the neighborhoods 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 if this were so, any visible-light variations would lag behind the X-ray variability, 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," asserts 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."
Strong magnetic fields represent the best candidate for the dominant physical process. 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 traveling 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.