The mechanism by which black holes expel powerful jets of particles at nearly the speed of light has long been speculated, but thanks to new observations of a blazar in action, these theories can now be substantiated.
Blazars are among the most energetic objects in the Universe, and are fueled by supermassive black holes at the core of certain giant elliptical galaxies. Periodically, they emit jets of high-energy plasma at almost the speed of light. The leading theory says the jets are accelerated by tightly twisted magnetic fields close to the black hole, and using the unrivaled resolution of the National Radio Astronomy Observatory’s Very Long Baseline Array (VLBA), astronomers have watched material winding a corkscrew outward path as dictated by this theory.
Illustration of a shock wave (bright 'blob' in the upper jet) following a spiral path (shown in yellow) as it moves away from the black hole and through a section of the jet where the magnetic field (light blue curved lines) is wound up in a coil. This caused the first brightening. Later, the shock passed through the stationary X-shaped compression in the jet and brightened a second time. Image: Marscher et al., Nature 453, 24 April 2008 issue.
Scientists from all over the world aimed a variety of telescopes at blazar BL Lacertae (BL Lac), which is located some 950 million light-years from Earth. Over a period of several years, optical, X-ray and radio observations were conducted. “Everything we see supports the idea that twisted, coiled magnetic fields are propelling the material outward," says Alan Marscher of Boston University, leader of the international research team. "This is a major advance in our understanding of a remarkable process that occurs throughout the Universe."
The researchers say that the outbursts of radiation from blazars are triggered near the black hole, where some explosive event, such as “reconnection” of magnetic field in places where oppositely directed magnetic fields come into contact, shoots extra energy down the jet, which probably forms a shock wave that moves down the jet in a spiral path. This light and other radiation emitted by the moving material would brighten when its rotating path was aimed most directly toward Earth. A few weeks later, after the emission has faded as the material cools and expands, the researchers predict a second brightening brought about by the compression of the material by a stationary shock wave created by a pressure difference between the jet and the gas of the surrounding galaxy.
"That behavior is exactly what we saw," says Marscher. "We got an unprecedented view of the inner portion of one of these jets and gained information that's very important to understanding how these tremendous particle accelerators work."
Scientists hope to get a closer look at blazar jets when NASA launches its Gamma-ray Large Area Space Telescope (GLAST) satellite observatory in May. “We'll be using GLAST data to examine this object and 28 others that we are observing in a similar way,” Marscher told Astronomy Now. “We are observing 5 of these with NASA's Rossi X-ray Timing Explorer as well, at least until the end of the year.”