An artist's conception of the blazar BL Lacertae at it spurts out jets of charged particles accelerated by corkscrew magnetic field lines.
Credit: Marscher et al., Wolfgang Steffen, Cosmovision, NRAO/AUI/NSF
While we may never know what it looks like inside a black hole, astronomers recently obtained one of the closest views yet. The sighting allowed scientists to confirm theories about how these giant cosmic sinkholes spew out jets of particles travelling at nearly the speed of light.
Ever since the first observations of these powerful jets, which are among the brightest objects seen in the universe, astronomers have wondered what causes the particles to accelerate to such great speeds. A leading hypothesis suggested the black hole's gigantic mass distorts space and time around it, twisting magnetic field lines into a coil that propels material outward.
Now researchers have observed a jet during a period of extreme outburst and found evidence that streams of particles wind a corkscrew path away from the black hole, as the leading hypothesis predicts.
"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," said Boston University astronomer Alan Marscher, who led the research team. The results of the study are detailed in the April 24 issue of the journal Nature.
The team studied a galaxy called BL Lacertae (BL Lac), about 950 million light years from Earth, with a central black hole containing 200 million times the mass of our Sun. Since this supermassive black hole's jets are pointing nearly straight at us, it is called a blazar (a quasar is often thought to be the same as a blazar, except its jets are pointed away from us).
The new observations, taken by the National Science Foundation's Very Long Baseline Array (VLBA) radio telescope, along with NASA's Rossi X-ray Timing Explorer and a number of optical telescopes, show material moving outward along a spiral channel, as the scientists expected.
These data support the suggestion that twisted magnetic field lines are creating the jet plumes. Material in the center of the galaxy, such as nearby stars and gas, gets pulled in by the black hole's overwhelming gravity and forms a disk orbiting around the core (the material's inertia keeps it spiraling in a disk rather than falling straight into the black hole). The distorted magnetic field lines seem to pull charged particles off the disk and cause them to gush outward at nearly the speed of light.
"We knew that material was falling in to these regions, and we knew that there were outbursts coming out," said University of Michigan astronomer Hugh Aller, who worked on the new study. "What's really been a mystery was that we could see there were these really high-energy particles, but we didn't know how they were created, how they were accelerated. It turns out that the model matches the data. We can actually see the particles gaining velocity as they are accelerated along this magnetic field."
The astronomers also observed evidence of another phenomenon predicted by the leading hypothesis ? that a flare would be produced when material spewing out in the jets hit a shock wave beyond the core of the black hole.
"That behavior is exactly what we saw," Marscher said.