Off-Peek: Radio Telescopes Edge In on Plasma Jet Spewing from Massive Black Hole

By John Matson-Scientific American


Black holes, by definition, emit no light. They are unseeable.

But astronomers would like to get as close as they can by zooming in on the region immediately surrounding a black hole. That is the objective of the Event Horizon Telescope (EHT), a network of linked radio telescopes around the globe.

An actual event horizon—the point beyond which light and matter alike become hopelessly lost to a black hole's pull—remains out of sight, but the telescope has now succeeded in piercing the veil of a nearby supermassive black hole to peer into unprecedented depths of its turbulent surroundings.

Researchers trained EHT radio dishes in Hawaii, Arizona and California on the giant elliptical galaxy M87, some 54.5 million light-years away. The galaxy features a dramatic jet, thousands of light-years long, emanating from its center and thus, presumably, from the galaxy's black hole. In a study published online September 27 in Science, Sheperd Doeleman of the Massachusetts Institute of Technology's Haystack Observatory in Westford, Mass., and his colleagues report that the dish network has resolved the base of M87's jet. The size of the jet at that position, close to its origin, in turn allowed the researchers to deduce some of the most fundamental attributes of both the galaxy's behemoth black hole, which weighs in at a mass of 6.6 billion suns, and the swirling disk of matter surrounding it.

Outside the event horizon of a black hole orbits a disk of material pulled in but not yet consumed by the gravitational pull of the black hole. That accretion disk grows quite dense and hot as infalling material collides and compresses, emitting copious amounts of radiation in the process. Accretion disks can also accelerate particles into a jet of plasma that propagates outward at a substantial fraction of light speed.

Doeleman and his colleagues measured the base of the jet in M87 to ascertain the inner edge of stability within the black hole's accretion disk, beyond which matter quickly falls inward to its doom. That edge, the densest and fastest-moving part of the accretion disk, can fling particles outward with ease. "The jets that we see from M87 are likely launched from right around this region," Doeleman says.

The EHT, with its superior resolving power from the long baselines separating its individual sites, allowed the researchers to measure a size for the jet's footprint of just 5.5 times the black hole's Schwarzschild radius. (The Schwarzschild radius is the size below which a given mass cannot be compressed without collapsing into a black hole.) "We saw something that was just impossibly small, startlingly small," Doeleman says.

The size of the jet—and, by inference, the size of the innermost stable orbit within the accretion disk—implies that the black hole is spinning, and that the accretion disk is rotating in the same direction. A nonrotating black hole would feature a much wider jet, and an accretion disk spinning counter to the black hole's rotation would launch a fountain that was broader still.

In measuring the jet's footprint the researchers had to account for distortions, caused by the warped spacetime of Einstein's general theory of relativity, inherent to observations of such massive objects. Because of the distortion, a particle jet measured by Earth-based tools can appear larger than it actually is. "The black hole acts as its own lens," Doeleman says. "That's just because the black hole is bending the light rays like taffy."

Simply visualizing the event horizon is not the only goal of the project. A central aim is to peer into an astrophysical environment dominated by supermassive objects to see if gravity works as predicted. "If Einstein's theory is going to break down, it's probably going to be near a black hole," Doeleman says, before acknowledging that the reigning theory of gravity has survived countless challenges before. "It is never wise to bet against Einstein," he adds. "I think the bookies in Vegas give you very long odds. But you have to try."

First images of particle jets at edge of a supermassive black hole

by John Timmer

for photo...... http://arstechnica.com/science/2012...le-jets-at-edge-of-a-supermassive-black-hole/

Supermassive black holes appear to occupy the center of almost all galaxies. When they are actively swallowing matter, these black holes can power energetic jets that shine brighter than the entire rest of the galaxy, and can shoot matter free of it. Despite the mass and energy involved, however, the origin of these jets has been extremely hard to image, both because they're relatively compact, and because they're situated in the crowded centers of distant galaxies.

Now, however, researchers are putting together an array of telescopes stretched across the globe with the specific goal of imaging the environment near these supermassive black holes. The team behind the Event Horizon Telescope has now used it to image the black hole at the center of the M87 galaxy, and returned the first details of the disk of matter that is being sucked into that galaxy's central black hole.

It's difficult to imagine the environment near a supermassive black hole. These objects are typically over a million times the mass of our Sun, but all of that matter is crammed into a space that may only be a fraction of the Sun's radius. Any matter falling into one piles up into an orbiting disk of material (called an accretion disk) that increases in density and energy as you get closer to black hole. Any matter that crosses a critical point, however, rapidly spirals inward to the black hole itself. The inner area of the disk is so energetic that it actually sends matter away from the black hole in a wind of particles.

But that's not the most energetic part. Even further inward, the intense magnetic field lines sometimes cross the event horizon of the black hole itself, propelling intense beams of charged particles away from the black hole. These jets interact with the wind of particles coming from the accretion disk, which focuses them into narrow beams that move at nearly the speed of light. These have so much energy that they are (in some cases) able to propel particles for hundreds of thousands of light years, sending them entirely out of the galaxy, where the particles eventually slow by interacting with the intergalactic medium.

Or that's what theoretical considerations seem to tell us. To actually image any of this, however, has been a serious challenge. It's what the Event Horizon Telescope was intended to solve. In a paper in this week's edition of Science, four of the telescope's instruments were pointed towards the center of M87: Hawaii's James Clerk Maxwell Telescope, the Submillimeter Telescope in Arizona, and two telescopes at CARMA in California. By carefully timing the incoming signals at each of these scopes (and using the two neighboring instruments in California to refine the signal), the researchers could turn these distant instruments into a single, giant telescope, one that could resolve details of the environment near the central black hole.

This system managed to image the area around the black hole down to a resolution on the scale of the Schwarzchild radius. And they were able to spot that the base of the high-energy jets is only a few times the size of the black hole itself (5.5 times the Schwarzchild radius), which "is consistent with scales on which energy is extracted from the black hole and accretion disk to feed the jet."

This also tells us something about the accretion disk. If the disk and black hole were rotating in opposite directions, the inner edge of the disk would be much further from the black hole itself than if they were rotating in the same direction. The size of the jets seen here is too small to arise from a system where the two bodies are rotating in opposite directions, so we can conclude that the disk is following the rotation of the black hole it orbits.

Even if the Event Horizon Telescope is improved, we're not likely to get a better picture of the black hole's environment, because the model built from the observation runs up against limits that arise from our uncertainties regarding the distance to M87 and the mass of the black hole within it. But the authors hope to be able to use the telescope to continue observations over longer periods of time, since the accretion disk probably contains an uneven distribution of matter, which could create periodic irregularities in the output.

Plus, eventually, they hope to turn the telescope on our own galaxy's black hole. It's not as active as M87's, but it still seems to be swallowing enough matter to make checking it out at high resolution worth our while.