Saturday, July 08, 2006

The wind from a black hole

Black holes are not black, of course. We can "see" many of them quite well -- the only controversy has been over whether what we "see" is the result of a black hole, as predicted by general relativity when a sufficiently large amount of matter is confined to a sufficiently small region of space (as defined by the so-called Schwarzschild radius).

Quasars are the most spectacular examples of electromagnetic radiation (including light) produced by black holes. In this case, the black holes can be as massive as entire galaxies (tens or hundreds of billions of stars as large as the sun). Some quasars are bright enough to be visible almost to the greatest distances our instruments can detect, more than 10 billion light-years away. Astrophysicists have estimated that as much as 25% of all the light in the universe is generated around supermassive black holes in the centers of most large galaxies. But even much smaller black holes, produced in supernova explosions of stars much larger than our sun, can be observed through very energetic (x-ray) electromagnetic radiation.

The paradox is that a defining characteristic of a black hole is the ability of its intense gravitational field to confine even electromagnetic photons within the Schwarzschild radius of the hole. So how does any light escape?

The answer, it has long been clear, is that the light (and other radiation) doesn't come from the black hole itself, but instead from the superheated matter in orbit around the outside of a black hole -- a configuration called an "accretion disk". Accretion disks of matter form around even ordinary stars, and eventually, in many if not most cases, result in the creation of planetary systems like our own.

Only in the presence of a very intense gravitational field is the matter of the accretion disk heated sufficiently to produce the radiation emitted by quasars or black holes that are supernova remnants. Neutron stars, the remnants of supernovae resulting from the death of stars not quite large enough to yield black holes, also have accretion disks capable of bright radiation -- which we see as "pulsars".

It should be noted that the radiation we are concerned with here is not the same as the so-called Hawking radiation, first predicted by Stephen Hawking in 1974. This radiation, if it exists at all, comes directly from a black hole itself, but must be vastly less intense, and has never been directly observed.

However, there is still a puzzle here. Like planets orbiting a star in an ordinary solar system, the matter of an accretion disk does not fall straight down into the object about which it orbits. (And even if it did, there is no mechanism which would cause the emission of electromagnetic radiation when the matter disappears within the Schwarzschild radius.) The reason that matter in orbit doesn't fall into the center is simply that, because it is in orbit, it has a quantity of angular momentum, and just as with linear momentum, the angular momentum of a system must be strictly conserved. Angular momentum lost by matter falling into the center of a system must somehow be transferred to other matter within the system, and the net result is that some of this other matter must flow outward from the system -- yielding a sort of wind away from the center.

The matter of an accretion disk which falls inward is eventually highly compressed near the Schwarzschild radius and therefore heated -- like any gas being compressed. Very near the black hole it becomes hot enough to generate very energetic electromagnetic radiation. But the angular momentum of this infalling matter must be balanced by an outwardly-directed wind that gains angular momentum. Indeed, it is actually the increased angular momentum of the matter forced outward which causes other matter to lose angular momentum and therefore to fall inward. This is very like the process (but in reverse) through which the Earth's rotational angular momentum has been lost due to tidal friction, only to be transferred to the Moon, which consequently orbits at a slowly but steadily increasing distance.

The puzzle, then, is what causes the outward wind. Astrophysicists have been able to think of only three plausible mechanisms. Two of these involve heating of matter in the accretion disk at a considerable distance from the center -- either by intense friction of turbulent motion in the matter or by the radiation coming from the gravitationally compressed matter near the Schwarzschild radius. The third mechanism involves forces generated by intense magnetic fields.

Recent research findings appear to rule out the first two possibilities, leaving, by a process of elimination, magnetic fields as the actual cause of the wind:

Black Hole Paradox Solved By NASA's Chandra
Gravity alone is not enough to cause gas in a disk around a black hole to lose energy and fall onto the black hole at the rates required by observations. The gas must lose some of its orbital angular momentum, either through friction or a wind, before it can spiral inward. Without such effects, matter could remain in orbit around a black hole for a very long time.

Scientists have long thought that magnetic turbulence could generate friction in a gaseous disk and drive a wind from the disk that carries angular momentum outward allowing the gas to fall inward.

Using Chandra, Miller and his team provided crucial evidence for the role of magnetic forces in the black hole accretion process. The X-ray spectrum, the number of X-rays at different energies, showed that the speed and density of the wind from J1655's disk corresponded to computer simulation predictions for magnetically-driven winds. The spectral fingerprint also ruled out the two other major competing theories to winds driven by magnetic fields.

The key observation that led to the conclusion was detection of a distinct absorption spectrum created when the intense x-rays passed through the material of the accretion disk. The doppler shift of this spectrum revealed an outwardly-flowing wind moving about 500 kilometers per second. This proved to be consistent with detailed computer simulations of the interaction between magnetic fields and the matter of the accretion disk.

At the same time, there was evidence to rule out explanations not involving magnetic fields. If thermal heating without magnetic fields were the cause, the temperatures in the disk would need to be about 5×1010 °K, but in fact they are more like only 106 °K. But at the same time, the matter of the disk is highly ionized, so that it is mostly transparent to electromagnetic radiation, which therefore cannot create the observed wind velocity.


Additional References:

GRO J1655-40: NASA's Chandra Answers Black Hole Paradox - images, animations, and other links

On the Hunt for Magnetic Field Winds with Jon Miller - interview with the principal investigator

Chandra solves black hole mystery - PhysicsWeb article

Magnetic fields snare black holes' food - NewScientist article

A Good Belch Helps Black Holes Feast - Sciencenow article (subscription rqd)


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Anonymous Organic Chemistry said...

Great post...thanks alot.

1/25/2007 06:00:00 AM  

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