Precession of the Accretion Disk

To explain the twisted jets, the accretion disk appears to be wobbling about the black hole, But why? Because the binary star companion of SS433 is feeding the voracious black hole an excessive diet of gas, mostly hydrogen, and at the same time pullmg on the accretion disk. That should be enough to cause slow precession, but there is more to it. The nearby star is not round! Due to the proximity of the black hole it is distorted, and therefore the gravitational influence from the rotating, distorted star is very nonuniform. Note that the accretion disk surrounding this four-solar-mass black hole is only solar-system-sized and the black hole a few kilometers in diameter. (All these numbers and details of the picture described here come from a tremendous amount of research by dozens of astronomers studying the spectral line shifts and a comparison of all the observations made at optical, radio, and X-ray wavelengths, which are then compared with theoretical calculations.)

The result of the tug-of-war between the ugly star and the accretion disk is precession, which is like the wobble of a top set spinning on a table. However, the entire disk does not really move as a solid object, because the gas is passing through the disk so rapidly that today's accretion disk is almost a new one compared to yesterday's.

Now the particles in two jets are ejected straight out into space, but because the orientation of the disk changes with time the direction of ejection also changes. Even as individual particles head straight outward, they create the corkscrew-like pattern of radio emission seen in Figure 9.2. Their trajectories may be likened to water streaming out of a rotating garden sprinkler. Each water drop heads straight out, but as the sprinkler head spins it creates an apparent spiral of ejected liquid.

Seen from earth, the velocity of material in the SS433 jets cycles through a range of values determined by the geometry of the twisted jets w ith respect to our point of view. Sometimes a jet would point more directly toward us, and days later it would be tilted away from us. This happens with the precession period as the accretion disk, once in 164 days.

To summarize, the strange spectral lines from SS433 are produced by two jets of incandescent gas driven out of an accretion disk surrounding a black hole, which is in orbit about a star that supplies the fuel! The jets are driven explosively outward by the energy created in supercritical accretion, which occurs when too much gas is made available for the black hole to swallow in one gulp. The jets, in turn, are propelled to the outskirts of the surrounding supernova remnant, which they keep fed with energy that makes the remnant shine.

This type of object is now known as a microquasar. The radio observations have led to a determination of the distance to SS433. Since the velocity of material along the jets is known, and the movement across the sky can be seen in the radio maps, the distance to the object can been determined—18,000 light-years, the distance to W50. The light from this remarkable object has been traveling since Homo sapiens dwelt in caves in the last ice age, when humans were utterly oblivious of the remarkable cosmic wonders that exist beyond the stars overhead. Such is progress!

The study of the microquasar that looked like an apparently innocuous little star called SS433 gave astronomers the first clear insight into the physical processes occurring near black holes. SS433 is still being thoroughly studied and the picture to account for its behavior is about as complete as any in astronomy, which is all too often spiced with mysteries which cannot be solved with present day observations and always seem to require bigger telescopes. SS433 is also a wonderful manifestation of the phenomenon occurring in radio galaxies and quasars (the next chapter), but here it is on a tiny scale, very close to home. Its discovery has reinforced the notion that jets and precessing accretion disks are enormously widespread in the universe.

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