Black Hole and its Accretion Disk

In SS433 two jets appear to be blasting away from some central compact object. A relatively normal star is in orbit about it, a conclusion founded on further observations, which showed periodic 13-day changes in the spectral line velocities from SS433. The source of the jets is therefore a member of a binary star system. But what causes the 164-day period? It cannot be blamed on a neutron star, because they spin at the rate of once per second. Could it be a black hole, an object so massive and so tightly compacted that matter has collapsed in on itself to the point that gravity prevents anything from escaping, even light A black hole would spin even faster (if its spin could be detected). On the other hand, a normal star rotating once every 164 days would not expel jets with such violence.

After several years of study the likely explanation for the SS433 system emerged: a normal star is in orbit about a black hole that contains the equivalent mass of about four suns. Due to the close proximity of the nearby star, the black hole lures material from the surface of the star and draws it toward the black hole. As the infalling material gains speed, it begins to accrete into a disk of matter that spins around the black hole en route to its final destination—nothingness. In the accretion disk the density grows larger and larger the closer to the black hole the matter comes. In so doing, the particles undergo increasingly violent collisions with each other. However, there is a wonderful twist to this story.

If too much material rushes into this accretion disk a condition known as supercritical accretion is reached and then things become very interesting indeed. An enormous increase in particle collisions suddenly heats the gas to the point where it contains so much energy that it explodes, driving material outward again, to escape the impending clutches of the black hole. But this material cannot blast through the surrounding material in the disk. It can only escape up the central hole of the doughnut-shaped accretion disk. So away we go; two jets blasting outward at a quarter of the speed of light, a very chaotic state of affairs.

The two jets tear into space, gathering up more material as they go. They also expand sideways at about 2,000 km/s and fan out slowly as they rush into the surrounding supernova remnant. The jets themselves appear to be something like long, miniature cylindrical supernova remnants! The hot material in the jet also emits X rays, as observed by X-ray astronomy satellites in 1976.

FIGURE 9.2. A beautiful radio image of the galactic microquasar SS433, which is located inside the supernova remnant W50 seen in Figure 9.1, showing the corkscrew motion of the material ejected from the vicinity of the black hole at its center. Investigators: K. Blundell and M. Bowler (Oxford). (Image courtesy of Katherine Blundell.) Reproduced by permission of the AAS.

FIGURE 9.2. A beautiful radio image of the galactic microquasar SS433, which is located inside the supernova remnant W50 seen in Figure 9.1, showing the corkscrew motion of the material ejected from the vicinity of the black hole at its center. Investigators: K. Blundell and M. Bowler (Oxford). (Image courtesy of Katherine Blundell.) Reproduced by permission of the AAS.

According to this picture, SS433 should show two nice straight jets pointed away from the central source, in which the velocity of material streaming outward would remain constant with time. But they don't look like that. The radio observations (Figure 9.2) show that the jets are shaped like corkscrews whose twisting motion can be followed from day to day. The reason for this cosmic corkscrew is related to the 164-day period in the jet velocities.

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