The energy comes from the black hole’s gravity. The agents for extracting it are magnetic friction and the disk’s gas.
The quasar’s bright light, seen by astronomers, comes from the disk’s heated gas, Lynden-Bell concluded. Moreover, the magnetic field accelerates some of the gas’s electrons to high energies; and the electrons then spiral around the magnetic force lines, emitting the quasar’s observed radio waves.
Lynden-Bell worked out the details of all this using a combination of the Newtonian, relativistic, and quantum laws of physics. He easily explained everything about quasars that astronomers had seen, except their jets. His technical article describing his reasoning and his calculations (Lynden-Bell 1979) is one of the great astrophysics articles of all time.
The Jets: Extracting Power from Whirling Space
Over the next few years, astronomers discovered many more jets sticking out of quasars and studied them in great detail. It soon became clear that they are streams of hot, magnetized gas ejected from the quasar itself: from the black hole and its accretion disk (Figure 9.2). And the ejection is extremely powerfuclass="underline" the gas travels out the jets at nearly the speed of light. As it travels, and when it plows into material far from the quasar, the gas emits power in light, in radio waves, in X-rays, and even in gamma rays. The jets are sometimes as bright as the quasar itself, a hundred times brighter than the brightest galaxies.
Astrophysicists struggled for nearly a decade to explain how the jets are powered and what makes them so fast, so narrow, and so straight. The answers came in several variants, with the most interesting in 1977 from Roger Blandford at the University of Cambridge, England, and his student Roman Znajek, building on foundations laid by the Oxford physicist Roger Penrose; see Figure 9.3.
The accretion disk’s gas gradually spirals into the black hole. When crossing the hole’s event horizon, each bit of gas deposits its bit of magnetic field onto the horizon, and then the surrounding disk holds it there, Blandford and Znajek concluded. As the black hole spins, it drags space into whirling motion (Figures 5.4 and 5.5), and the whirling space makes the magnetic field whirl (Figure 9.3). The whirling magnetic field generates an intense electric field like in a dynamo at a hydroelectric power station. The electric field and the whirling magnetic field together fling plasma (hot, ionized gas) upward and downward at near light speed, creating and powering two jets. The jets’ directions are held steady (when averaged over years) by the black hole’s spin, which is steady due to gyroscopic action.
In 3C273 only one jet was bright enough to see (Figure 9.1), but in many other quasars both are seen.
Blandford and Znajek worked out the full details, relying heavily on Einstein’s relativistic laws. They were able to explain most everything about the jets that astronomers see.
In a second variant of the explanation (Figure 9.4), the whirling magnetic field is anchored in the accretion disk instead of the hole, and is dragged around by the disk’s orbital motion. Otherwise, the story is the same: dynamo action; plasma flung out. This variant works well even if the black hole isn’t spinning. But we’re pretty sure that most black holes spin fast, so I suspect the Blandford-Znajek mechanism (Figure 9.3) is the most common one in quasars. However, I may be prejudiced. I spent much time in the 1980s exploring aspects of the Blandford-Znajek ideas and even coauthored a technical book about them.
Whence Comes the Disk? Tidal Forces Tear Stars Apart
Lynden-Bell, in 1969, speculated that quasars live at the centers of galaxies. We don’t see a quasar’s host galaxy, he said, because its light is so much fainter than the quasar’s light. The quasar drowns the galaxy out. In the decades since then, with improving technology, astronomers have found the galaxy’s light around many quasars, confirming Lynden-Bell’s speculation.
In those recent decades we also learned where most of the disk’s gas comes from. Occasionally a star strays so close to the quasar’s black hole that the hole’s tidal gravity (Chapter 4) tears the star apart. Much of the shredded star’s gas is captured by the black hole and forms an accretion disk, but some of the gas escapes.
In recent years, thanks to improving computer technology, astrophysicists simulated this. Figure 9.5 is from a recent simulation by James Guillochon, Enrico Ramirez-Ruiz, and Daniel Kasen (University of California at Santa Cruz) and Stephan Rosswog (University of Bremen).[22] At time zero (not shown) the star was headed almost precisely toward the black hole and the hole’s tidal gravity was beginning to stretch the star toward the hole and squeeze it from the sides, as in Figure 6.1. Twelve hours later the star is strongly deformed and at the location shown in Figure 9.5. Over the next few hours, it swings around the hole along the blue gravitational-slingshot orbit and deforms further as shown. By twenty-four hours the star is flying apart; its own gravity can no longer hold it together.
The star’s subsequent fate is shown in Figure 9.6, from a different simulation by James Guillochon together with Suvi Gezari (Johns Hopkins University). For a movie of this simulation, see http://hubblesite.org/newscenter/archive/releases/2012/18/video/a/.
The top two images are shortly before the beginning and shortly after the end of Figure 9.5; I enlarged these two images tenfold compared to the others, to make the hole and the disrupting star visible.
As the whole set of images shows, over the subsequent several years much of the star’s matter is captured into orbit around the black hole, where it begins to form an accretion disk. And the remaining matter escapes from the hole’s gravitational pull along a streaming, jetlike trajectory.
Gargantua’s Accretion Disk and Missing Jet
22
I changed the size of the hole to that of Gargantua and the size of the star to that of a red giant, and changed the time markers in Figure 9.5 accordingly.