What was clear was that the two partners of the Cygnus X-1 system must have completed an elaborate dance long before I arrived. I recalled the numerous articles that had speculated on this distressing topic. It was very likely, indeed, that the two members of the Cygnus X-1 system had been born together. They were fraternal, not identical, twins, for it was the remnant of the originally heavier twin that was now dining on its sibling. What was now the black hole had once been an ordinary star—30 or 40 times more massive than the Sun, hotter and a million times more luminous, but otherwise similar. Before its collapse to a black hole, it had spiraled so close to its companion—at the same time swelling in the normal decrepitude of stellar old age—that the two stars had mingled, with the dangerous result that most of the matter in the proto-black-hole had drained into its partner. Now that the matter was draining the other way, I wondered: Was this the payback for earlier sibling treachery?
It is usually assumed that the more massive star of a coeval duo will meet its maker first, because more massive stars burn up more quickly. Therefore, you will be forgiven if you think it paradoxical that the proto-black-hole died and collapsed after losing most of its mass to its companion. This common confusion results from a failure of many commentators to distinguish adequately between the initial mass of a star, which determines how fast its interior ages, and the composition and structure of the star’s core, which contains only a fraction of its mass. It is the latter that determines the star’s immediate fate, and in the case of Cygnus X-1, the proto-black-hole’s interior had aged too far to be rejuvenated merely by losing weight. And a good thing, too, for if the proto-black-hole had collapsed (very possibly with an explosive release of energy in its outer layers) before losing most of its matter to its neighbor, then we would not have had a binary system left to study today. The recoil would have sent the black hole careening off into space, bereft of its brother and left to starve alone.
I synchronized my approach with the rotation of the system as it swung around its center of gravity. It was obvious that the star that had not become the black hole bad aged considerably since its weight gain under dramatic circumstances. This was to be expected, because it was now quite a massive star. It was beginning to bloat, as more and more of its nuclear fuel was expended in producing its now-prodigious luminosity. It also looked far from normal, even for its age and mass. Because the black hole tugged more strongly on the hemisphere facing it, that entire side of the star was drawn into a vast, ungainly bulge, tapering almost to a point. I recalled the tides I had seen risen on the stars approaching collision in the Milky Way’s center and the more dramatic distention that preceded the destruction of the star that had ventured too close to the black hole in my daydream. But unlike those doomed bodies, this pointy bulge was not growing with time. It seemed to have reached some kind of stasis, with matter from the star flowing into the distension and then spilling through the point. This black hole, it appeared, was not tearing its companion star apart but instead was subjecting it to a more exquisite, drawn-out form of torture.
The stream of gas being sucked off the star was not going quietly. It was full of turbulent curlicues, swirls, and sprays, with an occasional sudden discharge of gas that reminded me of a colicky water faucet, I tried to focus on individual eddies—swirling, dark spots in the turbulent gaseous fluid—and to follow them as they passed through the constriction and toward the black hole. This was not easy; the contrasts were subtle and the flow complex and unsteady. But I was able to discern some regularity in the flow and saw that the matter did not fall straight toward the black hole once it had passed the constriction. On the side of the constriction toward the black hole, the flow was strongly deflected from making a beeline for the hole. I supposed that it retained the memory of its original orbital motion, which carried it so far forward that it overshot its mark. Despite the exotic setting, I recognized this as the same phenomenon that kept satellites from falling to Earth, and the name of this orbital memory, “angular momentum,” made it all seem commonplace. In a moment of panic, I feared that none of the matter spilling through the constriction would actually reach the black hole. Was this black hole, too, going to starve, despite being surrounded by a cornucopia of gaseous food? Then I remembered all those X-rays—obviously, something was feeding this black hole—and I pressed on.
6
Black Hole with a Mission
I followed the deflected stream as it gradually spiraled toward the hole. Now it was quite narrow and hence easier to follow, but I began to notice an increasing amount of swirling matter that was not part of the stream. The motion of this swirling gas was directed nearly at right angles to the straight path into the hole, and it did not diffuse through the entire space, as the tenuous corona surrounding the Galactic Center black hole had done. Rather, it seemed to be confined to a platter, or disk, hugging the plane in which the binary twins gyrated. The stream had to plow through this material, and eventually the resistance it encountered took its toll. The stream began to shred around the margins, to wobble in its path, and to spread from its well-focused trajectory. But it faced a still more formidable obstacle. About two-thirds of the way from the constriction to the black hole, the spiraling stream finished one complete circuit and crashed into itself. I quickly pulled away from the disk as I saw the great splatter of superheated gas looming ahead. Gas was thrown up out of the disk, and ripples spread out sideways as the swirling platter tried to come to terms with this unexpected disturbance.
This was the end of the stream—but the source of the disk. I watched the debris of the collision rock back and forth as it settled down, and I saw it spreading into a ring, which then broadened both toward and away from the hole. It thinned as it spread, until it merged into the very disk-like structure that I had encountered on the way in. The pattern behind its motion was now clear: The gas was distributing itself into nice, regular orbits about the black hole, just like a planet orbiting the Sun or a satellite orbiting the Earth. I thought of the Milky Way’s disk, and if I imagined each star as an atom of gas, I could perceive a similar pattern. Just as the stars marched nearly in lockstep about the Galaxy’s center of mass, so would the atoms of gas in the disk trace broad, stable circles about the black hole, oblivious to the danger they would face if they ventured too close. Because the disk was so much less massive than the hole, there was not even the risk that something akin to spiral arms or tumbling stellar bars would mar the regimental parade, as I had seen them do in the Galaxy. I knew that the gas could stay in that state of motion virtually forever. But clearly that could not be what was really happening, if gas was draining into the hole at the prodigious rate indicated by the X-rays. In order for it to spiral in, something had to be dragging on the orbiting gas, drawing angular momentum from it as it circled the hole. To find out what this viscous agent was, I had to enter the disk itself.
Did I dare to immerse Rocinante in the disk? It looked formidable, completely opaque, and not at all calm. Arches of luminescent gas kept erupting from beneath a roiling gaseous sea. Plugs of vapor would shoot out of the disk to various heights, spread out in striations along an arch, and then drain back toward the surface. Sometimes the arches would twist, merge, separate, or suddenly open up to the sky, lobbing their contents into space. The flitting network of arches looked like a pattern of iron filings created by a convention of drunken magnets, and even more like the prominences a colleague once showed me—through a specially designed telescope—erupting from the surface of the Sun. The latter flares were in fact magnetic in origin, as my colleague had found by measuring the polarized light (a sign of the helical gyration of electrons along the magnetic field).
The flares were obviously pumping out a lot of energy, because it was getting hot even before I reached the surface of the disk. The disk, it seemed, had an atmosphere of hot gas, presumably evaporated from the surface by the flares. I held my breath and descended through the surface. There was less resistance than I expected—the “s
urface” was more an impediment to vision than a physical boundary, although I could feel increasing discomfort as Rocinante was buffeted by the highly turbulent gas in the denser layers of the disk. The magnetic field was in evidence everywhere and stronger now than ever; lines of force and the striations they cause seemed to dominate the organization of the gas. Here they formed no arches, however, and they did not seem to be seeking the surface of the disk and freedom. They were tightly confined, combed out by the swirling orbital motion of the disk, much like marsh grass bent over flat in a raging flood.
The symbiosis between the magnetic field and the orbital motion—that was the key to how matter was able to spiral toward the black hole. I struggled to remember the theory that described the complex behavior of hot matter in the presence of magnetism. This subject had been my nemesis as a student. Magnetism, it turned out, gave hot matter an added elasticity and an odd kind of tension. It imbued ordinary gas with certain qualities of both rubber bands and the mainsprings of old-fashioned watches. Conversely, hot matter was able to grab onto magnetic lines of force and to stretch, twist, and bend them as though they were candy stripes frozen into toffee. This stretching was what combed out the field lines here, and the symbiotic back-reaction of the magnetic force was what was dragging on the gas, allowing it to spiral in.
Confident now that matter would flow all the way in, I surfed along the disk toward the black hole. I dipped in and out of the obscured disk interior, enjoying increased confidence in my mastery of Rocinante’s controls. The speed of the swirling gas increased, as predicted by the theory of gravity, and so did the temperature, the luminous glow inside the disk, and the violence of the stretching and churning magnetic fields both inside and out. The disk’s radiance grew so strong that it seemed to be bucking the dips in my inward motion, pushing me away from the disk’s central plane. This was probably an illusion, but it reminded me that radiation itself, when intense enough, can actually levitate matter against the pull of gravity.
I fully intended to reach the place in the center of the disk where matter departed from its gradual spiral path and made its final plunge into the hole—the “orbit of instability” that I had so carefully avoided in the center of the Milky Way. That place, I calculated, should be about 100 kilometers from the horizon of the black hole, which turned out to have a mass equal to that of 15 Suns. I was now so adept at navigating Rocinante that I would have felt comfortable going in nearly that close. Noting that the stream flowing in from the companion had merged into the disk at a distance of about 1 million kilometers from the hole, I relaxed and watched the kilometers tick off as I sped inward. Then, suddenly, I became violently ill. I had never been so sick in my life. I checked my distance from the hope: 10,000 kilometers. What could be happening? It felt as though my torso and upper body were being pulled away from my lower extremities. I was riding with my head toward the black hole and was able to achieve a minor increase in comfort by turning my body sideways. But the respite proved temporary. My craft was on autopilot, heading straight for the hole, and I soon felt as though my head were being pulled apart, front from back. And as for my gut. . . well, I will spare you the details.
I’m sure that I was about to pass out, and it was sheer luck that I managed—with. a great effort of will—to locate and activate the reverse lever. I pulled back out to 100,000 kilometers, up and away from the disk, and collapsed in a cold sweat. The disk formed an immense floor in my field of view, its center a barely discernible spot in the distance. As I regained my composure, I immediately realized what had happened to me. At first, the black hole’s gravity had not posed a problem, because I had been allowing Rocinante to fall almost freely toward the hole. Like any astronaut orbiting Earth, I had felt virtually weightless. But my head, being slightly closer to the black hole than my feet, had actually been subjected to a slightly stronger gravitational pull, and that difference is what had gotten me. When I came within 10,000 kilometers of the hole, the difference in gravitational pull had approached and then exceeded the normal gravity of Earth, under which a human body is designed to operate. I was indeed being pulled apart. It was as though I had somehow discovered a way to stand upright and upside down simultaneously! The blood (and every other fluid in my body) began to rush away from my middle and to pool in both extremities. A force comparable to my weight on Earth was stretching every tissue in my body. You can imagine the discomfort. When I turned sideways, the effect was diminished: Because I am thinner than I am long, the difference in the gravitational pull was correspondingly reduced. But as I approached still closer to the black hole, even the difference across my torso became intolerable. In a sense, I had suffered a milder version of the fate that befell Cygnus X-1’s companion, whose midsection had been extruded into a pointy bulge from which the accretion stream was pulled toward the black hole. I thanked the deity of mass transfer in binary stars that my encounter with extreme tidal forces had not proceeded to the gruesome outcome that was keeping Cygnus X-1 fed.
Apparently, I was not to close in on the black hole after all. I was up against physical laws that I could neither change nor circumvent. Fortunately I am a stoic at heart, and I knew that remote observation via telescope, though not the ideal way to collect data, sometimes has to do. One simply accepts that limitation as an astronomer. My ego needed to be reminded that, after all, I was closer to a black hole than anyone else had ever been. I could live with the disappointment of not getting quite as close as 100 kilometers.
Having come to terms with this unforeseen restriction on my movement, I surveyed the disk below and ahead of me. From this height the magnetic flares seemed less significant, little more than minor viscous eruptions like the bubbles and cheese-spouts that erupt from the surface of a simmering fondue pot or, given the disk’s thinness, a bubbling pizza (my harrowing encounter with the black hole’s gravity had unaccountably given me an appetite). I could see the underlying large-scale structure of the disk, stretching away toward the black hole. The disk was luminous everywhere, and the more so the closer one got to the hole. With increased radiance came the expected increase in temperature, manifested in the progression of “colors.” I have to put this word in quotation marks, because the progression far surpassed the sequence from red to yellow to blue-white that a human eye perceives when a metal bar is heated to thousands of degrees. Here the colors went off the human scale at violet proceeding through the ultraviolet, far ultraviolet, extreme ultraviolet, and on into the X-rays. Apart from these gradations of temperature and radiance, the disk was overwhelming in its flatness and featurelessness.
To get my bearings, I focused on the hottest and most luminous region of all, the area surrounding the black hole. I could see the blackness in the center and the disk continuing beyond. There the flatness appeared to be broken by a curious asymmetry. Just the other side of the hole, the disk seemed to warp upward, forming a distorted band that folded over on top of the gap that signified the location of the black hole. I moved around toward the far side to get a better look, but the distorted region of the disk seemed to compensate for my motions, always remaining exactly on the opposite side of the hole. It dawned on me that I was witnessing an illusion caused by the wrapping of light rays around the black hole as they crossed the intense gravitational field on their way to my instruments. I should have expected this. I knew that radiation, as well as matter, responds to gravity, though it was shocking to see the effect so forcefully displayed. There was another asymmetry as well, but the origin of this one was easier to visualize. I noticed that the disk was a little bit brighter, the X-ray luminescence just a little bit harsher, to the right of the hole than to the left. This, I immediately recognized, must be due to the “headlight effect,” the forward beaming of the radiation emitted by moving matter. In this case, the light would be beamed toward the direction of the disk’s spinning motion, and the brighter side had to be the side that was coming toward me. According to the theory of relativity, an asymmetry this pronounced coul
d mean only one thing: The matter that I was viewing swirled around the black hole at nearly the speed of light. These were all well-known theoretical predictions, but by this point I was so awed by the spectacle that I was disinclined to trust the intuition I had developed through years of painstaking study. As if to pinch myself mentally, I double-checked that the disk was spinning in a clockwise direction, as it would have to be if light from the right-hand side were really boosted by its oncoming motion. It was.
I tried to follow the gas as it disappeared through the black hole’s horizon. The horizon was a sphere with a radius of 45 kilometers. Was I right that the matter in the disk would break its slow inward spiral and plummet into the black hole from 100 kilometers farther out? I saw what looked like evidence of a dramatic change in the disk’s state of motion near the predicted place, but I couldn’t be sure; the hot atmosphere above the innermost parts of the disk became so thick that it all but obscured my view. At the other end of the death-plunge I could barely make out the gas fading just outside the horizon. The textbooks appeared to be right. The X-ray radiation regressed through the colors as it fought its way out through an increasingly debilitating gravitational field. As the signal faded, the color passed back through the varieties of ultraviolet, blue, yellow, and red, and then it was too faint to tell. What happened in the intervening space between the horizon and the orbit of instability? The artists’ impressions in my musty old textbooks had represented the disk as having a distinct inner edge, but this seemed unlikely. This disk was so rich with matter, right to the center, that it could not have thinned out enough to become transparent. I sighed with relief at this confirmation of my “theorist’s instincts.” Still, I felt one last pang of regret that I would be forever denied a closer view.
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