It could be that I was fortunate to have been forced to take a long view of the disk, for I began to perceive certain causal connections that had eluded me before. The black hole’s gravity caused motion, first the open spiral of the narrow accretion stream, pulled from the companion star into the black hole’s sphere of influence. I looked back, away from the black hole, and saw the distant panorama of the stream swinging one full turn around the system and crashing into itself, that big splash now dwarfed by the immensity of the disk. I saw the incipient disk spread away from the crashing stream, both inward and outward. Most of the gas spiraled inward and, as it did so, drifted ever deeper into the gravitational pull of the black hole, where it swirled faster and faster while its increasingly ferocious magnetic fields (stretched and amplified by the motion) tugged at it chaotically until it heated up. Thus motion begat heat, and heat begat luminosity. Four trillion tons of its companion’s substance was disappearing into the black hole of Cygnus X-1 every second, but 100,000 times the luminosity of the Sun was coming out. It seemed a fair trade. This black hole was an efficient engine for turning matter into energy, an intense flickering source of X-rays that finally reached Earth.
I pulled out my measurements of the Milky Way’s central black hole, expecting to find that it was starved of nearly all matter. That would surely explain why its environs emitted just the barest of glows rather than blazing away like Cygnus X-1. But the Milky Way’s black hole seemed not to be playing by the same rules. Despite its sparse surroundings, that huge black hole was actually swallowing matter more rapidly than Cygnus X-1 was gobbling its companion. Somehow the chain—matter plus gravity goes to motion to heat to radiation—had been broken, the trade of matter for luminosity not consummated. The bigger black hole was greedy, swallowing most of the heat along with the matter, before the heat could turn into luminosity and radiate into space. I pondered an old theoretical idea in the hope that it could explain such a difference. The Milky Way’s central black hole was 100,000 times heavier than Cygnus X-1, Its horizon was millions of kilometers across. I focused on this idea of relative size and what it might signify. The amount of matter flowing toward the black hole in the Galactic Center was indeed modestly larger, but the dimensions over which this matter was forced to spread were vastly larger. Could this be the clue I was looking for? Spread out so thinly, perhaps the matter never reached the concentrations at which heat could be generated and released efficiently.
It sounded like a plausible theory, and it gave me that temporary glow of self-satisfaction that theorists sometimes get just before delving further into an idea and realizing that it is much more complicated than it had seemed at first. True or not, I couldn’t think of any way to test this hypothesis without observing many more systems than just these two. I was more confident in my ability to explain what I had seen while snooping around the environs of Cygnus X-1. At least that seemed to make sense. Matter is drawn inward, and luminosity flows out in proportion to the matter flowing in. I decided to concentrate only on black holes with donor stars and orderly disks. Then I noticed something that threatened to unravel even this cozy fragment of my story.
7
SS 433
The problem was that not all of the matter approaching Cygnus X-1 made it into the black hole. I should have seen this coming, although in my defense, I must point out that I was looking for evidence of inflow, not outflow. I had already noted that a few wisps of gas managed to escape, expelled by the dynamic action of magnetic flares. A little evaporation here and there was no cause for alarm, merely another example of the normal wastage that always seems to accompany physical processes in the Universe. But these filaments were not isolated escapees. Together they wove a disturbing pattern in front of me as I peered toward the distant black hole. Only as I retreated, and after I had spent some time examining images from my visit, did I see how much of the gas in the disk departed from the inward drift of the great viscous spiral. There it was, a streamer of gas shooting away from the disk, and there another—several streamers at once, twisting together, then merging and finally forming a jet of matter screaming off at high speed, perpendicular to the disk. Could it be that black holes expelled matter as well as drawing it in?
Of course, I knew of a system that was an extreme example of this phenomenon. SS 433 was its surprisingly unromantic name, given how ingrained it had become in my generation’s store of iconic imagery. I hadn’t thought about it recently, but suddenly it seemed to pose a towering challenge to my developing worldview. The intensively gravitating body of SS 433 seemed to be expelling far more matter than it absorbed.
Everyone in my generation of scientists remembered the hoopla that surrounded the discovery of this object and its peculiar properties. It was one of the signal events of my early astronomical education. The fact that it ultimately proved to be representative of a very rare class of beasts may have knocked it out of the pantheon of astrophysical archetypes (few would rank it with pulsars or even with those mysterious bursts of gamma-ray radiation that were being so hotly debated at the time I left), but SS 433’s discovery had the element of surprise that reminds one that one’s view of the Universe may have to be revised on short notice.
I will never forget the first announcement, because I missed it. This was at one of those conference series that are named not for the place at which they are actually being held but for the place at which they were first held. I was attending a meeting associated with one such venerable institution—the famous Texas Symposium on Relativistic Astrophysics—in Munich in 1978. It was late in the week, and my attention was flagging. The lectures were dull, and little new was being reported, so I took the afternoon off to visit one of the art galleries for which the city was famous.
This was a big mistake. By the time I had slipped back to the enormous hotel ballroom in which the lectures were being held (in order to be seen dutifully nodding off during the concluding session), the buzz was just dying down, and all the astrophysicists were dashing off to the airport to report the new discovery to their groups. (Quaint world when there was no Internet!) Everyone was in such a hurry that I couldn’t get a straight story of what had happened, but I caught murmurs about “Doppler” and “anomalous redshift.” I slunk back to my home department, and there I encountered, for the first time as an insider in the profession, the kind of hyped-up astrophysical press coverage with which we all subsequently became familiar. Reports of the discovery were featured under multicolumn headlines in all the papers and newsmagazines, “Mystery Star Both Coming and Going” blazed a typical headline. As though eager to avert panic, venerable TV newsreaders relayed the opinion of astronomers that “this is some kind of star that’s in some terribly weird kind of trouble.” Like everyone else, we cobbled together the rumors and fragmentary observational reports that had come in since the first announcement and began work on our own theories, in order not to miss out on a possible “scoop.”
Rocinante signaled an abrupt heads-up from my nostalgic reverie. I was already heading into the environs of SS 433. There was no doubt that matter was being powerfully expelled from this system. Tens of light-years away from the destination itself, I encountered the vanguard of its effects on the surrounding regions. Like the bubbles around luminous stars that I had seen at various locations en route to the Galactic Center, SS 433 had inflated a hot cavity around itself, but with a difference. This cavity, instead of being spherical, sported a pair of highly elongated protrusions. The jets were pumping their energies and momenta in two diametrically opposed directions, boring into the interstellar matter like the high-pressure water jets sometimes used to excavate mine shafts. Like those pulsing water streams, the jets plashed vigorously against the working surface and, having spent only a fraction of their force, ricocheted into the main cavity, broadening it as an afterthought. I felt a minor jolt as I crossed the sharp boundary from undisturbed interstellar space into the pressurized cavity. As it happened, my route took me across the path of one o
f the jets, which was by this point much more diffuse and spread out than when it had left its source; nevertheless, there was no mistaking its impulse. With difficulty, I made out the pale, multicolor luminescence it left in its wake (here X-ray bright, there a mixed pinkish and ultraviolet glow from disturbed hydrogen).
The scene was grand, but there were no surprises here. I recalled the pictures I had seen of W50, the SS 433 cavity with “ears,” long before my visit. Identification, of the ears with the impacts of the jets had been one of the comforting verifications that had tied the whole picture so neatly together. But that had come well after the first flush of discovery. I thought back to the observation that had triggered the initial excitement, inspiring my colleagues’ hushed words about “abnormal Doppler shifts” and the like. I will not insult the quality of your liberal arts education with yet another disquisition on how the Doppler effect works. My style manual indicates emphatically that a discussion of the Doppler effect, or at least a description of its manifestations and uses, should appear in the first chapter of any astronomical memoir. I am perhaps treading dangerously close to new stylistic territory by not having informed you before Chapter 7 that I intend to dispense with this formality. If the mere squeal of an approaching train whistle or groan of a receding police siren does not elicit the expected Pavlovian response (thoughts of distorted ripples on a lake; mental images of line drawings in physics textbooks, with Pepto-Bismol-pink backgrounds), you may wish to consult Chapter 1 of any number of available works.
What I will describe is the dramatic (and unexpected) role that the Doppler effect played in the discovery of SS 433’s true nature. The designation itself connotes nothing extraordinary. The intention of the cataloguers (Messrs. Stephenson and Sanduleak) had been to record stars that were unusual only in their production of intense spectral “lines,” blips of extra emission at certain very precise colors. These lines are the products of electrons dropping between equally precise orbits in their respective atoms. For these lines to appear with intensity requires special but not exceptional conditions, and the available colors themselves are the well-documented properties of the chemical elements in various states of excitation and disarray. These intrinsic colors are not to be fiddled with.
Number 433 in the Stephenson and Sanduleak catalogue had a surprise in store. When measured accurately, the lines had the wrong colors. More frighteningly, the colors changed with time. Enter the Doppler effect, according to which these well-recorded lines are expected to have their original hues only if the luminous matter is at rest. If it is moving in the direction of the observer, the color will “squeal” toward bluer hues; if away, it will “groan” and turn redder. SS 433 showed three sets of lines—one groaning, one squealing, and one more or less in repose—and therefore seemed to exhibit three states of motion at once. As the headlines said (do they ever lie?), SS 433 was indeed both coming and going, not to mention standing still, and clearly had to be in some terrible sort of trouble.
Only it wasn’t really in so much trouble. Like the ending of a classical comedy, the resolution of the paradox showed that all was right with the world. The change of hues with time, which (in wilder moments of speculation) had seemed to threaten the foundations of physics, proved to be their salvation. After the lines had been monitored for a few months, a simple pattern emerged. The approach and recession of the “moving lines” oscillated, changing places twice every 164 days. A simple model fit the data perfectly: SS 433 was spraying out a pair of jets in opposite directions, and these rotated with the aforementioned period. The rotation of each jet was a kind of precession, like a searchlight plumbing the sky 20 degrees off the zenith. An early analogy to a rotary lawn sprinkler also proved apt, for analysis of the lines revealed that matter in the jets did not stream out smoothly but rather chugged out impulsively in bullets or blobs of fluid, several per day. Twice per rotation period, the matter in the jets would be moving perpendicular to our line of sight and, according to the classical theory, Should show no Doppler shift. But when these times came, the hues were still shifted to the red. Even this puzzle was quickly resolved. Long ago, the theory of relativity had predicted that time would slow down for a body in motion. Even though it was moving neither toward nor away from us, the matter in the jets was moving, and this dilation of time showed up, exactly as predicted, through a redshift—a slowing down of the frequency of light, Another prediction of Einstein’s theory was confirmed, the steel trap of its validity tightened. And the speed of the matter in the jets could be deduced with precision. It was a quarter of the speed of light.
While I ran through all this history in my mind, I was steadily approaching SS 433. The smeared-out jet that I had crossed as it plowed through the outer reaches of W50 now seemed to be resolving itself into much finer structure. Each bullet was emitted in a slightly different direction, as the sprinkler head chugged around, and thereafter traveled in a straight line. Far enough away from SS 433, the collective pattern of all these bullets (if they survived) should delineate the surface of a hollow cone. As I crossed back into the jet’s path, I found that the impulse was no longer spread widely but was confined to a conical surface, spread out by about 20 degrees from its axis of symmetry—just as predicted. Closer in, I could see the helical pattern traced by the sequence of bullets emitted during successive rotations, a pattern, that was washed out when viewed on large scales. I recalled how astounded I had been when my radio astronomer colleagues had imaged this very helical pattern, all the way from the 17,000-light-year distance of Earth.
It was all very familiar, the geometry and motions of the SS 433 jets, and yet the congruence between what I saw and what I expected made it spooky. This level of predictability and economy of assumptions was seldom attainable in models of real astrophysical systems. Aesthetics had always been a principal goal in my approach to theoretical astrophysics, but I had grown used to regular frustration of my hopes to attain this ideal. Astronomy as practiced was nearly always about data, which seemed without fail to sneak in a twist, a complication, an exception to every rule. The systems we studied were so complicated that elegant models seldom worked as well as they did in the case of SS 433. Maybe this, even more than the simple presence of its steadily precessing jets, accounted for the iconic power that SS 433 seemed to have over my colleagues and me.
In any case, this familiarity was soon to be shattered. The theory of SS 433 was cut and dried only insofar as its outward trappings were concerned. My present objective was to find the motive force of its jets. Here, where it counted, I ran into one obstacle after another.
From my present vantage point, I could see the basic structure of the binary system, which was not all that different from Cygnus X-1 in some respects. As in Cygnus, the star was being sucked away. It looked like a similar kind of star, too, one of those blindingly blue ones whose ultraviolet rays could give you a sunburn far worse than Earth’s star, even if you had the protection of a pea-soup atmosphere. (Rocinante, fortunately, had far better armor than that.) SS 433 and its companion were swinging around each other once every 13 days instead of every 5 ½ days, but this difference in detail seemed hardly worth mentioning. What was different was what appeared at first to be a general haze enveloping the SS 433 system. This, I saw, was actually a powerful wind rushing away from the system at speeds up to 2000 kilometers per second—70 times faster than Earth’s march around the Sun and 8000 times faster than a jet plane. The flowing gas glowed softly in X-rays, punctuated by arcs of extreme X-ray intensity and outward-rushing spots from which emanated the cool pink glow of hydrogen accented by yellow tints of helium. The arcs were telltale signs that this wind was violently turbulent and unsteady; X-rays were produced where the faster streams plowed full-force into more slowly moving matter. But what was the wind rushing away from? Was the wind coming off the accretion disk around SS 433, or off the star that was feeding it, or both? I couldn’t tell. The cool clouds reminded me of debris caught up in a gale, like the blowing palm
fronds and bits of plywood that had once terrified me when I was caught in a hurricane. But what was the source of the debris? Which part of SS 433’s binary system was being shredded and blown away by these gusts?
Through the light fog I could see the stream of gas crossing the gap between the tortured star and the accretion disk. It was more ragged and active than the stream in Cygnus X-1, and it looked like some of the debris—though not all—was being torn off its margins. Compared to this gushing firehose, the stream feeding the disk in Cygnus X-1 looked tame; thus it came as no surprise to me when I later deduced that matter was flowing across to SS 433’s cauldron at 100 times the rate at which mass was being transferred to Cygnus X-1. I peered ahead to see whether I could spot the hole at the center of the disk, if indeed there was a black hole there, but all I saw was an X-ray/ultraviolet shimmer behind a gauzy screen.
With difficulty I descended to trace the lay of the disk and saw that, unlike the swirling platter in Cygnus X-1, it was not flat. A rolling, warp, its spiral twist sweeping one turn around the center and coming out to envelop Rocinante as though in the trough of a gigantic sea swell, disoriented me and made me slightly woozy. The disk was apparently wobbling like a top, guiding the 164-day precession march of the jets, but for now the reasons eluded me. I cautiously crept inward, but the roller-coaster scene had triggered a panic attack that I had difficulty controlling. What if I should encounter those sickening tidal forces again? Still I inched toward the center of the disk, jaw clenched and palms moist. At 10,000 kilometers from the center I felt the first twinges of tidal stretching—and froze. I could go no farther.
Neither could I see any farther. The churning gas just in front of me became so dense that it formed a glowing wall, its outer boundary so indistinct that it seemed diffuse and opaque at the same time. There was no disk from here on in. What had been a thin orbiting structure (its warp superimposed on it like the warp in a potato chip) had ballooned into an immense quasi-spherical bulge, as though some force from within were trying to blow it apart. As if to underline its dispersive tendencies, this gaseous globe expelled a good fraction of the powerful wind and debris that I had puzzled over earlier. But the force I was familiar with, gravity, could not be doing this: Gravity was always attractive! The bulge hid the center from further scrutiny, and I had to pull up and creep along the diffuse surface to avoid flying blind. I noted that the bulge still retained a great deal of the disk’s rotation and thought that maybe, if I headed for the rotation axis, the flow would open up into an evacuated funnel, like a whirlpool in the sea or (more prosaically) in the water going down a drain. But like the maelstroms that wreck ships in classical sagas, this one had its own version of a waterspout shooting up through the center: the jet. As I came over the lip and one of the rotational poles came into view, I was blown away (almost literally) by the spectacle of this massive ejection. The jet wasn’t emerging calmly through an evacuated funnel lined with smoothly rotating gas; it was blasting its way through with a great deal of violence. True, the centrifugal force of rotation created a preferred alignment for the jet’s path, but once it had found the weak spot, the jet forced the gas aside, opening up a clear channel by its sheer impact. Through this channel a searchlight beam of X-rays and ultraviolet rays accompanied the jet out into open space, and the matter in the jet fluoresced in the glow of its own sheath of radiation. Even the protons and neutrons inside the nuclei of some of the atoms were disturbed by this tremendous agitation and emitted a spectrum of gamma rays. With atoms knocking together at a quarter of the speed of light, this was not hard to comprehend.
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