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Authors: Mitchell Begelman
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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 artistsâ 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 âtheoristâs
instincts.â Still, I felt one last pang of regret that I would be forever denied a closer view.
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 holeâs gravity caused motion, first the open spiral of the narrow accretion stream, pulled from the companion star into the black holeâs 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 companionâs 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 Wayâs 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 Wayâs 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 chainâmatter plus gravity goes to motion to heat to radiationâhad 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 Wayâs 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 couldnât 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
instincts.â Still, I felt one last pang of regret that I would be forever denied a closer view.
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 holeâs gravity caused motion, first the open spiral of the narrow accretion stream, pulled from the companion star into the black holeâs 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 companionâs 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 Wayâs 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 Wayâs 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 chainâmatter plus gravity goes to motion to heat to radiationâhad 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 Wayâs 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 couldnât 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
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