Last year's 100th anniversary of Wilhelm Roentgen's discovery of X-rays and the forthcoming centennial of Guglielmo Marconi's inauguration of radio communication via the ionosphere are reminders that those milestones forecast the astronomical revolution of the past half century. First radio astronomy and then rocket astronomy launched us on new pathways to the stars. Both new astronomies revealed a previously unimagined universe, breaking the confines of Galileo's visible universe of the preceding three and one-half centuries. Just as amazing is how fast high energy X-ray and gamma-ray astronomy have developed in less than 50 years of the space age and what remarkable phenomena have already been revealed.
Galileo set the course of the 17th century scientific revolution with emphasis on observation. "Our subject is the sensible world, not a world on paper," he said, referring to Aristotelian "common sense" natural philosophy unsupported by visible evidence. HLast year's 100th anniversary of Wilhelm Roentgen's discovery of X-rays and the forthcoming centennial of Guglielmo Marconi's inauguration of radio communication via the ionosphere are reminders that those milestones forecast the astronomical revolution of the past half century. First radio astronomy and then rocket astronomy launched us on new pathways to the stars. Both new astronomies revealed a previously unimagined universe, breaking the confines of Galileo's visible universe of the preceding three and one-half centuries. Just as amazing is how fast high energy X-ray and gamma-ray astronomy have developed in less than 50 years of the space age and what remarkable phenomena have already been revealed.is views seem especially relevant today when theorists postulate a universe made up largely of invisible mass in the form of exotic particles such as WlMPs, axions, gravitinos, and photinos that have never been observed.
But Galileo's astronomy was also severely handicapped by the observational restriction to visible light. For all of the elegant statistics on sunspots and the exciting resolution of detail seen in his telescopic images of the solar atmosphere, there was little progress in understanding physical processes of the transfer of energy from the sun into space. X-ray and gamma-ray astronomy have now revealed a new sun and produced an explosion of knowledge that has spurred theorists to devise over a hundred models to explain X-ray pulsars, black holes, quasars, and active galactic nuclei. But theoretical enigmas persist. Observational astronomers search for still more definitive clues to these cosmic sources.
Following Marconi's demonstration of transatlantic radiowave transmission came the discovery of the reflecting ionosphere in the 1920s. Solar astronomers and geophysicists were challenged to explain the coupling between the sun and the electrified upper regions of the terrestrial atmosphere. The far ultraviolet extension of the visible light of the sun offered no simple clues since the major constituents of the terrestrial atmosphere were immune to ionization by the resonance radiations of the most abundant elements of the solar atmosphere. Theoretically, only "soft" X-rays (of such low energy that a sheet of paper is sufficient to stop them while the hard X-rays employed in medical and industrial radiography can penetrate the body or a slab of steel) from the sun, absorbed in the rarefied upper atmosphere of the Earth were likely to ionize the air above 60 km.
Precisely because they were absorbed at such height, however, X-rays were off limits to observation from the ground. Rocket astronomy finally lifted the veil in 1949. X-ray astronomy came as a spin-off from the rocket weapons of World War II. For the first time, captured German V-2 rockets made it possible to place X-ray detectors above Earth's absorbing atmosphere that hid the X-ray sun. Until then, astronomers were blind to the X-ray/ gamma-ray universe.
In 1949 my colleagues and I mounted an array of detectors in a V-2 rocket and succeeded in observing solar X-rays from the base of the E-region ionosphere at 90 km to its top at 140 km above the earth. The intensity of X-rays is sufficient to electrify the E-region, serves as a mirror for the reflection of radio waves. This observation marked the beginning of X-ray astronomy. For the next ten years solar X-ray measurements showed pronounced variations keyed to sunspots and solar flares. However, the possibility of extending X-ray astronomy beyond the sun, deep into the Galaxy, seemed very remote. If galactic sources of X-rays are like the sun, it would take X-ray detectors millions of times more sensitive than I had been using to observe them.
But by increasing the size of detectors by factors of only ten to a hundred, teams at American Science and Engineering Co. (ASE) and at the Naval Research Laboratory (NRL) began to achieve some success in probing the Galaxy. In 1962 the ASE group reported a broad signal from the general direction of the galactic center which they soon corrected to the constellation Scorpius, about 40 degrees away. Less than a year later the NRL group positioned the Scorpius X-I source to within one degree by restricting the field of view seen through mechanical baffles and also detected emission from the general direction of the Crab Nebula at about one-seventh the brightness of Sco X-l. The first positive identification of an X-ray source with an optical counterpart came in 1964 when I was alerted to the occurrence of a lunar eclipse of the Crab Nebula by the moon, a once-in-ten-years event. A rocket was flown in coincidence with the eclipse, and the X-ray flux was observed to fall off precisely as the moon crossed the Crab.
The first decade of rocket astronomy produced an NRL catalogue of about three dozen X-ray sources that included extragalactic detections of the quasar 3C-273 and the giant galaxies M87 and NGC1275. These observations showed immediately that X-ray stars and galaxies could be orders of magnitude more luminous than their optical counterparts. Many of the X-ray stars radiated 1000 times as much energy in X-rays as in visible light. A big leap forward came with the first X-ray astronomy satellite, Uhuru (''freedom'' in Swahili), launched from the San Marco platform, an old oil rig three miles off the coast of Kenya on Dec. 12, 1970, Kenyan independence Day. The mission, led by astronomer Riccardo Giacconi, carried an improved rocket-class payload designed by the ASE group. In its two years of operating life, Uhuru produced a catalogue of 339 discrete sources and led to the discovery of periodic variations of intensity of a number of sources, proving that they belonged to double-star systems and that they were powered by gravitational accretion processes.
We now recognize that about 50 percent of the stars in our Galaxy belong to double-star binaries and that in the normal course of stellar evolution one member of a binary will eventually collapse to a compact object-white dwarf, neutron star or black hole-and become a powerful X-ray source by accretion of mass from its companion.
Uhuru marked the end of "small science" X-ray astronomy, characterized by payloads of a few hundred pounds carried on small rockets, and the beginning of an era of "big science" in which truck-size payloads of 20 thousand pounds or more are launched on space shuttles.
A revolutionary change in observational technology came with the development of the reflecting X-ray telescope. X-rays striking a surface head-on are absorbed without reflection. At glancing incidence the X-rays ricochet off the surface so that an X-ray telescope approximates a cylinder with a very slight taper. The most successful imaging of the sun has been accomplished with a soft X-ray telescope on the Japanese satellite Yohkoh (Sunbeam). Imaging with 2 arcsec resolution is accomplished by means of two consecutive hyperboloids of revolution. The mirrors are unusually short (4.5 cm) and only 23 cm in diameter with a 1.5 m focal length. The X-ray sensor is a CCD array of a million pixels with a spectral range of 3-45Å. Time resolution of 0.5 sec is achieved. Viewed as time-lapse movies, the X-ray sun sparkles with bright points and displays a whirlwind of eruptive prominences, surges, and flares.
A major advance for galactic and extragalactic astronomy came with NASA's Einstein Observatory, launched in 1978. With a nested set of mirrors 0.6 m in diameter, it was highly successful but will be superseded by AXAF (Advanced X-ray Astronomy Facility), scheduled for launch in about two years. An interim success has been ROSAT (German Roentgen Satellite), equipped with a 30-cm diameter mirror for very soft X-rays. It has mapped 50,000 discrete X-ray sources. From a few dozen X-ray sources spotted in 1962-64, the field of X-ray astronomy can now claim a catalogue of several tens of thousands of objects. ROSAT has demonstrated that almost all normal stars are detectable X-ray emitters.
One of the fascinations of X-ray astronomy is that it focuses on the most violent events in the universe. For this brief view of the X-ray scene I limit my discussion to the diffuse X-ray background, neutron stars, black holes, and X-ray/gamma-ray bursters. I regret that space does not permit me more than a passing mention of active galactic nuclei and X-ray quasars. Furthermore, I shall not discriminate between soft X-rays, hard X-rays, and gamma rays when the physical mechanisms are essentially the same and only the energy ranges differ. Continuum radiations merge one range into another. The visible universe is dominated by thermal radiation excited by heat generated from nuclear reactions whereas X-rays are attributed to gravitational collapse, magnetic field confinement, and collisional interactions at high temperatures. The difference in physical mechanisms of radiation is emphasized by the fact that none of more than 100 visibly most luminous stars match the 100 most luminous X-ray stars.
At X-ray wavelengths, the black night sky doesn't exist. If we could see X-rays, a diffuse glow would shine in all directions. Perhaps half of this glow is made up of unresolved X-ray stars, and the rest is truly diffuse. Galaxies of a hundred billion stars hang together in clusters that may contain thousands of galaxies covering a vast celestial scene. In what resembles a cosmic Mixmaster, the galaxies race around a common center of a cluster, usually marked by a giant galaxy. The chaos and turmoil created by galaxies plowing through the intergalactic gas heat the medium to 200 million degrees and generate diffuse X-rays. Clouds about a million light-years across fill the cluster volume. The diffuse gas is spread more thinly than any laboratory high vacuum on Earth but the total mass of gas is comparable with the sum of all the stars in all the galaxies of the cluster.
To those of us who learned about stars as giant gas bags, the neutron star with its solid crust millions of times as stiff as steel is an almost incredible stellar architecture. It is the end product of the collapse of a star whose core weighs less than three suns when its life support of nuclear energy is exhausted. Death and transfiguration transpire in a catastrophic convulsion. In less than one second, the burned-out core comes crashing down and a rebounding shock wave turns the body of the star inside out. For several days a titanic explosion-a supernova-shines with the combined brilliance of all the stars in the Milky Way. The compacted remnant of the supernova undergoes instant metamorphosis to a neutron star, all its protons and electrons squeezed together in a gravitational vise to make one giant atom about 20 km in diameter with the density of an atomic nucleus. A sphere of neutron star material the size of the period at the end of this sentence would weigh about 20 million pounds. At the surface of the neutron star the gravitational field is about 100 billion times stronger than the gravitational field at the Earth. Any outside matter near the neutron star will be captured. When initially formed, its surface temperature may exceed a few million degrees and X-ray emission should be detectable at distances up to thousands of light-years. Such thermal emission has been observed in only a few cases, because the cooling time is of the order of a few thousand years. The supernova explosion, however, sprays the vicinity with energetic particles that generate more powerful X-rays than does the hot neutron star.
More than 50 X-ray sources are known to be associated with the exploded nebulas surrounding supernova remnants in the Milky Way, and as many more have been detected in the adjoining Magellanic Clouds and the nearby Andromeda galaxy. If the neutron star is part of a binary pair, its intense gravity attracts mass flow from the normal star. Two scenarios are possible. (1) A powerful magnetic field, trillions of times as strong as the Earth's, funnels mass down onto the magnetic poles of the rapidly spinning neutron star to produce an X-ray pulsar. (2) If the magnetic field is weak, an X-ray burster results. Mass settles everywhere onto the surface of the neutron star, building up to the combined temperature and density required to ignite a thermonuclear explosion and a burst of X-rays. That process repeats at a rate corresponding to the reloading time.
The notion of a black hole first appeared in the scientific literature when astronomer John Michell, in 1783, proposed to the Royal Society of London that a star 500 times as large as the sun and of the same average density would prevent even light from escaping the force of gravity. Pierre Simon de Laplace, the renowned French astronomer and mathematician, reached the same conclusion in 1796. Today we know of no stars heavier than about 100 suns, but General Relativity makes a theoretical case for highly curved regions of spacetime about collapsed stars of more than three solar masses that can prevent the escape of light. In 1960, John Wheeler of Princeton University coined the name black hole.
Although there may be millions of black holes in our Galaxy, blackness hides them from view. When the mass of a collapsing star is greater than eight suns, no force can stop the implosion. Bereft of a sufficient sustaining source of heat, it falls forever into its black hole, never reaching the bottom of the abyss as it vanishes from the universe. All that remains to signal its ghostly existence externally is the tug of gravity. Black holes are where General Relativity is all-powerful and offer a challenging theoretical laboratory for the physics of gravitational collapse.
The first candidate black hole to be discovered was Cygnus X-l. Observations in 1964 and a year later in 1965 showed extreme variability. An intensive follow-up of observations revealed that it was a double-star system of a normal star and a compact object. Furthermore, the mass of the compact star had to exceed 3.5 solar masses, well above the limit of a neutron star. Rapid variability implied that the emitting region must be less than one light-millisecond or 300 km in diameter. Enormous X-ray energy was radiated by the system, about one hundred thousand times the total luminosity of the sun. All the evidence excluded nuclear energy, which could not sustain the system for even a few decades. Only gravity was a plausible power source with a lifetime as much as a million years. In the last decade several sources of the nature of Cyg X-I have been discovered.
Soft X-ray transients are a newly identified class of black hole candidates among double-star systems. They flare suddenly to become, for a few weeks, the brightest X-ray objects in the galaxy and then fade away in a matter of months. In the 1992 transient, V404 Cygni, the minimum mass of the compact component was 6.3 solar masses, far above the limit for a neutron star.
A black hole is a cosmic Disposal. The more mass it swallows, the bigger it grows and the more it devours. Its appetite is insatiable. There is strong evidence for the existence of black holes with the mass of billions of suns in the nuclei of quasars and in the cores of active galactic nuclei. They can cannibalize entire galaxies through tidal forces that capture matter into wheeling accretion disks. As the whirling gas funnels toward the core of the hole, like a vortex of bathwater spiraling into the drain, it accelerates almost to the speed of light. Friction heats the in-falling gas to hundreds of millions of degrees so that it radiates a flood of X-rays.
Astronomers now believe that most galaxies harbor quiescent black holes, many as massive as millions of suns, in their nuclei. They could have formed in the first billion years after the Big Bang to become brilliant quasars, eventually exhausting the diet of stars in their surroundings. Toward the end of time, all of the mass of the universe may be captured into one super-massive black hole. Jonathan Swift in his poem "On Time" in 1727 seemed to be describing a black hole:
Ever eating, never cloying.
All devouring, all destroying,
Never finding full repast
Till I eat the world at last.
The discovery of gamma-ray bursts was a spin-off of U.S. military interest in detecting clandestine Soviet tests of nuclear bombs in space after an atmospheric test ban treaty was signed. It was thought that secret tests were possible in deep space. An atomic bomb detonation there would not create the bright fireball of an atmospheric explosion. Its signature instead would be a great X-ray flash. The weapon could be triggered behind the moon to conceal the flash. Only when the cloud of radioactive debris spread beyond the lunar mask could gamma rays and neutrons become detectable. The Vela Project was established to provide continuous monitoring of space for X-ray/gamma-ray flashes. By 1967 Vela Satellites made their first detection of a gamma-ray burst, but its time variation and energy range did not fit a nuclear explosion. Since then, the study of X-ray/gamma-ray bursts has been pursued in various spacecraft for purely scientific reasons without much diagnostic success until the present Compton Gamma Ray Observatory (CGRO) was launched in 1991.
Whereas visible light photons have energies between 2 and 3 electron volts (eV) that encompass all the colors of the rainbow, the highest energy gamma rays in space reach a thousand trillion eV. The bursts recorded by the Burst and Transient Source Experiment (BATSE) include energies up to 10 million eV. At the rate of about one per day, more than one thousand gamma-ray bursts have been observed by (BATSE) on CGRO in three years of operation. Not one has been identified with a known astronomical source. Not one has flashed from the same spot twice. They last for a matter of seconds, then vanish from the sky. Most of the speculations about origin associate the bursts with neutron stars, but neutron stars in the Milky Way cluster close to the galactic plane. Instead, the distribution of bursts is nearly isotropic with as many bursts coming from far above the plane and below the plane as in the plane as though they come from a remote distance far beyond the Milky Way.
Twenty-five years ago Russian astronomer Joseph Shklovsky proposed that neutron stars formed in supernovas receive a hard kick away from the center of compression because of slight asymmetries in the collapse. Like watermelon seeds pinched between the fingers, they squirt out at high speed. Some have been clocked as fast as 1000 km per second, fast enough to escape the gravity of the Milky Way. Over eons of time they could form an extended halo. Bursts originating in a far-reaching halo of neutron stars would exhibit a uniform space distribution.
If an extended galactic corona is the source, why don't the nearby Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) and especially the Andromeda galaxy, which is twice as large as the Milky Way, show up as substantial enhancements in the general distribution? The locus of bursts is baffling and the mechanism of bursts remains to be explained. Speculations have abounded, such as neutron star quakes, asteroid collisions, and encounters of comets and neutron stars. Twenty years of burst observations have not established a single example of coincidence with any known astronomical object, and their distances still remain uncertain by twelve orders of magnitude. The energy released in a burst at cosmic distance, lasting a few seconds, may exceed the total radiation from the sun in a thousand years, yet rapid variations in the sharp burst profiles imply that the size of the source must be less than that of the sun. The difference between the energy attributed to a burst in the Milky Way and one that is extragalactic is like the difference between a candle and a nuclear bomb.
As fantastic as the suggestion of colliding neutron stars may sound, I am intrigued by it. Professor Joseph Taylor of Princeton University received the Nobel Prize for his study of a binary pair of neutron-star pulsars that are slowly losing their energy of revolution to the radiation of gravitational waves. As their orbit shrinks, the speed of revolution increases. At the end of perhaps 100 million years, the two solar mass objects may speed up to 1000 revolutions per second as they reach contact and crash in a great catastrophe. They could merge to form a black hole, and the reverberations would reach us in a brilliant flash of X-rays and gamma rays. Neutron stars may collide in the Galaxy once in a million years. If the bursts are cosmic, once a day is not unreasonable.
In Einstein's words, "The most beautiful experience we can have is the mysterious." Certainly, the burster problem is one of the deepest mysteries that now confront astrophysicists. It begs for a beautiful explanation.
Astronomy regularly brings us amazing discoveries, and the potential for new knowledge seems inexhaustible. We can be assured of fascinating scientific progress if research support is adequate, but the future is very much in doubt. The federal science budget has been declining steadily for several years. The deficit reduction plan of the Clinton administration and Congress proposes to reduce the United States' science programs by 35 percent by the year 2002. Astronomy may be hardest hit since it cannot guarantee the quick payoff demanded by many politicians. It's small consolation that the current funding tribulations of science are not new. Consider the conversation between Socrates and his disciple Glaucon from Plato's Republic 111, circa 370 B.C.:
Socrates: Shall we make astronomy the next study? What do you say?
Glaucon: Certainly. A knowledge of the seasons, months and years is beneficial to everyone, commanders as well as farmers and sailors.
Socrates: You make me smile, Glaucon. You are so afraid that the public will accuse you of recommending unprofitable studies.
Herbert
Friedman ('64) has been affiliated with the Naval Research Laboratory for 57
years. He now holds the position of Chief Scientist Emeritus of Hulburt Center
for Space Research. He led the way in developing the field of rocket astronomy
and followed by space research using satellites. He has published more than
300 scientific papers and several books. Among his honors: the National Medal
of Science and the Cosmos Club Award.
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