Rosat in review.
A century later orbiting X-ray telescopes are proving indispensable for understanding the structure and evolution of the macro-world of stars, galaxies, and the universe as a whole. One of the most productive of all X-ray observatories is the Rontgen-satellit (Rosat), appropriately named in Rontgen's honor.
Rosat was proposed by scientists at the Max Planck Institute for Extraterrestrial Physics in 1975. In view of its large cost, Rosat became a joint venture among Germany, the United Kingdom, and the United States. The observatory was launched by NASA from Cape Canaveral on June 1, 1990, aboard a Delta II rocket and put into an orbit 580 kilometers above the Earth. The first goal of the mission was to perform an all-sky survey, to paint a comprehensive picture of the sky in the low-energy X-ray band. (For details on the instruments aboard, see Sky & Telescope for August 1990, page 128.)
TRIALS AND TRIBULATIONS
After a period of trouble-free operation one of the star sensors used to point the telescope failed, but thanks to the redundancy that is essential for a successful space mission another sensor was available. All went according to plan for the first six months: more than 60,000 previously unknown X-ray sources were discovered, and valuable data was accumulated on several thousand known sources.
Then disaster struck on January 25, 1991. Apparently a cosmic ray, one of the high-energy charged particles that stream through space, struck an onboard computer chip. A glitch ensued that sent the satellite spinning uncontrollably without shutting down the instruments. As the telescope slewed past the Sun, one instrument was zapped by intense solar radiation and another was seriously degraded.
Control was regained by reprogramming the onboard computers. But a few days later one of the gyroscopes used to orient the spacecraft began to drift. The situation worsened, and in May another gyro failed completely. For four tense months scientists and engineers at the German Space Operations Center searched for a fix while Rosat carried out a limited schedule of observations. They conceived an ingenious solution whereby a combination of star-trackers, magnetometers, and Sun sensors would allow them to maneuver the satellite reliably. New software was uplinked to the spacecraft, and on November 4, 1991, normal pointing operations resumed. Rosat had been pulled back from the edge of oblivion.
PROBING THE STARS
The near destruction of Rosat by accidental exposure to solar X-rays has a certain black irony. One of the satellite's principal goals was to study the emission from stars similar to the Sun at various stages in their evolution. The source of X-rays from all normal stars (those that have not collapsed to become white dwarfs, neutron stars, or black holes) is their hot gaseous atmosphere, or corona, heated to millions of degrees.
The mechanism for producing hot coronas is not well understood, but it is known to be related to conditions deep in stellar interiors. In a star with a surface temperature below about 7,000 [degrees] Kelvin the outer third or more of its interior is in a rolling, boiling turmoil called convection. The corona is thought to be heated by the interplay between this convection, the rotation of the star, and its magnetic field. The up-and-down motion of convection, coupled with the round and round motion of rotation, twists and amplifies the magnetic field. Helical, magnetized loops of gas rise high above the stellar surface. There they unwind, unleashing stored energy in a manner analogous to a twisted rubber band when it is released.
Based on this general picture we would expect extensive coronas around rapidly rotating stars with vigorous convection zones. Some very young stars, with ages less than about 100 million years, are thought to possess both these characteristics. Rosat observations of star-forming regions in the Orion Nebula, the gas clouds in Chamaeleon, the Pleiades, and the Hyades provide dazzling confirmation of these expectations. Rosat's ability to make sensitive observations of extended regions of the sky has pulled hundreds of very young stars out of obscurity and onto the center stage of scientific research.
Rosat is also providing insight about the final stages of evolution of stars like the Sun. When solar-mass stars exhaust the supply of nuclear fuel in their cores they collapse to become white dwarfs, dense objects about the size of the Earth. A white dwarf is so named because at birth it is white hot from the heat generated by its collapse. Over billions of years these dwarfs will slowly cool and fade from sight. During the early stages of cooling most of their energy comes out at ultraviolet wavelengths. Fortunately, Rosat also carries a separate camera that is well suited to study this radiation.
The results will interest physicists and cosmologists, as well as experts on stellar evolution, because the details of the cooling process depend on esoteric nuclear physics that involves the neutrino. This is an invisible but abundant particle that darts in and out of cosmological theories like the ghost at Macbeth's banquet. The importance of the neutrino's role depends upon whether this particle has mass or not, and the latest experiments may suggest it does (April issue, page 10).
So far the Wide-Field Camera has detected more than 100 white dwarfs. Surprisingly, this sample includes far fewer high-temperature (greater than 20,000 [degrees] K) stars than expected. Could it be that white dwarfs cool faster than first believed because of some unknown physical process in their interiors, or is the situation related to an unsuspected absorption process in their atmospheres? Preliminary indications suggest the latter.
One of the great enigmas of modern-day astrophysics is the apparent existence of large amounts of "dark matter" - matter that radiates too weakly to be seen but whose presence can be inferred because of its gravitational attraction. One of the reasons that distant, dim white dwarfs have been rejected as a candidate for dark matter is that counts indicate there aren't enough of them. Is it possible that rapid cooling or atmospheric absorption has hidden most white dwarfs from view, and they really exist in numbers great enough to account for the dark matter in our galaxy? Perhaps further study will tell us.
A star much more massive than the Sun does not become a white dwarf. Instead it explodes violently as a supernova. The outer layers are ejected into space while the inner core collapses to form a superdense neutron star. X-ray telescopes make a unique contribution to the study of this process. The shock waves generated by a supernova explosion excite the interstellar medium to radiate strongly at X-ray wavelengths for thousands of years, while the neutron star left behind can be an X-ray source for hundreds of thousands of years. One of the most significant achievements of the mission has resulted from Rosat's ability to detect the radiation from neutron stars.
More than 20 years ago a strong source of gamma radiation was detected in the constellation of Gemini. It was the third brightest object in the gamma-ray sky after the pulsars in the Crab and Vela supernova remnants. Yet no obvious counterpart of the Gemini source could be identified at optical or radio wavelengths. The object was dubbed Geminga, a contraction of Gemini gamma-ray source that also means "there's nothing there" in a Milanese dialect. Investigations lasting two decades only deepened the mystery until scientists working with Rosat data cracked the case.
Jules P. Halpern (Columbia University) and Stephen S. Holt (NASA-Goddard Space Flight Center) detected a weak X-ray source near Geminga pulsating with a period of 0.237 second. With this information and data from NASA's Compton Gamma Ray Observatory, Goddard's David L. Bertsch and his collaborators demonstrated that the gamma-ray emission pulsed with the same period. The Italian team that discovered Geminga then took a second look at their original data and concluded that it too was cyclic, albeit with a slightly shorter period. This is consistent with Geminga being a rapidly spinning neutron star, or pulsar, that is gradually slowing down as it ages. The rate of spin-down implies that Geminga was created in a supernova explosion about 300,000 years ago. Over such a long time the remnants of the explosion merged into the interstellar medium and faded from view.
The solution of the Geminga mystery was unexpected, for Rosat was specifically designed to make major advances in studying other components of a supernova explosion - the incandescent shells and filaments of matter heated by supersonic shock waves rumbling across light-years of interstellar space. Supernovae are the creative flashes that renew the galaxy. They seed the interstellar gas with heavy elements essential for life, heat the gas with the energy of their radiations, stir it with the force of their blast waves, and cause new stars to form.
As data on supernova remnants continues to pour in, a clearer understanding of supernova explosions and their interaction with interstellar gas is emerging. The most spectacular example is provided by the Cygnus Loop, the remnant of an explosion that took place roughly 15,000 years ago. Combined Rosat and optical, images (see page 40) reveal a rich dynamic tapestry of rippled, glowing sheets having a thickness barely resolvable with the Hubble Space Telescope yet thick enough to enfold the solar system; also, fiery streamers are seen to encircle searing bubbles of high vacuum, scores of light-years in diameter. All this suggests a complex interplay of shock waves moving through clouds, reflecting off clouds, and colliding with one another.
Although this seems to be pretty esoteric stuff, a detailed understanding of cosmic shock waves would apply to a number of important problems. These include the theory of expanding stellar atmospheres, nova explosions, the formation of high-speed jets of particles in active galaxies, and the triggering of bursts of star formation.
GALAXIES AND BEYOND
An exciting development in astrophysics has been the discovery of the starburst phenomena. That is, some galaxies form millions of stars in their central regions within only a few million years.
Such a starburst is presumably caused by a chain reaction that begins when a cluster of massive stars condenses out of a dense cloud of dust and gas. The most massive of these stars race through their evolution in less than a million years and explode as supernovae. Shock waves from the explosions trigger more star formation and more blasts, and so on. This wave of star formation sweeps through the central regions of the galaxy until most of the dust and gas is used up or blown away by shock waves. Infrared telescopes are best for studying the vast, warm clouds of dust associated with a starburst, while X-ray telescopes can map the enormous bubbles of hot gas produced by the supernovae.
The nearest starburst galaxy to Earth is M82, about 10 million light-years away in Ursa Major. Rosat observations reveal an oblong bubble of hot gas some 40,000 light-years across streaming away from the galaxy in a direction perpendicular to its plane (shown at top on page 41). A supernova explosion every few years in the galaxy's center - a hundred or more times the rate expected in a similar-size region of a normal galaxy - is required to produce this outflow.
Many astronomers believe that starbursts are usually triggered by a near collision with another galaxy. If this is true, then 10 or so billion years ago, when the universe was much smaller and galaxies were younger and much closer together, near collisions should have been frequent; many, perhaps most, galaxies would have experienced a starburst. In some instances the starburst might have been so intense as to blow most of the interstellar gas out of the galaxy.
Indirect evidence for this can be found in groups and clusters of galaxies. These associations provide an opportunity to study the "social behavior" of the members. Galaxies in groups and clusters are markedly different from galaxies that evolve in isolation, due to close encounters and interaction with the thermal bath of multimillion-degree gas spread throughout most clusters. The only direct way to study this gas is through the X-rays it emits. Observations reveal that the mass of the gas in clusters is considerable, comparable to - and often much larger than - the mass of all the stars in all the member galaxies.
Slightly more than half the gas is presumably left over from the formation of galaxies from a primordial cloud, or accreted onto the cluster from intergalactic space. The gas has traces of heavy elements such as iron, and the only known mechanism for producing this element is through thermonuclear reactions inside stars having masses more than twice the Sun's. The amount of iron in the intergalactic gas of a typical cluster indicates that between a quarter and a half of the gas was processed in massive stars and later blown out of the host galaxy in a starburst event.
The expulsion of gas from member galaxies and the gradual collapse of the cluster will increase the intergalactic gas density. Galaxies moving through this gas at speeds of several million kilometers per hour will experience aerodynamic drag. A process analogous to the stripping of leaves from a tree in a windstorm will strip the gas from the individual galaxies.
Since gas is the raw material from which new stars form, these galaxies will age prematurely. Observations show that most of the galaxies in the central regions of clusters are excessively red due to the lack of new stars. Second- or third-generation stars such as our Sun would never have formed if the Milky Way had been subjected to the scouring effects of the intergalactic gas in a rich cluster.
A cluster of galaxies is in dynamic balance between gravitational energy and the motion (kinetic) energy of the galaxies themselves. Collisions between members cause an inexorable change in the character of the cluster. Over the course of eons the cluster, which may have started out with a loose irregular shape, will become more compact and spherical.
The distribution of hot gas provides an accurate map of the gravitational forces within the cluster. A region of relatively strong gravitational force will confine a relatively high-density region of gas, producing a bright spot on an X-ray image. Rosat provides striking evidence for the evolution of the clumping process - from many individual centers associated with small groups of galaxies to the end stage in which the groups (and clumps) have merged into a single smooth distribution. One of the surprises of the Rosat mission was that the Coma Cluster, long thought to be the epitome of a smooth, spherical, well-evolved cluster, has definite structure to it (S&T: July 1993, page 10).
The capability of X-ray observations to trace the gravitational field has placed X-ray astronomy in the forefront of determining the nature and extent of dark matter in groups and clusters of galaxies. Unless we are seeing a cluster at a very special time when gas is exploding out of it, the pressure of the hot gas must be balanced by the gravity of the cluster.
Observations with previous X-ray telescopes have shown that the hot gas in clusters cannot be confined by the combined gravity of gas and the galaxies themselves. An additional force, due to dark matter, must be postulated. The implied amount of dark matter is enormous, about three to 10 times as much as that we can see.
Rosat's sensitivity to low-energy X-rays makes it most effective in determining the gravitational force in small groups of galaxies, where the gas temperatures are about 10,000,000 [degrees] K - no more than one-fifth that of rich clusters. Observations of a couple of well-studied groups indicate that most (75 to 95 percent) of their mass is in the form of dark matter.
Since approximately half of all galaxies belong to small groups, it is important to make further observations to narrow the range of uncertainty of the amount of dark matter and to determine whether the results characterize the majority of groups. The outcome will have much to say about the total mass density of the universe, which is one of the factors that determine its past and future evolution.
Other important clues to the evolution of the universe are found in X-ray background radiation. One of the first discoveries of X-ray astronomy was an unexpectedly strong and uniform background glow. This suggests the radiation is not coming from nearby galaxies but from a distance so great that all local irregularities, such as galaxies and clusters of galaxies, have been smoothed.
If, as appears likely, the X-ray background is composed of many individual sources too distant to be resolved, then as we develop instruments that can probe ever fainter, the discrete sources of radiation should begin to pop out. One of Rosat's principal goals was to detect such sources through deep surveys of blank fields - sky areas empty of radio, optical, and known, discrete X-ray sources.
One such survey was carried out by a group of astronomers from Leicester, Durham, and Cambridge Universities in England, and Johns Hopkins in the United States. About 200 previously unknown sources were detected; 110 of these were later identified with the pointlike powerhouses of radiation known as quasars. Other research supports the idea that quasars result from supermassive black holes swallowing up prodigious amounts of matter in the central regions of galaxies. The X-ray quasars discovered in this survey include many more distant ones than those found in any previous large-scale survey. The results indicate that the peak of X-ray activity in quasars took place about 3 billion years after the Big Bang. It is still not clear whether quasars produce all of the X-ray background or if something else is a significant contributor, such as galaxies undergoing intense starbursts. What is clear is that, as this and other deep surveys are analyzed further, we will know much more about quasars, the X-ray background, and the events that long ago led to the formation of the elegant, beautiful, and still mysterious patterns of galaxies, clusters, and superclusters of galaxies.
Bartusiak, M. Through a Universe Darkly. New York: Harper, 1993.
Charles, P., and F. Steward. Exploring the X-ray Universe. Cambridge: Cambridge University Press, 1995.
RELATED ARTICLE: ALEXIS Flies High
Los Alamos National Laboratory's Array of Low-Energy X-ray Imaging Sensors satellite (ALEXIS) has begun returning observations that complement the data from missions such as Rosat.
The 240-pound spacecraft was launched in April 1993. Its mission - to survey the sky at three different wavelengths in the extreme ultraviolet and ultrasoft X-ray bands - appeared to be in jeopardy when it reached orbit damaged and out of contact (S&T: August 1993, page 14). Only recently has the satellite been restored to a condition allowing routine operations.
ALEXIS's six wide-field telescopes, each the size of a coffee can, use concave spherical mirrors with multilayer coatings to reflect X-ray and extreme ultraviolet light directly, much as optical telescopes do. As the satellite spins its telescopes sweep across half of the sky - ideal for monitoring outbursts and other transient events. Detectors time the arrival of individual photons and ship the data to an onboard processor for later transmission to the ground.
The ALEXIS satellite and telescopes are currently operated, and its data processed, by a team of only nine persons. ALEXIS exemplifies the type of small, flexible, and efficient program NASA is aiming for.
Among mission highlights so far, ALEXIS captured the cataclysmic variable star VW Hydrus when it underwent an intense outburst in late May and early June last year. Astronomers obtained a unique light curve of the episode, tracking the variable over several days, and also caught another minor outburst this February.
The telescopes' narrowband filters, centered at 66, 71, and 95 electron volts, make the instruments sensitive to what should be the dominant emission from the hot gas (nearly 1,000,000 [degrees] Kelvin) encircling a region of space close to the Sun. If the elements within the gas have similar abundances to those that exist in the Sun, ALEXIS should see a strong signal.
To date observations with ALEXIS have enabled the mission team to set a firm upper limit on the emissions from the gas. If ongoing analysis further reduces this upper limit the result will send theorists back to ponder the origin and composition of the local interstellar medium. The ALEXIS team also hopes to find structure in the diffuse emissions from the gas as the sky maps are further refined.
JEFFREY J. BLOCH is the ALEXIS project leader and instrument principal investigator at Los Alamos National Laboratory.
RELATED ARTICLE: Rosat Mission Update
The first phase of Rosat's mission - the all-sky survey using the Position Sensitive Proportional Counter (PSPC) - was followed by a second when observers used the satellite to study a variety of targets. At the end of 1993 the PSPC ran low on the gas it needed to detect X-rays. Since then, Rosat has depended on its other main detector, the High Resolution Imager (HRI).
Like the PSPC, the HRI makes images of the sky in the soft (lower energy) X-ray band, detecting photons with energies between 0.5 and 2 kiloelectron volts. The HRI employs a microchannel plate to register accurately the position of each incoming X-ray photon. It produces the highest-ever spatial resolution for a satellite-borne X-ray imager, with source positions accurate to a few arcseconds. Unfortunately, the sharpness of the images is limited by camera shake - Rosat jitters in a manner that scientists have been unable to counter with sufficient accuracy. The high angular resolution also gives better contrast against the cosmic X-ray background for faint, diffuse structure.
The HRI-only phase of the mission was meant to begin in December 1993, but software problems following a gyro failure in November meant that the satellite spent almost a month on hold. The HRI observing program began belatedly in March 1994, and Rosat has been operating without any further serious problems since then. By the end of the first half of the fifth annual observing period this April, the HRI had imaged just over a thousand astronomical objects, including 170 quasars, 120 galaxies, 130 groups and clusters of galaxies, 60 supernova remnants, and 420 stars.
Rosat is now in a 545-kilometer orbit, only 25 km lower than at its launch five years ago, so reentry due to atmospheric drag is not an immediate threat. U.S. project scientist Robert Petre (NASA-Goddard Space Flight Center) reports that the spacecraft continues to operate well with no age-related problems.
Nevertheless, much of the onboard redundancy has been used up, and a serious hardware failure could terminate the mission without warning. Barring such problems, both U.S. and German space agencies plan to operate the spacecraft until at least late 1997, and astronomers hope it will survive until the launch of NASA's Advanced X-ray Astrophysics Facility in 1998.
JONATHAN McDOWELL is an astronomer at the Harvard-Smithsonian Center for Astrophysics and writes a weekly electronic newsletter on the space program.
Wallace Tucker splits his time between the Harvard-Smithsonian Center for Astrophysics and the University of California, San Diego. He is the coauthor with Riccardo Giacconi of The X-Ray Universe (Harvard University Press) and, with his wife, Karen, has written two popular books on astronomy: The Cosmic Inquirers (Harvard University Press) and The Dark Matter (William Morrow).
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|Title Annotation:||includes related articles; Rontgen satellite|
|Publication:||Sky & Telescope|
|Date:||Aug 1, 1995|
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