X-Ray Variability in Astrophysics

X-Ray Variability in Astrophysics tudy of the variability of x-ray arguing over its interpretation. and gamma-ray sources is a rel- Many commented that the workshop atively mature field. providing was one of the most exciting scientific astrophysicists with a wealth of meetings they had ever attended. Not only informations about the workings of binary did the workshop pave the way for plan- stellar systems and the internal structure ning use of the next generation of x-ray of the component stars. Recently, interest in these rapidly changing systems has soared as a result of new data gathered by the European Space Agency’s satellite EX- OSAT, launched in 1983. EXOSAT has been able to gather invaluable data on rapid and repeating transients as well as on periodic behavior and more subtle effects such as frequent, small shifts in the period of an oscillatory phenomenon. Orbital periods and partial eclipses in binary sys- have been more fortunate. During the tems, x-ray bursts. and even changes in the weeks preceding the workshop, unex- direction of flow of accreting matter onto pected new results from EXOSAT kept neutron stars are among the new details pouring in, requiring repeated overhaul of that can now be studied. the schedule to accommodate reports on Until recently, luck has been a big factor two newly discovered phenomena. One in discovering transient behavior. But that was quasiperiodic oscillations in the x-ray will change when the new Japanese output of some neutron-star binaries (See satellites, but it served as a forum for ASTRO-C satellite is launched in 1987 “Quasiperiodic Oscillations”); these os- discussing the startling discoveries being and the new American XTE satellite is cillations provide the first direct probe of made with present instruments. launched in the early 1990s (see “The Next the transition region between the neutron This report summarizes many of the Generation of Satellites”). The plan is to star and the accretion disk present in these discoveries and developments discussed equip these satellites with all-sky sensors systems. The other was evidence that the in the sessions of the workshop, to which that will be able to discover and monitor famous neutron star in Hercules X-1 (Her all participants contributed. The illustra- transient behavior anywhere in the sky. In X-1) is freely processing, which could ex- tions and tables arc adapted from material addition, the satellites will carry large-area plain the origin of its enigmatic 35-day presented at the workshop. The selection detectors that will allow the spectral and cycle (see “Her X-1: Another Window on of topics and the views expressed arc ob- temporal behavior of cosmic x-ray sources Neutron-Star Structure”). viously those of the authors alone. The to be studied with unprecedented pre- In view of the importance and con- participants and the subjects of their cision. Moreover, the satellites will be troversial nature of the evidence on Her scheduled talks are listed on page 37. specially designed so that observers can X-1, a special effort was made to gather for quickly turn the large-area detectors the first time those scientists with major X-Ray Astronomy and Sco X-1 toward sources showing unusual behavior. interest in the 35-day cycle. Almost all Both the quantity and quality of informa- were able to attend the workshop, making The brightest stellar x-ray source in our tion about transient systems should there- possible in-depth discussions of the dra- sky, Scorpius X-l (Sco X-1), has been fore increase dramatically in the coming matic new evidence for precession in this studied since the beginning of x-ray as- years. system. It became apparent that several tronomy. The story of this object, dis- It was thus felt that the summer of 1985 possible stumbling blocks to acceptance of covered early but not easily understood, would be an opportune time for a work- this interpretation could be set aside. illustrates the development of x-ray as- shop to review the accomplishments of the Similarly, the workshop was the first tronomy, last decade, to discuss plans for the new meeting at which almost all the scientists Sco X-1 shines so intensely that it de- satellites, and to prepare for the expected involved in the discovery and interpreta- livers to earth about a thousand x-ray wave of new, more detailed data on tion of quasiperiodic oscillations were photons per square centimeter per second, variable x-ray and gamma-ray sources. present. Participants stayed up late into ten times more than the next brightest x- The timing of the workshop could not the night, discussing the new data and ray star, It was detected during a 1962 4 Spring 1986 LOS ALAMOS SCIENCE .. X-Ray Variability in Astrophysics rocket experiment led by Riccardo Giac- coni that was designed to look for solar x rays scattered from the moon. The discovery of such a bright x-ray source outside the solar system came as a great surprise. Sco X-1 was found to radiate x rays with ten thousand times the total power of our sun. What sort of object could be such a powerful source of x rays? One suggestion was that Sco X-1 is a neutron-star system in which matter is heated to millions of degrees as it falls into the enormous gravi- tational potential of the neutron star, producing x rays. Without direct evidence of a neutron star in Sco X-1, this accretion model remained highly speculative until that is, the period of the intensity variation the early 1970s when similar but pulsed x- also varies. As a result, the Fourier trans- ray sources were discovered. The rapid form of intensity squared versus time, namely, the power-density spectrum, gives a broad rather than a sharp peak. The centroids of the observed peaks lie in the region of the spectrum between 6 and 50 cycles per second (Hz). The cause of the oscillations cannot be simply rotation of the neutron star—-a very precise clock—but could be produced by interac- tion of the magnetosphere of the neutron star with the surrounding disk—a sloppy clock. These quasiperiodic oscillations are the first direct evidence of what is happen- ing near the neutron star and confirm our what little we know is deduced from the expectations that study of x-ray variability shapes of its x-ray and optical spectra and will reveal the detailed dynamics of these their variations with time. For example, binary systems. continued on page 8 the existence of an accretion disk around regular pulsing of these sources signaled the neutron star in Sco X-1 was confirmed the presence of a rotating magnetic neu- by the discovery in 1981 of simultaneous tron star. Many of these pulsed x-ray rapid changes in the optical and x-ray sources were found to be neutron-star intensity. The lack of delay between the binaries in which a close companion star is two signals meant that the x rays are gener- the source of the accreting matter. Al- ating optical light in an accretion disk near though Sco X- I exhibits no strong x-ray the neutron star as opposed to generating pulsations. regular variations in the Dop- optical light by traveling to and heating the pler shifts of its spectral lines and in the companion star. brightness of its optical light, discovered Even an old friend can surprise you. Sco in 1975, show that it too has a compan- X-1 did just that early in 1985 when it was ion—one that circles it every 0.787 days. shown to exhibit quasiperiodic oscilla- Sco X-1 has revealed its nature only tions in its x-ray intensity. This phenome- grudgingly. The system is too small and far non is a variation of several per cent in the away for its components to be resolved: x-ray output that is not perfectly periodic, LOS ALAMOS SCIENCE Spring 1986 5 The Next Generation of Satellites The Next Generation of Satellites ithin the next few years two planned satellites, one Japanese w and one American, should begin gathering data vital to a deeper un- derstanding of x-ray sources and their variability, The instruments aboard the satellites will allow both detection of previously invisible sources and much more detailed studies of bright sources. The Japanese satellite, ASTRO-C (Fig. 1), which is being designed by the Japanese Institute of Space and Astronautical Sci- ence, will be launched in 1987. Its key instrument will be a proportional counter, called the large-area counter (LAC), with an effective area of 0.45 square meter. A proportional counter, the workhorse of x- ray astronomy, was chosen as the main instrument because it can be made with a large effective area, is rugged, and has good background rejection. It has mod- Signals from extraneous sources and the diffuse glow of the x-ray sky will be ex- tions in the background. The field of view Fig. 2. ASTRO-C, Japan’s third x-ray as- cluded with mechanical vanes that restrict will be similar to that of ASTRO-C, but tronomy satellite. The large-area counter the field of view to 0.8 by 1.7 degrees. The the use of xenon rather than an argon- (LAC) will consist of eight proportional LAC will be sensitive to energies from 1.5 xenon mixture will allow detection of counters containing a mixture of argon to 30 keV, a range that includes most of higher energy x rays (2 to 60 kcV).

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