Instrumentation for X-ray astronomy

Less than five decades ago, the first X-ray observations of the sky were made using simple devices such as film and geiger counters with crude collimators. These instruments were carried aloft by sounding rockets and made observations lasting only a few minutes at most. Today, orbiting observatories, utilizing high-resolution CCDs at the focus of arc sec optics, have lifetimes measured in years. To maintain the pace of discovery in X-ray astronomy, detectors must continue to evolve into devices of ever increasing sensitivity and sophistication. Further progress depends upon a host of technologies: grazing incidence optics, proportional counters, semiconductors, calorimeters, etc. In this article we present a brief qualitative overview of these technologies and of the principles behind them, as well as some examples of how they are employed in scientific missions for X-ray observations at energies up to 100 keV.     

Some problems of very high-energy gamma-ray astronomy

The latest achievements in very high-energy (VHE) gamma-ray astronomy are discussed. Types of candidate objects for the sources of very high-energy gamma-quanta are considered, and pulsars, as the most probable ones, are anticipated. The objectives of VHE gamma-ray astronomy are presented, outlining the pressing need for complex observations of individual objects.     

Gamma-ray bursts and very high energy gamma-ray astronomy

The current status of gamma-ray burst astronomy is reviewed briefly, with emphasis on experiments which might determine the burster distance scale. Recent observations by the EGRET experiment aboard the Compton Observatory, indicating the presence of delayed, high energy emission, are summarized. It is shown that searches for TeV emission from burst sources are feasible, and may be fruitful. Detection of such emission might resolve the question of whether burst sources are at cosmological distances.     

The relevance of molecular observations to gamma-ray astronomy

The interaction of cosmic rays with interstellar clouds may produce some of the observed gamma-ray sources. The use of molecular observations to estimate the cloud masses, which are used to derive cosmic-ray fluxes, is reviewed. Molecular diagnostics of high cosmic-ray ionization rates are discussed, and a detailed application of those diagnostics is summarised and presented as evidence that second-order Fermi acceleration is important in old supernova remnants and can produce cosmic rays of too low energy to induce gamma-ray emission.Proceedings of the XVIII General Assembly of the IAU: Galactic Astrophysics and Gamma-Ray Astronomy, held at Patras, Greece, 19 August 1982.Royal Society Jaffé Donation Fellow.     

A space observatory for astrophysical explorations in the REALM of gamma-ray astronomy

Conclusion The space observatory whose major systems are outlined in the present paper is being designed to ensure astrophysical explorations in the field of -ray astronomy during the course of one year. The observatory is supposed to travel along a circular orbit up to 400 km in altitude, inclined at 51.6°. Investigations to be accomplished in the observatory will enable us to make substantial progress in gaining a deeper insight into a wide variety of astrophysical phenomena.     

All-sky monitors for X-ray astronomy

We discuss the rationale for a semi-permanent all-sky X-ray monitor, and investigate a variety of options for its implementation. We conclude that the Space Station offers an excellent opportunity for hosting such a monitor, and that a set of pinhole cameras can be configured to provide an effective and economical monitor system. A baseline of six independent pinhole modules, each of which requires approximately one cubic foot, 30 pounds, 2 watts, and 100 bits per second, can provide full sky coverage with scientifically interesting sensitivities. No other resources or special accommodation (such as detailed alignment registration, time-tagging or on-orbit servicing) would be required. The baseline system can locate bright sources to a few arc min, and can simultaneously measure each of the several hundred sources in the sky brighter than a few thousandths the intensity of the Crab nebula every day for decades.     

Infrared astronomy

This review paper is a survey of infrared astronomy up to early 1969. The techniques and photometric standards are mentioned briefly, and results cover solar, lunar, and planetary observations. Point sources and extended sources both within and beyond the Galaxy are included, ending with the problem of cosmic background radiation. It is concluded that great progress will be possible when large infrared telescopes are placed above the atmosphere in orbit for extended periods of time.This work was conducted under the McDonnell Douglas Corporation Independent Research and Development Program.     

Gamma-ray line astronomy

Recent gamma-ray line observations and their interpretations are reviewed and prospects for future line detections are discussed.     

Position sensitive multiwire proportional counters for X-ray astronomy

The state of the art of the existing position sensitive multiwire proportional counters (PSMWPC) will be briefly reviewed. In particular taking into account the contribution these detectors have already made to our knowledge of the X-ray sky and the expected future steps that will only be possible with new detector development.Finally a new design of multiwire proportional counter will be described. The new generation of this kind of PSMWPC detectors, when operated with a modulating mask or a single grid rotation modulation collimator or, in the future, with a combination of these two techniques will provide images of the high-energy sky with an angular resolution of l min of arc.     

Wide angle X-ray optics for use in astronomy

Recently there was a suggestion in the literature to apply the principle of the lobster-eye to X-ray astronomy imaging (J.R.P. Angel, Ap. J. 233, 364, 1979). Our own suggestion for a wide angle X-ray telescope made earlier (W.K.H. Schmidt, Nucl. Instr. Meth.127, 285, 1975) is very similar to the above one. It consists of one or two sets of plane mirrors used in a grazing incidence configuration. The advantages of this type of X-ray optics over other systems for particular astronomical observations will be discussed.     

Aspects of neutrino astronomy

A broad overview of the current status of experimental neutrino astronomy is presented. Particular emphasis is given to the major recent developments that have occurred during the last few years. It is concluded that these developments and the next generation of experiments currently being installed signifies the coming of age of neutrino astronomy.     

A high-resolution Ge spectrometer for Gamma-Ray burst astronomy

The Transient Gamma-Ray Spectrometer (TGRS) to be flown aboard the WIND spacecraft is primarily designed to perform high resolution spectroscopy of transient -ray events, such as cosmic -ray bursts and solar flares over the energy range 25 keV to 8.2 MeV with an expected spectroscopic resolution of 3 keV at 1 MeV. The detector itself consists of a 215 cm³ high purityn-type Ge crystal kept at cryogenic temperatures by a passive radiative cooler. The geometric field of view defined by the cooler is 1.8 steradian. To avoid continuous triggers by soft solar events, a thin BeCu Sun-shield around the sides of the cooler has been provided. A passive Mo/Pb occulter, which modulates signals from within ±5° of the ecliptic plane at the spacecraft spin frequency, is used to identify and study solar flares, as well as emission from the galactic plane and center. Thus, in addition to transient event measurements, the instrument will allow the search for possible diffuse background lines and monitor the 511 keV positron annihilation radiation from the galactic center. In order to handle the typically large burst count rates, which can be in excess of 100 kHz, burst data are stored directly in an onboard 2.75 Mbit burst memory with an absolute timing accuracy of ±1.5 ms after ground processing. The memory is capable of storing the entire spectral data set of all but the largest bursts. WIND is scheduled to be launched on a Delta II launch vehicle from Cape Canaveral on November 1, 1994. After injection into a phasing orbit, the spacecraft will execute a double lunar swing-by before being moved into a controlled halo orbit about theL1 Lagrangian point (250R _e towards the Sun). This will provide a 5 light-second light travel time with which to triangulate gamma-ray burst sources with Earth-orbiting systems, such as those on-board the Gamma-Ray Observatory (GRO). The response of instrument to transient -ray events such as GRB's and solar flares will be presented as well as the expected response to steady state point sources and galactic center line emission.     

Extragalactic γ-ray astronomy

Space Science Reviews - Extragalactic γ-ray astronomy is in its infancy. In this paper a review is presented of the γ-ray emission of the few individual objects observed to date and some...     

On-Line Computer for Transient Turbine Cascade Instrumentation

A 32-channel computer based data acquisition and processing system em has been developed for use with the new type of transient cascade facility at Oxford. This is used for testing turbine blades and nozzle guide vanes at full-scale engine Reynolds and Mach numbers ers with correct wallto-flow temperature ratios. A novel technique for processing transient heat transfer data from thin film surface resistance thermometers has been developed. Measurements of surface ace pressure around blades, and of the upstream turbulence level have been made. The cascade and instrumentation are shown to have advantages both in cost and effectiveness over continuous running cascades.     

Cosmic gamma-ray bursts

The observational characteristics of gamma-ray bursts are reviewed, concerning their spectra as well as their temporal structure and spatial distribution. From this data, it is suggested that the sources belong to a thick halo population (scale height > 3 kpc), and that the mean recurrence time for one source is greater than 5 yr. The implications of these results are discussed, concerning the future experimental perspectives of detection of gamma-ray bursts, and also the constraints on theoretical models.     

Techniques in balloon X-ray astronomy

The first balloon observation of a cosmic X-ray source, the Crab Nebula, was made in 1965, only three years after the initial discovery of such sources by rocket observations. Since then balloon data has provided much information on the positions, spectra, time variability and pulsed nature of localized sources, and on the spectrum and isotropy of diffuse galactic and universal components. Measurements are limited to energies above about 20 keV by atmospheric attenuation at 2–3 g cm ^–2 depth and to below several hundred keV by detector sensitivity. Detectors usually consist of large area NaI or CsI scintillation counters with anticoincidence collimators for rejection of charged particles and scattered X-rays. Proportional counters are occasionally used at lower energies and solid state detectors are used where extreme energy resolution is important. The instruments require a pointing capability on the order of 1.0 to 0.1°, depending on the collimator aperture. Digital data is either recorded on board or telemetered using a PCM technique. Exploratory work in the 0.2–10 MeV -ray range is starting now, and balloon observations may be expected to make important contributions in the near future.     

Point-like gamma-ray sources

Various models are examined, which could give rise to point-like gamma-ray sources, at the present time indistinguishable, experimentally, from true point sources. These models involve energetic processes associated with interstellar clouds, e.g. supernova-cloud interactions, neutron star accretion inside interstellar clouds, cloud collisions, etc. The dynamical evolution of such systems is discussed and physical processes are described in a mathematical framework which can be solved. Statistical arguments are presented, where possible, on the likelihood that the scenarios may actually occur in our Galaxy. The visibility of the systems at other wavelengths, e.g. infrared, X-ray, radio etc., and further consequences, e.g. gamma-ray line emission, special radio emission line features, absorption features etc. are also discussed. Finally, a limited attempt at identification of some gamma-ray objects is made based on the theoretical predictions.Proceedings of the XVIII General Assembly of the IAU: Galactic Astrophysics and Gamma-Ray Astronomy, held at Patras, 19 August 1982.