The current state of gamma-ray astronomy is reviewed in terms of the most recent experimental results that define, in either measured fluxes or upper limits, the cosmic photon spectrum in the energy range from 0.5 MeV to 1016 eV. Methods of research developed during the last 10 yr are discussed and specific results are cited. Cosmic gamma rays have been observed in two regions of the spectrum – from 0.5 to 6 MeV and from about 50 MeV to several hundred MeV. Above 50 MeV a strong concentration of intensity is observed in a band of directions around the galactic equator. In both spectral regions a diffuse background gamma ray flux, apparently isotropic and of extragalactic origin, is observed. Evidence of ~ 50 MeV gamma-ray pulses from the Crab pulsar has been reported. No definite evidence of any other discrete gamma-ray source besides the galaxy itself has been observed at any energy.

The detecting systems used in high energy astrophysics are generally more similar to particle detectors than to optical devices. The basic design of the gamma ray instrument depends on whether the energy range is below about 10 MeV and therefore in the region where the Compton effect predominates in the absorption of the gamma-rays, or above that energy where electron-positron pair production is most important. The most usual approach to the detector system in the lower of the two energy intervals is to use a scintillation counter in the center of the detector system to absorb the photons and permit a measure of their energy, and to surround it by another detector which is employed as an active anticoincidence shield to discriminate against charged particles. In the gamma-ray interval above about 10 MeV, the very low flux of gamma rays and the high particle background has directed the development of high energy gamma-ray telescopes towards complicated techniques and large detector arrays. As a result, several investigators have now turned to the spark chamber as the heart of a detector system. Generally, it is surrounded by an anticoincidence system and is triggered by a counter telescope.

The ESRO satellite COS-B carries one single experiment aiming at the measurement of arrival direction and energy of celestial gamma rays with energies between 25 MeV and 10 GeV. The experiment is conventional in design and consists of a veto counter, a wire spark chamber, a telescope and an energy calorimeter.

The energy measurement is obtained by a CsI scintillation crystal of 4.7 radiation length thickness. The expected energy resolution at 100 MeV is 50% FWHM. The other detector elements are designed as to cause the least possible degradation of the energy measurement.

The possibilities for the detection of a small contribution of π -origin gamma rays in the presence of a power-law type background spectrum will be discussed.

A high energy solar gamma-ray telescope incorporating a lenticular Čerenkov for directional measurement and an energy calorimeter is described. The instrument is included in the payload of the TD-1 ESRO spacecraft to be launched into a sun-pointing orbit during spring 1972. The results of laboratory and accelerator tests are presented and the sensitivity and measurement capability to solar flare gamma rays is discussed.

A low energy gamma-ray telescope (0.2–3 MeV) has been studied in our laboratory. It is made up of an active honeycomb collimator of Nai(T1), with a geometrical angular aperture of 1.5°, and of a detector composed of 73 Nai(T1) sticks (0.5 cm diam.–5 cm length) incorporated in a plastic gabarit used in anticoincidence.

The directivity of the telescope is obtained at low energy by the collimator, and at higher energy by the directivity which exists in the gamma-ray interaction through the Compton and photoelectric effects. The electrons emitted in these interactions which are not contained in the sticks of Nai(T1) are detected by the plastic anticoincidence scintillator. Only laboratory tests are presented here, for the study of the galactic gamma-rays, however, the system must subsequently be protected by a 4 cm thick anticoincidence well. In fact the detector directivity enables a great increase in the signal to noise ratio to be expected without greatly increasing the thickness of the shielding.

According to the results obtained the estimation of the minimum exposure time for the ‘Crab Nebula’ would be about 2h30m to have a signal at 1 MeV with 2σ of confidence above the background.

A stilbene scintillator detector allowing gamma-ray spectrometry in the range 0.5–5 MeV is presented. A complete elimination of charged particles is obtained by a plastic scintillator anticoïncidence jacket. Separation of gamma rays from neutrons is made by pulse shape discrimination technique with over 99% efficiency. This detector which has a 4 π field of view has been made as light as possible to avoid perturbation due to secondary production in the apparatus. The correction of the edge effects and the method of conversion from experimental Compton electron spectrum to gamma-ray spectrum are explained.

Results from balloon launchings at three latitudes (Kourou Guyana: 10 N, Aire sur l'Adour: 46°N and Oboziersky U.S.S.R.: 62 °N) are briefly presented. The detection possibility with balloons of galactic gamma rays at equatorial latitude is shown. The atmospheric part of the flux at the equator is deduced from the measurements at higher latitudes, (46 °N and 62 °N) where the galactic component is of negligible importance. Assuming a power law spectrum and after correction of the atmospheric absorption we obtain for the galactic spectrum the expression dN/dE = 1.1 × 10−5E−1, 2 photons/cm2 s sr keV. This spectrum agrees with the results of ERS 18 satellite given by Vette et al. showing an excess of flux for energies higher than 1 MeV.

Problems of making a device for directional measurements of extraterrestrial gamma rays in the MeV-range are discussed – especially background counting rates. A detector system is described which is planned to be flown in a series of balloon experiments in order to provide a better understanding of the sources of background and their elimination.

The registration of gamma rays in a spark chamber has been simulated by Monte-Carlo-calculation technique. The spark chamber pictures of these gamma-ray events having a known energy and direction of incidence have been analysed for determinability of direction of incidence. The values obtained for angular resolution depending on energy are compared with values derived by other authors.

Experimental upper limits of the high-energy gamma-ray flux (above 100 MeV) from point sources are about 10−5 cm−2 s−1, and realistic theoretical predictions for the strongest sources are an order of magnitude lower than this, while the diffuse background flux is about 4 × 10−5 cm−2 s−1 sr−1 in space, and 100 times higher at balloon altitudes. To meet the need for instrumental sensitivity and angular resolution adequate to measure the small but important gamma-ray source strengths, a telescope of large area (5 m2) and fine angular discrimination (0.5 deg at 300 MeV, 0.3 deg at energies above 1 GeV) has been developed.

A gas-Čerenkov detector sensitive to gamma radiation above 10 MeV is currently undergoing final testing. The detector relies on the conversion and Compton scattering of gamma rays in a plastic scintillator and detecting the resulting electrons via the Čerenkov light they emit in a 2-m propane-gas column. Spectral information can be attained by varying gas pressure during the flight. The present detector is approximately 34″ in diameter, 91″ in length and weight 450 lb. At 20 MeV, an angular resolution of 6° half angle is expected. With an efficiency of 1 to 2%, a 10 hr balloon-borne system should achieve a point-source sensitivity of approximately 5× 10−5 photon cm−2 s−1 above 20 MeV. A satellite version of the detector is expected to have a sensitivity of approximately 1.3 × 10−5 photon cm−2 s−1 above 10 MeV for a 1-month galactic-plane scan mode. (Helmken and Hoffman, 1970.)

A gamma-ray telescope with a 850 cm2 multiplate spark chamber as a detector, has been used in an experiment of six balloon flights to investigate the Crab Nebula Pulsar 0532. The triggering signal is provided by a plastic scintillator-directional Čerenkov counter system. The spark chamber events are photographed by a stereo camera. The time of arrival of each event is recorded in UTC time with one millisecond accuracy, to search for a possible pulsed gamma emission from NP0532 above 50 MeV.

I shall review recent observational results on discrete X-ray sources and on the diffuse background component. Thus far, approximately forty sources have been reported. It is to be noted that the sources can not be classified in only one or two categories and that there are a variety of the sources. Figure 1 shows the situation.

The limitations imposed in X-ray astronomy observations by the use of sounding rockets with their inherent short time for observations and restricted payload capabilities will soon be surmounted when the new generation of X-ray experiments carried aloft by earth orbitting satellites is launched.

In the early 1970's several payloads included in the spacecrafts SASA, OSOH, SAS C, OSO 1, UK 5 and ANS will perform high sensitivity surveys of the entire celestial sphere. As a result of these experiments we expect to: increase the sensitivity of our observations by a factor of approximately 100; to improve the measurement of location on all known sources and on newly discovered ones to an accuracy of ½ to 6 arc min; to acquire more definite information on rapid time variations of the X-ray sources; to measure the polarization of the incoming X-ray radiation to several per cent for the strongest sources; and, finally, to determine for the strongest sources the existence of emission lines or absorption edges to the accuracy required to confirm or disprove present theories of source of emission processes and interstellar absorption.

Beyond these surveys a new generation of X-ray instrumentation utilizing focussing X-ray telescopes is in the planning stage. These instruments will yield a qualitative advance in the type of observations that can be performed. Location and structure of X-ray sources can be determined with these techniques to precisions of a few arc seconds to a limiting surface brightness of 10−14 erg/cm2 s (arc s2). Spectroscopy of the strongest sources can be carried out with energy resolutions λ/Δλ of 103 to 104. Polarization measurements can be carried out to a few per cent on the Crab and rapid varying events can be followed with a time resolution of 10−4 s.

A summary of the instrumentation will be given from the point of view of the developments and improvements which are being carried out to achieve the observational goals mentioned above.

High energy astrophysical observations supported by the National Aeronautics and Space Administration will be conducted primarily from the Small Astronomy Satellites (SAS) and the High Energy Astronomy Observatories (HEAO). At the present time, three Small Astronomy Satellites have been approved for flight. The first (SAS A) will carry a set of collimated proportional counters to conduct a high sensitivity, high spatial resolution, all sky survey for X-ray sources, to search for temporal variations in the source intensity, and to measure the spectral distribution of sources in the energy range 1 to 20 keV. The spacecraft will carry two collimator systems of 1 by 10 deg and 10 by 10 deg fullwidth. The proportional counters are gas-filled beryllium window tubes. SAS A is planned for launch in late 1970.

Work has been in progress for many years at University College London to determine how the intensity spectrum of emitted X-rays changes as a function of time and position on the solar disk. Early spectroheliographs consisted of grazing incidence parabolic mirrors which focussed radiation onto proportional counters from which spectral data were obtained. Such instruments, built in collaboration with the University of Leicester Group, have been flown on two rockets (Negus et al., 1969) and on the OSO V satellite (Herring et al., 1971).

In the last year there have been several proportional counter observations of Sco X-1 which give an indication of an emission feature at around 7 keV (Holt et al., 1969; Acton et al., 1970). The emission lines of Fe+24 and Fe+25, which are very strong in radiation from solar flares of comparable plasma temperature, may be responsible for this feature. Two factors, poor energy resolution and the steeply varying continuum spectrum whose shape is not known in detail, make it impossible to obtain with proportional counters an unambiguous identification of emission features. For this reason we have constructed a Bragg crystal spectrometer to scan the spectrum of Sco X-1. The instrument, flown successfully aboard an Aerobee-170 rocket in April 1970 in conjunction with the Kitt Peak National Observatory, scanned the spectral range from 2.4–2.9 keV which includes the Lyman-α line of hydrogenic sulphur (2.62 keV). The spectrum obtained shows no feature at this energy. The sensitivity of the instrument may be judged from the upper limit (3σ) of 0.08 photon/cm2/s which we are able to place on the line intensity. This value can be compared with the flux of 0.55 photon/cm2/s predicted by Tucker (1967) for the S+15 line intensity from an isothermal, optically thin plasma at 5× 107K. If the line had had this theoretical intensity, we would have observed a 21-σ signal.

One of the outstanding problems in X-ray astronomy is the detection and understanding of a number of important phenomena that are expected at photon energies in the range from 100 eV to 1000 eV. One expects to be able to detect interstellar matter in both absorption and emission. It is of critical importance to cosmology to determine the spectrum, isotropy and intensity of extragalactic radiation in this energy region. It is also expected that a large number of discrete sources with temperatures in the range from 105K to 106K will be detected. Such sources could result from neutron stars that are cooling or from the accretion of interstellar matter onto condensed objects. Matter falling into ‘black holes’ may be heated sufficiently to be detectable at long X-ray wavelengths. Extended objects, such as the Cygnus Loop and the Magellanic clouds, are known to be luminous at low X-ray energies (Grader et al., 1970; Gorenstein, 1970); other such objects are almost certainly present. There may be local sources of diffuse radiation. Finally, it should be possible to detect X-ray zodiacal light.