Introduction

The discovery of γ-ray bursts was serendipitous, as was that of pulsars, which were discovered at about the same time. Pulsars were first detected in 1967 in an experiment designed to study interplanetary scintillation of compact radio sources, and the discovery paper (Hewish et al. 1968) was subsequently published; the first γ-ray burst (GRB) was also seen in the year 1967 (although not reported until six years later; see Strong and Klebesadel 1976 for an account of the chronology) in a satellite-borne detector intended to monitor violations of the nuclear explosion test ban treaty. The publication of the discovery of GRBs was first made in 1973 by Klebesadel, Strong and Olson (1973). The detector comprised six caesium iodide scintillators, each of 10 cm3, mounted on each of the four Vela series of satellites (5A, 5B, 6A and 6B), these vehicles being arranged nearly equally spaced in a circular orbit with a geocentric radius of ∼ 1.2 × 105 km. The detectors were sensitive to individual γ-rays in the approximate energy range 0.2−1.5 MeV and the detector efficiency ranged from 17 to 50%. The scintillators had a passive shield around them; background γ-ray counting rates were routinely monitored. A statistically significant increase in the counting rates initiated the recording of discrete counts in a series of quasi-logarithmically increasing time intervals. The event time was also recorded. Data were telemetered down to the ground-based receiving stations.

The popularity of the subject of gamma-ray astronomy has led to the need to update the material presented in the first edition, and this we are pleased to do.

The subject is in an exciting state in the lower energy region, below some tens of GeV, with the successful launch of the Gamma Ray Observatory in April, 1991. Already, sufficient data have appeared to show that, barring unforseen accidents, the subject will march forward at these energies. It is unfortunate that the Soviet GAMMA 1 satellite did not meet its design specifications – a reminder of the difficulties still inherent in satellite experiments.

The supernova SN 1987A continues to provide data of interest to the gamma-ray astronomer, and the results achieved so far have been included in this edition.

At the higher energies, advances have been less spectacular; indeed, there is some disappointment that many of the claimed TeV and PeV sources have still not been confirmed. Our view is that time variability of genuine sources married with some spurious signals probably accounts for the situation. Nevertheless, the subject is so important that continued, indeed enhanced, effort is needed.

The rate of publications in the field of gamma-ray astronomy at all energies is several times higher now than in 1985, when the manuscript for the first edition was turned in to the editors. Although we have made every effort to make the presentation in the second edition up to date (till the end of July, 1991), we apologise for inadvertent omission of any important results prior to that date.

Gamma-ray astronomy comprises the view of the Universe through what is essentially the last of the electromagnetic windows to be opened. All other windows from radio right through to X-rays have already been opened wide, and as is well known their respective astronomies are quite well developed – and the views there are very rich. Gamma-ray astronomy promises to be likewise; the strong link of γ-rays to very energetic processes and the considerable penetration of the γ-rays see to that.

Admittedly one deals with a small number of photons in this new window and yet a considerable amount of progress has already been made; hopefully this progress will shine through in what follows.

It is usually necessary to make a selection of topics when writing a book, and the present one is no exception. The selection made here reflects both the interests of the authors (both of whom are cosmic ray physicists) and the perceived needs of the subject. The authors' interests and, no doubt, biases show through in the areas in which they have themselves contributed (Chapters 4 and 5). There appears to be a contemporary need for a comprehensive review of γ-ray bursts and this is the reason for an extended Chapter 3. We have not included in Chapter 2 any material relating to γ-ray lines in solar flares – a very important subject in its own right – as we felt that it was outside the character of this book, dealing as it does with source regions exclusively beyond the solar system.

Introduction

The spectroscopy of γ-ray astronomy is, understandably, an area where important advances are to be expected, an expectation born of similar previous experience with other regions of the electromagnetic spectrum. Technical difficulties are considerable at present, however, due to low line fluxes aggravated by serious background problems; nevertheless, a promising start has been made and several interesting observations have already appeared.

As with astronomy in general, a distinction can be made between observations of ‘discrete’ objects (such as stars, supernovae, other galaxies, etc.) and signals from more extended regions, in particular the interstellar medium (ISM).

In the first category, γ-ray lines from the Sun – due to energetic protons and heavier nuclei interacting with the solar atmosphere – provide interesting and important information about a variety of solar phenomena. This subject of solar γ-ray spectroscopy is distant from the main stream of topics discussed here, and the reader is directed to a number of useful reviews by Ramaty and Lingenfelter (1981), Trombka and Fichtel (1982), Ramaty and Murphy (1987), and the books by Chupp (1976) and Hillier (1984).

In the non-solar region, which is of main concern here, only a few γ-ray lines have been detected from non-transient celestial sources so far. These include the lines at 1809 keV from the Galactic Equatorial Plane, the line at 511 keV from the Galactic Centre region and the one at 1369 keV from the object SS 433; these will be described in Sections 2.2, 2.3 and 2.4, respectively.

Introduction

Studies of ultra high energy gamma-rays (UHEGR) i.e. γ-rays at energies greater than 100 GeV, provide us with information on the conditions existing in remote celestial regions, such as magnetic and electric fields, matter and radiation densities, and on the acceleration mechanisms of charged particles. Additionally such studies have an important bearing on the problem of the origin of the cosmic radiation. There is, as yet, no universally accepted identification of either the sources or the mechanisms of production of cosmic rays, though, as was pointed out in Chapter 4, there are strong arguments made in favour of some. The problem is confounded by the fact that cosmic rays, almost all of which are charged particles, undergo frequent deflections in the interstellar magnetic fields, making it impossible to know the source directions. Thus, even a primary cosmic ray proton of energy as high as 1015 eV has a Larmour radius in the ISM of only ∼ 0.3pc and has its initial direction almost isotropised. Electrically neutral radiation is free from this problem. The more commonly occurring neutral particles are neutrons, neutrinos and γ-rays. Neutrons are unstable; they would not survive in most cases from source to Earth even after allowing for relativistic time dilatation, with a decay mean free path of only 9.2 (E/1015 eV) pc. Neutrinos, being weakly interacting, are not easy to detect, γ-rays, on the other hand, are ideal as their production and interaction cross sections are rather high and they are stable.

Introduction

Thermal noise is unavoidable and sets the fundamental limit to the detectability of the response of an oscillator to any gravitational effect, but it is not the only disturbance to which an oscillator may be subject. Other forces may act on the mass of a torsion pendulum if it is subject to electric or magnetic or extraneous gravitational fields. The point of support of a torsion pendulum or other mechanical oscillator may be disturbed by ground motion. Ground motion is predominantly translational and so might be thought not to affect a torsion pendulum to a first approximation. However, all practical oscillators have parasitic modes of oscillation besides the dominant one, and although in linear theory normal modes are independent, in real non-linear systems modes are coupled. Thus, even if in theory seismic ground motion had no component of rotation about a vertical axis, none the less there would be some coupling between the primary rotational mode of a torsion pendulum and its oscillations in a vertical plane. In practice, therefore, any disturbance of a mechanical oscillator may masquerade as a response to a gravitational signal.

External sources of noise can be avoided with proper design of experiments. In this chapter we shall discuss both the sources of external disturbance and also the ways in which oscillators of different design respond.

Ground disturbance

Sources of ground noise

We begin with a discussion of seismic motions that move the point of support of a pendulum.

Introduction

For reasons concerned with the availability of contemporary γ-ray data, the lower limit for ‘medium energy’ quanta can be taken as 35 MeV (this is the lower limit for the important SAS II satellite experiment). The upper limit again comes from satellite data availability and is rather arbitrarily taken as 5000 MeV, the upper limit of the highest COS B satellite energy band; in fact, the photon flux falls off with energy so rapidly that our knowledge about γ-rays above 1000 MeV from satellite experiments is virtually nil. As will be discussed in Chapter 5, however, knowledge blooms again above 1011 eV, where Cerenkov radiation produced by γ-ray-induced electrons in the atmosphere allows detections to be made.

Although there are some who still believe that unresolved discrete sources contribute considerably to the diffuse γ-ray flux, the majority view is that the sources are responsible for only 10−20% of the γ-ray flux and that the predominant fraction arises from cosmic ray (CR) interactions with gas and radiation in the interstellar medium (ISM). In fact, some 30 years ago, both Hayakawa (1952) and Hutchinson (1952) had made estimates of the CR–ISM-induced γ-ray flux and had shown it to be within the scope of experimental measurement.

The foregoing is not to say that the discrete sources are unimportant, indeed the reverse is true, and there is considerable interest in ways of explaining the observed γ-ray flux from identified sources (the Crab and Vela pulsars) and the unidentified but definite sources such as Geminga (2CG 195 + 04 in the COS B source catalogue of Hermsen 1980, 1981).

Introduction

The essence of the principle of equivalence goes back to Galileo and Newton who asserted that the weight of a body, the force acting on it in a gravitational field, was proportional to its mass, the quantity of matter in it, irrespective of its constitution. This is usually known as the weak principle of equivalence and is the cornerstone of Newtonian gravitational theory and the necessary condition for many other theories of gravitation including the theory of general relativity. In recent times, however, it was found that the weak principle of equivalence was not sufficient to support all theories and the principle has been extended as (1) Einstein's principle of equivalence and (2) the strong principle of equivalence.

Following a brief discussion of the principle of equivalence, this chapter is devoted to an account of the principal experimental studies of the weak principle of equivalence.

Einstein's principle of equivalence

Gravitation is one of the three fundamental interactions in nature and a question at the heart of the understanding of gravitation is whether or how other fundamental physical forces change in the presence of a gravitational force.

Einstein answered this fundamental question with the assertion that in a non-spinning laboratory falling freely in a gravitational field, the non-gravitational laws of physics do not change. That means that the other two fundamental interactions of physics – the electro-weak force and the strong force between nucleons – all couple in the same way with a gravitational interaction, namely: in a freely falling laboratory, the non-gravitational laws of physics are Lorentz invariant as in the theory of special relativity.

Introduction

In tests of the weak principle of equivalence, exact calculations of the attractions of masses are not necessary, but they are essential in experiments to test the inverse square law and to measure the gravitational constant. In fact, the calculation of the gravitational attraction of laboratory masses is usually not at all simple, because the dimensions of the masses are comparable with the separations between them, so that neither the test mass nor the attracting mass can be treated as a point object. In the following sections we discuss the gravitational attractions of laboratory masses with various common geometrical shapes. We present the results in terms of the gravitational efficiency, that is, the ratio of the gravitational attraction of a laboratory mass at a certain separation to that of a point mass with the same mass and separation. Furthermore, the precision demanded in measurements of separations of masses, the most difficult measurements in the determination of G and the test of gravitational law, depends on the geometry of the masses. These effects can have a strong influence on the conduct and final results of an experiment and it is essential to discuss in detail the calculation of potentials and attractions before going on to describe experiments.

Masses of three forms are often used in the laboratory: spheres, cylinders and rectangular prisms. The formula for the gravitational attraction of a sphere is well known and simple, but in practice it is not possible to manufacture an ideal sphere, the practical problem is usually how the real precision of manufacture affects the results; cylinders and prisms can be made very precisely but calculating the attraction is difficult.

Introduction

Although the weak principle of equivalence has been verified for ordinary macroscopic matter to very high precision, two questions remain open:

1. Is the principle valid for antimatter? Although indirect evidence from virtual antimatter in nuclei and short-lived antiparticles suggests that antimatter may have normal gravitational properties, no direct tests of the validity of the weak principle of equivalence for antimatter have been made.

2. Is the principle valid for microparticles? As the test bodies in macroscopic experiments are formed of neutrons, protons and electrons bound in nuclei, there is no doubt about the validity of the weak principle of equivalence for bound particles. However, the possibility of the principle of equivalence being violated for free particles should be studied.

Two main features characterize laboratory tests of the weak principle of equivalence for free elementary particles, both the consequence of their small masses. (1) When forces on substantial masses of bulk material are compared, a null experiment based on comparing different test bodies of two kinds of material can be devised. That is not possible for microscopic particles, and the gravitational accelerations have to be measured directly and subsequently compared with the acceleration of ordinary bulk matter to obtain the Eötvös coefficient. (2) The gravitational forces are very weak, even in the field of the Earth (which is the strongest attractive field), and so the accuracy of any experiment is very poor compared with Eötvös-type experiments using bulk masses.