Supernovae

Classification of novae and supernovae

The ancient astronomers had already noted that sometimes new stars became visible in the sky and after some time disappeared again. In the Middle Ages the astronomers called these stars novae, which is the Latin word for new stars. Some of these new stars were exceedingly bright, and were later called supernovae. Three of these supernovae were observed in historic times: Tycho de Brahe's supernova, which occurred in the year 1572, Kepler's supernova which became very bright in the year 1604, and a supernova which was observed by Chinese astronomers in the year 1054. At the location of the Chinese supernova we now see the Crab nebula in the constellation of Taurus. The nebula got its name from its appearance which reminds us of a crab. The Crab nebula still expands with velocities of about 1400 km s−1, showing that a truly gigantic explosion must have occurred 900 years ago.

What are these novae and supernovae? How often do they occur? What kinds of objects are their progenitors? What leads to such gigantic explosions? What distinguishes novae and supernovae? Are all supernovae similar events or do we have to distinguish different kinds of novae or supernovae? These are questions for which we would like to find the answers.

Both novae and supernovae are objects which suddenly increase their light output by many orders of magnitude.

General discussion

In the previous sections we have discussed stars which are generally considered to be normal stars, which means their spectra fit into the two-dimensional classification scheme according to spectral type and luminosity. True, the weak-lined stars, or population II stars, do not fit into that scheme, but generally their peculiarity can be understood by the change of just one parameter, the ratio of the metal abundance to the hydrogen abundance, though recently it has been found that this may not always be the case. More than one parameter may actually be necessary to describe the abundances of the heavy elements. The population II stars are still generally considered to be ‘normal’ stars because we believe that all their peculiarities can apparently be traced back to different chemical abundances. For the stars we are going to discuss in this chapter, this does not seem to be the case. There are, of course, a large number of different kinds of peculiar stars, but we are not able to discuss all of them in the framework of this introduction to stellar astrophysics. We shall only discuss the most frequent kinds of peculiar stars and those which are of special interest in the framework of understanding stellar structure and evolution.

Peculiar A stars, or magnetic stars

The observations

In the previous section we saw that some stars with very strong magnetic fields are found among the early A stars.

Introduction

Probably the most radical advance in X-ray instrumentation in the past five years has been the development of the single photon calorimeter, in which X-rays are detected via the temperature pulses they induce in a small (< 1 mm3) absorber, cooled to a fraction of a degree kelvin.

The detection of individual 5.9 keV X-rays (fig. 6.1) was first demonstrated by groups at NASA's Goddard Space Flight Center (GSFC) and the University of Wisconsin in 1984, using a silicon microcalorimeter operating at 0.3 K (McCammon et al, 1984). This work was specifically directed towards the production of a high-efficiency, non-dispersive focal plane spectrometer with energy resolution comparable to that of a Bragg crystal. It can, however, still be seen as the culmination of several decades’ research in fields other than X-ray astronomy, originally in nuclear physics and latterly in infrared astronomy. Andersen (1986) and Coron et al (1985a) trace calorimeter development back as far as 1903, and the radioactivity studies of Pierre Curie. They record how, by the mid-1970s, the sensitivity (in detectable watts) of IR bolometers operating at liquid helium temperatures, where heat capacities are very low, had reached the point where Niinikoski and Udo (1974) could identify the extraneous spikes seen in the output of balloon-borne bolometers with local heating produced by the passage of cosmic rays. Niinikoski and Udo appear to have been the first to suggest that it might be possible to thermally detect single photons or particles, rather than continuous fluxes.

Introduction

This chapter describes the uses-past, present and proposed-of three types of solid X-ray converter in soft X-ray astronomy: scintillators and phosphors, which work by the conversion of X-ray energy into visible light and negative electron affinity detectors (NEADs), which rely on external photoemission from a surface activated to a state of negative electron affinity. Although the terms scintillator and phosphor are formally synonyms (Thewlis, 1962), we shall adopt the usage prevalent in the detector literature and distinguish between bulk, crystalline materials such as Nal(Tl) and CsI(Na) (scintillators) and thin, granular layers of, for example, the rare earth oxysulphides (phosphors). Phosphors are often identified by a commercial T-number’. A partial list of such numbers is given by Gruner et al (1982).

The use of luminescent solids in nuclear physics has a long tradition. Rutherford's nuclear model of the atom (1909), for example, had as its experimental basis the observation, by eye, of α-particle induced light flashes (scintillations) on a zinc sulphide screen. The substitution of a photomultiplier tube for the human observer, which first occurred towards the end of the second world war, produced a sensitive electronic counter for γ-rays and particles, whose operation is described in texts such as those of Curran (1953), Birks (1964) and Knoll (1979).

The first use of a scintillation counter in X-ray astronomy was in a balloon-borne observation of the Crab Nebula in 1964 (Clark, 1965). As described in section 1.2, such balloon payloads were limited, because of atmospheric opacity, to the spectral band E > 20 ke V, where source fluxes decrease rapidly with increasing X-ray energy.

Introduction

MicroChannel plates (MCPs) are compact electron multipliers of high gain and military descent which, in their two decades as ‘declassified’ technology (Ruggieri, 1972), have been used in a wider range of particle and photon detection problems than perhaps any other detector type.

A typical MCP consists of ∼ 107 close-packed channels of common diameter D, formed by the drawing, etching and firing in hydrogen of a lead glass matrix. At present, the most common values of D are 10 or 12.5 μm, although pore sizes as small as 2 μm have begun appearing in some manufacturer's literature. Each of the channels can be made to act as an independent, continuous-dynode photomultiplier. MicroChannel plates (or channel multiplier arrays or multichannel plates, as they are sometimes known) are therefore used, in X-ray astronomy as in many other fields, for distortionless imaging with very high spatial resolution.

The idea of replacing the discrete dynodes (gain stages) of a conventional photomultiplier (Knoll, 1979) with a continuous resistive surface dates from 1930 (Ruggieri, 1972). It was only in the early 1960s, however, that the first channel electron multipliers (CEMs), consisting of 0.1-1 mm diameter glass or ceramic tubes internally coated with semiconducting metallic oxide layers, were constructed in the USSR (Oshchepkov et al. 1960) and United States (Goodrich and Wiley, 1962). Somewhat later, parallel-plate electron multipliers (PPEMs) were developed with rectangular apertures more suited to the exit slits of certain types of spectrometer (Spindt and Shoulders, 1965; Nilsson et al. 1970).

Introduction

Gas proportional counters have been the ‘workhorses’ of X-ray astronomy throughout the subject's entire history. The roots of proportional counter development, however, go back much further, to the pioneering counters of Rutherford and Geiger (1908), to the first quantitative gas ionisation studies of J. J. Thomson (1899) and beyond.

The physics of gas-filled particle and X-ray detectors was very intensively researched during the four decades up to 1950. The classic texts of Curran and Craggs (1949), Rossi and Staub (1949) and Wilkinson (1950) describe a highly developed field at the zenith of its importance: before first Nal scintillators (in the 1950s) and then semiconductor detectors (in the early 1960s) replaced gas detectors in many areas of nuclear physics research.

Outside X-ray astronomy, proportional counter fortunes began to revive in the late 1960s when position-sensitive variants of the single-wire proportional counter (SWPC) were introduced as focal plane detectors for magnetic spectrographs (Ford, 1979). Multi-wire detectors (first developed, but not fully exploited, at Los Alamos as part of the Manhattan Project – Rossi and Staub, 1949) then rapidly evolved to provide an imaging capability in two dimensions over large areas. Here, the impetus was provided by the particle physicists (Charpak et al, 1968) who continue to dominate the field of gaseous detector development.

This chapter does not attempt to give a complete account of gaseous electronics, nor does it describe in detail related detector developments in fields such as particle physics (Fabjan and Fischer, 1980; Bartl and Neuhofer, 1983; Bartl et al, 1986).

The first cosmic X-ray source was discovered in June, 1962, during the flight of an Aerobee sounding rocket from the White Sands missile range in New Mexico (Giacconi et al, 1962). As the rocket spun about its axis, three small gas-filled detectors scanned across a powerful source of low-energy X-rays in the constellation of Scorpius, in the southern sky. Even though the position of the source (later designated Sco X-l) could only be determined to within an area of some hundred square degrees, cosmic X-ray astronomy had begun.

As usually recounted, the story of Sco X-l and the birth of X-ray astronomy bears a not inconsiderable resemblance to the story of X-rays themselves. The element of serendipity seems all-important in both discoveries. Wilhelm Roentgen, in 1896, had been intent on measuring the aether waves emitted by a low-pressure gas discharge tube when, by chance, he discovered his new and penetrating radiation. In 1962, the expressed aim of the American Science and Engineering (AS&E)-MIT research group led by Riccardo Giacconi was to detect the X-ray emission, not of distant stars, but from the moon.

Detailed consideration undermines this neat parallel. There is in fact a clear evolutionary line linking X-ray astronomy and the pioneering solar studies carried out in the USA by the Naval Research Laboratory (NRL) group under Herbert Friedman. Friedman's solar X-ray observations had begun with the flight of a captured German V2 rocket in September, 1949.