Discovery and nature of neutron stars

Until the discovery of radio pulsars by Jocelyn Bell and Antony Hewish in 1967 (Hewish et al., 1968), neutron stars had existed only in the minds of theoretical physicists. First proposed as an end state of stellar evolution by Robert Oppenheimer and George Volkoff (1939), they are now accepted as the only explanation for radio pulsars. The discovery was serendipitous. No one had conjectured or even dreamed that this sort of signal might be generated.

Bell, then a student, had just spent months wiring antennas for a new radio telescope. In the course of testing, she noticed ‘a bit of scruff’ in the recorded signal. This ‘scruff’ was found to repeat, not every 24 hours (the solar day), but every 23 hours and 56 minutes (the sidereal day), showing that the source was anchored in the sky, not to the rotating Earth. Furthermore, when present, the signal was periodic with a remarkably stable period of 1.337 s. Thus CP 1919, the first of the radio pulsars, was discovered. The stable period could only be explained by rotation, and only a small object with strong gravity could rotate this fast without breaking up. A neutron star was the only reasonable explanation (Gold, 1968).

The discovery of X-rays

On the second story of the building at Röntenring 8 in Würzburg, Germany, there is a plaque: ‘In diesem Hause entdekte W. C. Röntgen im Jahre 1895 die nach ihm benannten Strahlen’ – In this building, in the year 1895, W. C. Röntgen discovered the radiation named for him. Here was the laboratory of Wilhelm C. Röntgen, a 50-year-old professor of physics, who was studying phenomena associated with electrical discharge in gasses. On the afternoon of 8 November, working alone in his laboratory, he noticed a curious phenomenon. When high voltage was applied to the electrodes in the partially evacuated glass discharge tube, he noticed a faint glow from a fluorescent screen placed at the other end of the laboratory table. The room was dark and he had previously covered the tube with black cardboard so no light would escape. Why was the screen glowing?

That evening he verified that the discharge tube was indeed the source of the energy that caused the screen to glow, and that no visible radiation was escaping from the shrouded tube. He quickly found that the unknown radiation would pass through paper, wood, and aluminum but was stopped by heavy metals. Then, when holding a lead disc in front of the screen to observe its shadow, Röntgen also saw the shadow of bones in his hand! In a week he had measured the basic characteristics of this new form of radiation.

Introduction

Cataclysmic variables are remarkably similar to the low-mass X-ray binaries (LMXBs) described in Chapter 11 but are significantly less luminous in X-rays, particularly at the higher X-ray energies of the first X-ray satellites. Indeed, only one previously known CV, EX Hya, was found in the Uhuru survey of the X-ray sky. But these objects have a long history that goes back well before the era of X-ray astronomy, as they include among their number both dwarf novae and novae. As we shall see in this chapter, dwarf novae and novae are powered by fundamentally different processes than occur in supernova events. Supernovae represent the final, and irreversible, moments in the lives of massive stars, when they collapse rapidly under gravity and then explode catastrophically. For any given object it happens once, whereas nova and dwarf nova eruptions can and do recur. Indeed, it is hypothesised that all novae recur, but the typical recurrence time is long: at least hundreds, perhaps thousands, of years.

Cataclysmic variables are interacting binaries similar to LMXBs, except that the compact object is a white dwarf, accreting material from its (usually) cool, late-type companion star in a short period (approximately hours) binary system. They are one of the few classes of object in this book that were known and observed prior to the twentieth century. Novae have been known to mankind throughout history, and dwarf novae were first recorded in the mid-nineteenth century.

Early observations, 1965

The X-ray background was not anticipated. It was discovered in 1962 during the rocket flight which first detected Sco X-1, the first successful attempt to detect X-rays from sources other than the Sun or Earth. An uncollimated detector viewing about 10 000 square degrees of the sky was used. Giacconi et al. (1962) concluded that the background was of ‘diffuse character’ and due to X-rays of about the same energy as those from Sco X-1. The observed diffuse signal in this detector could have been generated by a few moderately strong point sources spread over the sky. The next observations, however, with detectors collimated to observe only 100 square degrees, showed the background to be indeed diffuse and of uniform brightness to at least 10 per cent.

There was no doubt that this background existed. The signals observed were strong and unmistakable. When detectors in rocket payloads were uncovered, pointed at any part of the night sky, the count rate always increased. All early observations, without exception, showed a few bright sources embedded in a uniform X-ray glow. The night sky at X-ray wavelengths was uniformly bright! Sources appeared superposed on this background, rather like stars viewed with the naked eye on a night with a full moon; when the faint stars disappear into the background of moonlight scattered from the atmosphere. Because no structure was observed and the emission was apparently uniform, this phenomenon has been called the ‘diffuse X-ray background’.

The Sun

The Sun is close and has been studied intensively. It radiates strongly from radio- to X-ray frequencies and, because of solar-terrestrial effects, has been monitored by an armada of spacecraft for 50 years. There were the OSO spacecraft (which also observed other cosmic sources) (1962–1978), Skylab (1973), Solar Max (1980–1989), Yohkoh (1991–2001), SOHO (1995–), TRACE (1998), and Hinode (2006). Solar X-ray emission is now continuously measured by a series of GOES spacecraft, and current data are available online almost instantaneously (NOAA/SWPC, 2009a). In this section we show only a few observations which illustrate things to keep in mind when considering the emission of other stars. The data are spectacular, and we regret not having room to include more. For a more thorough overview of solar observations and theory, there is an excellent book by Golub and Pasachoff (1997). Movies of EUV and X-ray images of the Sun can be viewed on several websites (e.g. TRACE, 2009; XRT, 2009).

An historical puzzle

Why should there be detectable X-rays from the Sun at all? Certainly not on the basis of its everyday visible appearance. The optical spectrum of the Sun can be represented quite well by a simple blackbody at a temperature of about 6000 K. Such an object should produce no detectable X-ray flux, whereas the amount actually seen implies the presence of material at a temperature of at least 1 million degrees!

Astrophysical mechanisms for generating X-rays

There are three radiation processes – thermal, synchrotron and blackbody – that are the dominant mechanisms for producing X-rays in an astronomical setting, and whenever high-energy electrons are present, we must add inverse Compton scattering of microwave background photons into the X-ray regime. The spectral signature of each process is unique and is therefore one of the first clues to the nature of an unknown X-ray source. If the spectrum can be measured with high resolution over a broad energy band, then usually both the emission process and the physical conditions within the source can be deduced.

Thermal emission from a hot gas

Consider a hot gas of low enough density that it can be described as thin and transparent to its own radiation. This is not difficult to achieve for X-rays. At temperatures above 105 K, atoms are ionised, and a gas consists of positive ions and negative electrons. Thermal energy is shared among these particles and is transferred rapidly from one particle to another through collisions. Indeed thermal equilibrium means that the average energy of all particles is the same and is determined only by the temperature. When an electron passes close to a positive ion, the strong electric force causes its trajectory to change. The acceleration of the electron in such a collision causes it to radiate electromagnetic energy, and this radiation is called bremsstrahlung (literally, ‘braking radiation’).

X-ray detectors

The first instruments used for X-ray astronomy were developed originally for the detection of charged particles and γ rays emitted by radioactive material. These detectors respond to energy deposited by photoelectrons and, for higher energies, Compton electrons (discussed in Chapter 2). A fast electron creates a track of ionised material in the active volume of the detector. The detector collects either this charge or light from recombination of the ions. Electronic circuits then amplify this signal and record the time and amplitude of the event.

The proportional counter

The proportional counter is not only an efficient X-ray detector but also measures the energy of every photon detected. It was the workhorse of early cosmic X-ray observations and is still being used in modern instruments. However, the modifications necessary to adapt the simple laboratory counter to an X-ray detector capable of operating in the harsh environment of space were challenging.

The detector must have a large area to collect photons from weak cosmic sources and obviously a window thin enough to transmit X-rays. Yet the window has to be strong enough to keep the gas inside the detector from leaking into the nearvacuum of space and well supported to withstand the force of the gas pressure inside the detector. Many an early observation was lost by the failure of detector windows during rocket ascent out of the atmosphere and upon first exposure to space.

Introduction

The discovery of binary behaviour

The very existence of the bright cosmic X-ray sources discovered in the 1960s represented an exciting and challenging astrophysical problem. No physical process known then was capable of generating the enormous X-ray luminosities observed. The subsequent optical identifications of Sco X-1 and Cyg X-2 stimulated theorists and observers alike to learn more about these new ‘X-ray stars’. Why were these extremely powerful X-ray sources associated with such apparently unremarkable optical objects (see Chapter 1)? They were rather faint (13th to 15th magnitude) and did not stand out on optical photographs. However, the optical spectrum of Sco X-1 had similarities with the cataclysmic variables that were being intensively monitored by amateur groups and had been shown, a few years earlier, to be interacting binary systems (see Chapter 10).

As shown in Fig. 11.1, Sco X-1 displayed a smooth blue continuum with superposed emission lines of hydrogen and ionised helium. The absence of absorption features, as in normal stellar spectra, indicated that little or none of the light was coming from a main sequence star. The presence of ionised helium indicated that the source of excitation of the lines was very hot and very likely to be connected with the X-rays. However, despite many observing campaigns dedicated to Sco X-1, which revealed substantial variability on all timescales, no indication of binary behaviour was found. The same was true for Cyg X-2.

The production of planetary X-rays

Planets are small and, compared to the cosmic subjects of other chapters, extremely weak sources of X-rays. Nevertheless, X-rays have now been detected from five planets, moons of Earth and Jupiter, several comets, and diffuse material in the solar neighborhood. These results have been scientifically useful and often surprising. The strongest X-ray source in the Solar System is, of course, the Sun. As in the visible band, orbiting solid objects shine with reflected solar energy. The soft X-ray luminosity of the solar corona is ∼4 × 1027 erg s−1, and that of the planets is a factor of ∼1014 weaker. Cometary X-rays are produced by collisions of energetic solar-wind particles with material in the comet. Some planets have magnetospheres which provide a mechanism for generating auroral X-rays. The energy that drives almost all these X-ray production processes originates in the Sun.

The observations are difficult, as targets move appreciably during the observation and are very bright optically; so bright that star sensors for aspect determination sometimes cannot be used. Soft X-ray detectors are also sensitive to visible light, which makes data reduction difficult. This chapter will cover Solar System objects in approximate order of X-ray detection.

Earth

In some of the very first X-ray astronomy observations, solar X-rays scattered from the upper layers of the Earth's atmosphere were detected with rocket-borne proportional counters (Harries & Francey, 1968; Grader et al., 1968).

Introduction

On the largest scale, the distribution of matter in the Universe is uniform, but on an intermediate level, galaxies are found in gravitationally bound aggregates. These ‘groups’ and ‘clusters’ exist in sizes ranging from a few galaxies to 10 000 galaxies. The gravitational potential which binds galaxies in a cluster also binds a cloud of hot gas which fills the space between and around the galaxies. This gas, the intracluster medium (ICM), has a temperature of tens of millions of degrees. It coexists with the galaxies and, although very diffuse, is a strong source of X-ray emission.

This hot gas was discovered unexpectedly in 1971 through the analysis of X-ray observations. Modern observatories have now measured the X-ray luminosities of hundreds of galaxy clusters, and the morphology of emission from many brighter clusters has been well mapped. The shapes of the gravitational potentials of these clusters have been derived and the mass of X-rayemitting gas determined. (The deeper the gravitational potential well, the faster the motion of the galaxies within the cluster and the greater the concentration of hot gas at the centre.) The mass of hot gas is typically 3–10 times greater than the mass derived from the visible luminosity of the galaxies.

The cluster gravitational potential which fits both X-ray and optical measurements requires the existence of a large hidden mass.

Discovery

In 1963, to lessen the rapid proliferation of nuclear weapons, the United States and the Soviet Union signed a treaty prohibiting testing such weapons in the atmosphere and in space. To assure that there were no violations of this treaty, in the late 1960s the United States deployed a series of spacecraft, the Vela satellites, as monitors. Several spacecraft were positioned so that all of near- Earth space was always viewed by at least one set of detectors.

A nuclear explosion in space produces an intense prompt burst of X-rays, neutrons and γ rays. This signal is bright enough, and with a distinctive enough time signature, that there should be no confusion with natural events. Also, as in a supernova explosion, debris is ejected in all directions at high velocity. The primary detectors on the Vela spacecraft were designed to detect and recognise the prompt signals. Still, a clandestine test might be hidden from the promptburst detectors by detonating the device behind the Moon. The debris, however, which contains highly radioactive, rapidly decaying fission fragments, would be thrown from the vicinity of the explosion and free of the Moon's shadow. Gamma-ray detectors were therefore included which were capable of detecting radiation from nuclear debris.

In 1972, after 3 years of operation, the Los Alamos group responsible for the various detectors realised that the system was detecting bursts of γ rays that were real events, not some strange combination of background noise.

Introduction

This chapter describes phenomena caused by truly large explosions: catastrophic events in which large stars disintegrate completely. Vast clouds of stellar debris are ejected and are rapidly heated to temperatures of millions of degrees. This is a very important mechanism in astronomy, as it enriches the ISM with heavy elements, out of which new stars and planetary systems (such as our own) can be formed. These expanding clouds of hot gas are strong sources of X-ray and radio radiation. They shine clearly as extended objects with a great variety of shapes and are referred to as remnants of the supernovae.

Every few centuries there is a supernova close enough to be seen with the naked eye, and some of these have been spectacular. On 1 May 1006, a new star appeared in the constellation Lupus and, within a matter of days, became the brightest star observed in all of recorded history. According to records kept by Chinese and Arabic scholars at that time, this star seemed ‘glittering in aspect, and dazzling to the eyes’. ‘The sky was shining because of its light’. ‘Its form was like the half Moon, with Pointed rays shining so brightly that one could see things clearly’. This nearby supernova (a very bright ‘new star’) was awe inspiring. It was probably visible for 3 months during daylight, and only after 3 years did it fade below naked-eye visibility at night.

O stars

The most luminous, most massive stars are the O stars. Starting with more than 25 M⊗ of material (possibly ~100 M⊗), they burn their nuclear fuel at a prodigious rate. They live only a short time and end in a brilliant supernova explosion. The surrounding space is left full of stellar debris enriched in heavy elements. Our bodies all contain elements made in these massive stars.

These are not common stars, and none are nearby. The brightest ones visible to the naked eye are δ and ζ Orionis at the two ends of Orion's Belt. Both are 1600 pc distant and spectral type O9.5; ζ Puppis is 2400 pc distant and type O5. Because the nuclear fuel is consumed rapidly, the lifetime is, astronomically speaking, short. In a few million years an O star changes character, becoming perhaps a red giant or a Wolf-Rayet star. We see, with naked eye or telescope, only the outer layer, which gives little information about events in the core. Hidden from view, the central region evolves rapidly until the nuclear fuel is exhausted. As explained in Chapter 8 on supernova remnants, the core collapses and the gravitational energy released powers the resulting supernova. That is the end of the O star.

Astronomers originally believed all stars evolved along the main sequence. In this scheme a star would start life as a hot O star and, as it aged, would change into progressively cooler spectral types.