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.

Introduction

Concept of a black hole

Black holes have attracted people's imaginations perhaps more than any other kind of object in the cosmos. Remarkably the concept of a black hole dates back more than 200 years. In 1783, the Cambridge cleric John Michell speculated in a lecture to the Royal Society about the effects of the Sun's gravity on the light it was radiating. Michell was aware of the finite speed of light (determined by Roemer in the seventeenth century from observations of eclipse timings of Jupiter's moons) and believed that photons from the Sun (he called them ‘corpuscles’) would be slowed down as they left the Sun due to its gravity. His speculation was to point out that if the Sun's diameter were 500 times larger and of the same density, then its mass would be 108M⊗, and gravity would prevent light from escaping the Sun at all. A similar conjecture was put forward by Laplace in 1795.

However, our modern concept of a black hole stems from Einstein's theory of general relativity (GR) and the first exact solutions, derived by Karl Schwarzschild in 1916, of Einstein's equations. Under GR, the effect of a massive body's gravity is to curve the space-time around it, forcing light to follow a curved path (called a ‘geodesic’). If the body is sufficiently massive and compact, then this curvature closes in on itself, and any light emitted by the body will never escape – hence the term black hole.

X-ray sources in the Milky Way

Our Galaxy, the Milky Way, is a ‘normal’ spiral galaxy. It does not have a currently active nucleus (see Chapter 14), nor is it unusually luminous at any wavelength. Since we live in it, we find this a pleasing situation.

The first X-ray sources discovered within our Galaxy were naturally the brightest: the accretionpowered binaries. As time progressed, other Milky Way objects were also found to emit X-rays. Most of the emission from our Galaxy seemed to be from discrete objects, not from some galacticsized diffuse region. This is also true for most other ‘normal’ galaxies. The populations of discrete sources in other galaxies is a major topic of interest. For any particular class of source we would like to know how the number of sources varies with X-ray luminosity. This function, the XLF, can be compared with theoretical results concerning the nature and evolution of the sources.

The most luminous galactic X-ray sources are the accretion-powered binaries, which consist of a compact object and a companion star. If the companion is spectral type A or later (mass <~1 M⊗), the system is a low-mass X-ray binary or LMXB. If the companion is spectral-type O or B (mass >~10 M⊗), the system is a high-mass X-ray binary or HMXB (Chapter 11).

Luminosities range from very low up to ~ 1038 ergs s−1. Those with high luminosity are thought to be operating close to the Eddington limit, where the pressure of in-falling material is balanced by the pressure of outflowing radiation.

Introduction

Normal galaxies like our own, when viewed from great distances, appear to be peaceful and unchanging aggregations of stars, whose wellbeing is only slightly disturbed by the occasional supernova explosion. However, violent processes far more powerful than supernovae have been known since early in this century. The optical jet emanating from the giant elliptical galaxy M87 (the dominant galaxy in the relatively nearby Virgo cluster of galaxies) was found in 1917, but its significance was not understood for many years. After the Second World War, the founding of radio astronomy led to the discovery of luminous extragalactic radio sources such as Cygnus A. Also, short-exposure optical photographs showed that some apparently normal spiral galaxies actually had very bright, almost starlike nuclei, the prime example of which is NGC 4151 (Fig. 14.1), hence the term active galactic nuclei, or AGN.

Such galaxies are referred to as Seyfert galaxies, after their discoverer, Carl Seyfert. But even these exotic objects paled in comparison with the enormous energy output at all wavelengths of quasi-stellar objects (better known as quasars, or QSOs), discovered originally through their radio emission in the early 1960s and so-called because of their ‘stellar’ appearance. However, the discovery in 1963 of their very high red-shifts (see Box 14.1) implied that QSOs were immensely distant, and hence they were the most luminous objects in the Universe (see Fig. 14.2).

The Interstellar Medium

The space between stars is not empty. It is full of gas and dust which are collectively called the Interstellar Medium or ISM. The ISM accounts for ~10 per cent of the mass of our Galaxy. To see anything beyond the Solar System, we must look through the ISM, and thus all observations are filtered and modified. In our Galaxy the gas forms a disc in the plane of the Milky Way, with diameter ≈30 kpc and thickness ≈0.7 kpc. The density averages about 1 atom/cm3, a far better vacuum than any that could be created here on Earth. This does not sound like much, but it is enough to absorb soft X-rays from most galactic sources. The composition of the gas is close to the usual cosmic abundance: 90 per cent H (by number), 10 per cent He and 0.1 per cent heavier elements. However, it is far from being a uniform medium. The neutral gas exists in a very large range of density, n, and temperature, T. A diffuse cloud might have n ~100 and T ~80 K. The medium between clouds might have n ~1 atoms cm−3 and T ~8000 K. There is also warm (8000 K) and hot (300 000 K) ionised material. Our interest here is in the neutral gas, which does most of the absorbing.

Neutral H in the ISM has, for more than 60 years, been directly observed in the radio band at a wavelength of 21.106 cm.

This is a book about X-ray astronomy. We take a historical perspective because this is how we saw it happen and because this gives a feeling for the observable universe. In a table listing all members of a class of objects, the brightest source does not stand out, but in the first observation, it is a splendid object and remembered fondly by those involved in the discovery.

Some 50 years ago X-rays from stars other than our Sun were unknown and unexpected by all but a few pioneering scientists. Since the discovery of cosmic X-rays in 1962 the field has grown at an astonishing rate. Since the first edition of this book, published in 1995 and including results from the first X-ray telescopes, the sensitivity of X-ray observations has increased dramatically. In 1999 the Chandra and XMM X-ray observatories were launched and, in 10 years of operation, have produced X-ray images of comparable angular resolution to those obtained by the largest ground-based observatories. More importantly, X-ray spectroscopy of sufficient resolution to allow comparison with spectra at other wavelengths has become possible. Technical improvements in dispersive spectroscopy mean that high resolution X-ray spectra of faint sources have become available for the first time. This has helped propel X-ray astronomy to its rightful place as a sub-discipline of astronomy, where a knowledge of truly multiwavelength results is necessary for the study of any class of objects. This book, however, is about X-rays.