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.

An observer with much experience with eclipses, Steve Edberg, gazed silently at the Sun when its corona appeared near midday on July 11, 1991. Just stared. Even though he had seen the corona several times before, he was overwhelmed with its size and beauty. This was to be a long total eclipse – almost seven minutes of totality – so he had the chance to preserve this moment. “Well now,” he muttered after the moment. “Perhaps I should take a picture or something?”

No matter how well you prepare for it, unless you are a person of extreme discipline, the onset of totality will drag you away from your planned program. You just want to gaze at it. That is actually the right approach, and no matter how much or how little time totality will last, you do want to have some time for just taking it all in. If totality is long, as it was in 1991 and in 2009, then you have some time for this. If totality is short, then make the time. The shortest totality I've ever experienced was the hybrid eclipse, annular in places and total in other places, in April 2005. Even then I gave myself about five of the 29 seconds of totality to enjoy the view before I resumed my plan.

It is possible that the single most delightful observing session can happen when there is a penumbral eclipse of the Moon. Such an event is as far removed from a total solar eclipse as one can get, and still have an eclipse of some sort. In a penumbral lunar eclipse, the full moon enters, travels through, and then exits the outer shadow of the Earth. This part of the shadow is called the penumbra, and its effects can range from absolutely nothing to, at best, a slight darkening of one edge, or limb, of the Moon. For almost all observers, that is all. For Thomas Hardy, whose wonderful poem “At a Lunar Eclipse” appears at the start ofChapter 13, the total eclipse he saw in 1903 was preceded by a penumbral shading.

However, for me there is much more. At no other time do the rays surrounding the younger craters, like Tycho and Copernicus, appear so obviously. These big craters are certainly not young by human standards; Tycho was formed about 100 million years ago by the sudden impact on the Moon of a comet or an asteroid, and Copernicus is a bit older, but both are young geologically. The Moon's oldest craters were formed probably by impacts mostly during a period about 3.9 billion years ago, and is remembered today as the period of late heavy bombardment.

Similitudes

Watching a lunar eclipse is like many things. It is like reading Hamlet and understanding every word, every cadence. It is like having a candlelit dinner at home. It is like watching your favorite movie. But even more suggestive is what a lunar eclipse is not like. It is not like a total solar eclipse, with its massive emotional buildup, brief exultation at totality, then a partial letdown after. An eclipse of the Moon is more like a two-hour long massage, pure relaxation followed by joy at the event's having taken place over your head. So long as the weather is clear, you really can't go wrong with a lunar eclipse.

In the spring of 1997, as Wendee and I were preparing for our March 15 wedding, we faced the problem of our on-campus ceremony being marred or postponed by crowds attending a basketball NCAA playoff game. We telephoned Mike Terenzoni at the Flandrau Science Center, the place we had chosen for our nuptials. “What about changing it to one week later, March 23?” we asked. “Not then,” his answer came. That's Hale–Bopp.”

I considered the impact of having a wedding on a night near closest approach to the Earth of the great comet of 1997. “Actually,” I explained, “Hale–Bopp will be bright for so long that it really doesn't matter.” But Michael was already looking at his calendar. “And,” he added, “there's a lunar eclipse that night.

Of the many eclipses of the Moon that I have enjoyed, none can approach, or will likely ever come within reach of the unbelievable blackness of the eclipse of December 30, 1963. That eclipse had the Moon pushing its way through an atmosphere clouded with dust from a recent volcanic eruption. Constantine Papacosmas, an experienced observer who saw it with friends, barely saw the Moon at all, and then only faintly through binoculars. He estimated that the eclipsed Moon was no brighter than a fifth-magnitude star. From where I was, finding the Moon in a city sky was almost impossible. I recall enjoying the clear, bitterly cold night, then rushing indoors to sit atop an electric heater.

Solar eclipses

In the next 20 years, eclipses of the Sun will cross a variety of paths over the world. Here is a list of what's in store (total eclipses are in bold).

Remember: Do not ever look at the Sun without proper protection for your eyes. Permanent blindness can result from even a quick look. Normally the Sun is so bright that you are forced to squint, then quickly turn away, as a built-in protection. But during an eclipse, when the Sun is partly obscured by the Moon, you are tempted to look at it longer and more intensely. The Sun's ultraviolet rays can actually burn a hole in your retina, resulting in permanent, partial blindness. A welder's glass (No. 14 strength), or specialized eclipse glasses that are available from telescope stores, will block enough of the Sun's ultraviolet rays to make it safe to look through.

During the total phase of a solar eclipse, when the Sun is completely covered by the Moon, it is completely safe to look at the Sun. Protection must be in force again, however, right after the end of totality.

When the Moon, during its monthly orbit of the Earth, directly crosses the position of the Sun, the result is a central eclipse. If, at the time of its crossing, the Moon is close enough to us that its angular diameter is greater than that of the Sun, a total eclipse is the result. These are the eclipses we travel around the world to see, and will be the subject of Chapters 7, 8, and 9. But what if the Moon's angular diameter is less than that of the Sun? Then we have what is called an annular eclipse, which we can also call a ring eclipse. During these moments the Moon's black silhouette is surrounded by a brilliant ring of sunlight.

As exciting as an annular eclipse might be, it is still a partial eclipse. Thus, all the strict rules about looking directly at the Sun unfortunately apply. And more: because there appears to be so little sunlight, observers are tempted to look directly at the ring of Sun. Thus, if the annularity lasts more than a few seconds, blindness can result. Therefore, use a filter whenever looking at the Sun during all phases of an annular eclipse.

The annular eclipse of September 2005

Of the several annular eclipses I have seen over the years, none was as inspiring as the October 2, 2005 eclipse in Madrid, Spain.