High Energy Astrophysics
Third Edition
Providing students with an in-depth account of the astrophysics of high energy phenomena in the Universe, the third edition of this well-established textbook is ideal for advanced undergraduate and beginning graduate courses in high energy astrophysics.
Building on the concepts and techniques taught in standard undergraduate courses, this textbook provides the astronomical and astrophysical background for students to explore more advanced topics. Special emphasis is given to the underlying physical principles of high energy astrophysics, helping students understand the essential physics.
The third edition has been completely rewritten, consolidating the previous editions into one volume. It covers the most recent discoveries in areas such as gamma-ray bursts, ultra-high energy cosmic rays and ultra-high energy gamma rays. The topics have been rearranged and streamlined to make them more applicable to a wide range of different astrophysical problems.
Malcolm S. Longairis Emeritus Jacksonian Professor of Natural Philosophy and Director of Development at the Cavendish Laboratory, University of Cambridge. He has held many senior positions in physics and astronomy, and has served on and chaired many national and international committees, boards and panels, working with both NASA and the European Space Agency. He has received much recognition for his work, including a CBE in the millennium honours list for his services to astronomy and cosmology. He is a Fellow of the Royal Society of London, the Royal Society of Edinburgh, the Academia Lincei and the Istituto Veneto di Scienze, Arte e Literatura.
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High Energy Astrophysics
Third Edition
MALCOLM S. LONGAIR
Emeritus Jacksonian Professor of Natural Philosophy, Cavendish Laboratory,
University of Cambridge, Cambridge
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cambridge university press
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C M. Longair 2011
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First published 2011
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or will remain, accurate or appropriate.
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For Deborah
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Contents
Preface page xiii
Acknowledgements xvii
Part I Astronomical background
1 High energy astrophysics – an introduction 3
1.1 High energy astrophysics and modern physics and astronomy 3
1.2 The sky in different astronomical wavebands 4
1.3 Optical waveband 3× 1014 ν 1015Hz; 1 μm λ 300 nm 5 1.4 Infrared waveband 3× 1012 ν 3 × 1014Hz; 100 λ 1 μm 9 1.5 Millimetre and submillimetre waveband 30 GHz ν 3 THz;
10 λ 0.1 mm 14
1.6 Radio waveband 3 MHz ν 30 GHz; 100 m λ 1 cm 17 1.7 Ultraviolet waveband 1015 ν 3 × 1016Hz; 300 λ 10 nm 21 1.8 X-ray waveband 3× 1016 ν 3 × 1019Hz; 10 λ 0.01 nm;
0.1 E 100 keV 22
1.9 γ -ray waveband ν 3 × 1019Hz;λ 0.01 nm; E 100 keV 25
1.10 Cosmic ray astrophysics 27
1.11 Other non-electromagnetic astronomies 32
1.12 Concluding remarks 34
2 The stars and stellar evolution 35
2.1 Introduction 35
2.2 Basic observations 35
2.3 Stellar structure 39
2.4 The equations of energy generation and energy transport 43
2.5 The equations of stellar structure 47
2.6 The Sun as a star 50
2.7 Evolution of high and low mass stars 59
2.8 Stellar evolution on the colour–magnitude diagram 68
2.9 Mass loss 70
2.10 Conclusion 75
3 The galaxies 77
3.1 Introduction 77
3.2 The Hubble sequence 78
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3.3 The red and blue sequences 80
3.4 Further correlations among the properties of galaxies 86
3.5 The masses of galaxies 89
3.6 The luminosity function of galaxies 95
4 Clusters of galaxies 99
4.1 The morphologies of rich clusters of galaxies 99
4.2 Clusters of galaxies and isothermal gas spheres 102
4.3 The Coma Cluster of galaxies 106
4.4 Mass distribution of hot gas and dark matter in clusters 109
4.5 Cooling flows in clusters of galaxies 110
4.6 The Sunyaev–Zeldovich effect in hot intracluster gas 114 4.7 Gravitational lensing by galaxies and clusters of galaxies 116
4.8 Dark matter in galaxies and clusters of galaxies 123
Part II Physical processes
5 Ionisation losses 131
5.1 Introduction 131
5.2 Ionisation losses – non-relativistic treatment 131
5.3 The relativistic case 136
5.4 Practical forms of the ionisation loss formulae 141
5.5 Ionisation losses of electrons 145
5.6 Nuclear emulsions, plastics and meteorites 146
5.7 Dynamical friction 151
6 Radiation of accelerated charged particles and bremsstrahlung of electrons 154
6.1 Introduction 154
6.2 The radiation of accelerated charged particles 154
6.3 Bremsstrahlung 163
6.4 Non-relativistic bremsstrahlung energy loss rate 166
6.5 Thermal bremsstrahlung 167
6.6 Relativistic bremsstrahlung 173
7 The dynamics of charged particles in magnetic fields 178
7.1 A uniform static magnetic field 178
7.2 A time-varying magnetic field 180
7.3 The scattering of charged particles by irregularities in the magnetic field 184 7.4 The scattering of high energy particles by Alfv´en and
hydromagnetic waves 187
7.5 The diffusion-loss equation for high energy particles 189
8 Synchrotron radiation 193
8.1 The total energy loss rate 193
8.2 Non-relativistic gyroradiation and cyclotron radiation 195
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8.3 The spectrum of synchrotron radiation – physical arguments 198 8.4 The spectrum of synchrotron radiation – a fuller version 202 8.5 The synchrotron radiation of a power-law distribution of electron
energies 212
8.6 The polarisation of synchrotron radiation 214
8.7 Synchrotron self-absorption 217
8.8 Useful numerical results 222
8.9 The radio emission of the Galaxy 224
9 Interactions of high energy photons 228
9.1 Photoelectric absorption 228
9.2 Thomson and Compton scattering 231
9.3 Inverse Compton scattering 237
9.4 Comptonisation 243
9.5 The Sunyaev–Zeldovich effect 257
9.6 Synchrotron–self-Compton radiation 260
9.7 Cherenkov radiation 264
9.8 Electron–positron pair production 270
9.9 Electron–photon cascades, electromagnetic showers and the detection
of ultra-high energyγ -rays 272
9.10 Electron–positron annihilation and positron production mechanisms 275
10 Nuclear interactions 279
10.1 Nuclear interactions and high energy astrophysics 279
10.2 Spallation cross-sections 282
10.3 Nuclear emission lines 287
10.4 Cosmic rays in the atmosphere 292
11 Aspects of plasma physics and magnetohydrodynamics 298
11.1 Elementary concepts in plasma physics 298
11.2 Magnetic flux freezing 304
11.3 Shock waves 314
11.4 The Earth’s magnetosphere 319
11.5 Magnetic buoyancy 321
11.6 Reconnection of magnetic lines of force 323
Part III High energy astrophysics in our Galaxy
12 Interstellar gas and magnetic fields 333
12.1 The interstellar medium in the life cycle of stars 333
12.2 Diagnostic tools – neutral interstellar gas 333
12.3 Ionised interstellar gas 340
12.4 Interstellar dust 347
12.5 An overall picture of the interstellar gas 353
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12.6 Star formation 361
12.7 The Galactic magnetic field 369
13 Dead stars 378
13.1 Supernovae 378
13.2 White dwarfs, neutron stars and the Chandrasekhar limit 394
13.3 White dwarfs 401
13.4 Neutron stars 401
13.5 The discovery of neutron stars 406
13.6 The galactic population of neutron stars 419
13.7 Thermal emission of neutron stars 421
13.8 Pulsar glitches 422
13.9 The pulsar magnetosphere 424
13.10 The radio and high energy emission of pulsars 427
13.11 Black holes 429
14 Accretion power in astrophysics 443
14.1 Introduction 443
14.2 Accretion–general considerations 443
14.3 Thin accretion discs 451
14.4 Thick discs and advective flows 461
14.5 Accretion in binary systems 464
14.6 Accreting binary systems 473
14.7 Black holes in X-ray binaries 486
14.8 Final thoughts 492
15 Cosmic rays 493
15.1 The energy spectra of cosmic ray protons and nuclei 493 15.2 The abundances of the elements in the cosmic rays 496
15.3 The isotropy and energy density of cosmic rays 502
15.4 Gamma ray observations of the Galaxy 503
15.5 The origin of the light elements in the cosmic rays 507 15.6 The confinement time of cosmic rays in the Galaxy and cosmic ray
clocks 515
15.7 The confinement volume for cosmic rays 517
15.8 The Galactic halo 520
15.9 The highest energy cosmic rays and extensive air-showers 522
15.10 Observations of the highest energy cosmic rays 524
15.11 The isotropy of ultra-high energy cosmic rays 529
15.12 The Greisen–Kuzmin–Zatsepin (GKZ) cut-off 531
16 The origin of cosmic rays in our Galaxy 536
16.1 Introduction 536
16.2 Energy loss processes for high energy electrons 536
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16.3 Diffusion-loss equation for high energy electrons 540 16.4 Supernova remnants as sources of high energy particles 545 16.5 The minimum energy requirements for synchrotron radiation 549 16.6 Supernova remnants as sources of high energy electrons 553
16.7 The evolution of supernova remnants 554
16.8 The adiabatic loss problem and the acceleration of high
energy particles 556
17 The acceleration of high energy particles 561
17.1 General principles of acceleration 561
17.2 The acceleration of particles in solar flares 562
17.3 Fermi acceleration – original version 564
17.4 Diffusive shock acceleration in strong shock waves 568
17.5 Beyond the standard model 574
17.6 The highest energy cosmic rays 580
Part IV Extragalactic high energy astrophysics
18 Active galaxies 585
18.1 Introduction 585
18.2 Radio galaxies and high energy astrophysics 585
18.3 The quasars 586
18.4 Seyfert galaxies 592
18.5 Blazars, superluminal sources andγ -ray sources 596 18.6 Low Ionisation Nuclear Emission Regions – LINERS 598
18.7 Ultra-Luminous Infrared Galaxies ULIRGs 598
18.8 X-ray surveys of active galaxies 600
18.9 Unification schemes for active galaxies 602
19 Black holes in the nuclei of galaxies 610
19.1 The properties of black holes 610
19.2 Elementary considerations 611
19.3 Dynamical evidence for supermassive black holes in galactic nuclei 613
19.4 The Soltan argument 623
19.5 Black holes and spheroid masses 625
19.6 X-ray observations of fluorescence lines in active galactic nuclei 626 19.7 The growth of black holes in the nuclei of galaxies 633
20 The vicinity of the black hole 637
20.1 The prime ingredients of active galactic nuclei 637
20.2 The continuum spectrum 637
20.3 The emission line regions – the overall picture 640
20.4 The narrow-line regions – the example of Cygnus A 641 20.5 The broad-line regions and reverberation mapping 646
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20.6 The alignment effect and shock excitation of emission line regions 653
20.7 Accretion discs about supermassive black holes 656
21 Extragalactic radio sources 661
21.1 Extended radio sources – Fanaroff–Riley types 661
21.2 The astrophysics of FR2 radio sources 666
21.3 The FR1 radio sources 675
21.4 The microquasars 676
21.5 Jet physics 678
22 Compact extragalactic sources and superluminal motions 681
22.1 Compact radio sources 681
22.2 Superluminal motions 683
22.3 Relativistic beaming 686
22.4 The superluminal source population 693
22.5 Synchro-Compton radiation and the inverse
Compton catastrophe 697
22.6 γ -ray sources in active galactic nuclei 699
22.7 γ -ray bursts 704
23 Cosmological aspects of high energy astrophysics 714
23.1 The cosmic evolution of galaxies and active galaxies 714
23.2 The essential theoretical tools 715
23.3 The evolution of non-thermal sources with cosmic epoch 720 23.4 The evolution of thermal sources with cosmic epoch 729
23.5 Mid- and far-infrared number counts 737
23.6 Submillimetre number counts 740
23.7 The global star-formation rate 743
23.8 The old red galaxies 746
23.9 Putting it all together 749
Appendix Astronomical conventions and nomenclature 753
A.1 Galactic coordinates and projections of the
celestial sphere onto a plane 753
A.2 Distances in astronomy 755
A.3 Masses in astronomy 759
A.4 Flux densities, luminosities, magnitudes and colours 760
A.5 Diffraction-limited telescopes 764
A.6 Interferometry and synthesis imaging 771
A.7 The sensitivities of astronomical detectors 774
A.8 Units and relativistic notation 779
Bibliography 783
Name index 825
Object index 829
Index 831
Preface
Ancient history
It was a challenge to write this third edition of High Energy Astrophysics. Writing the first edition was great fun and that rather slim volume reflected rather closely the lecturing style I adopted in presenting high energy astrophysics to final-year undergraduates in the period 1973–7. Although the material was updated when the manuscript was sent to the press in 1980, the book remained in essence a lecture course (Longair, 1981). The reception of the book was encouraging and in due course a second edition was needed. The subject had advanced so rapidly during the 1980s and early 1990s that the material could not be comfortably contained within one volume. The aim was originally to complete the task in two volumes, but by the time the Volumes 1 and 2 were completed, I had only reached the edge of our own Galaxy (Longair, 1997b,c).1Volume 3 was begun, but for various reasons, was not completed – the whole project was becoming somewhat unwieldy.
In the meantime, I completed three other major book-writing projects. The first of these was a new edition of Theoretical Concepts in Physics (Longair, 2003). Then, I completed The Cosmic Century: A History of Astrophysics and Cosmology (Longair, 2006). Finally, in 2008, the new edition of Galaxy Formation was published (Longair, 2008).
The new edition
Since the second edition of High Energy Astrophysics, many of the subject areas have changed out of all recognition and new areas of astrophysical research have been opened up, for example, ultra-high energy gamma-ray astronomy. The publication of Theoretical Concepts in Physics, The Cosmic Century and Galaxy Formation have made it feasible to condense the original plan of a three volume work into a single volume. In reorganising the material, some hard decisions had to be taken, but the convenience of including everything in one volume is worth the sacrifice of some of the material from the second edition. The principal decisions were as follows:
1The original volumes of the second edition were first published in 1992 (Volume 1) and 1994 (Volume 2). Major revisions and corrections were included in the 1997 reprints of both volumes. I regard the 1997 reissues as the definitive versions of the second edition.
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r Much of the relevant historical material has been included in The Cosmic Century and so that material will not be repeated here. I make references to the appropriate sections of The Cosmic Century and other historical texts. I do this with considerable reluctance since the historical development of high energy astrophysics has influenced strongly the way in which the astrophysics has developed intellectually. History will not disappear completely, but it will not be as prominent as in the earlier editions.
r Much of the necessary material needed to obtain a modern view of galaxies and the large scale structure of the Universe is included in Galaxy Formation. In particular, there is no need to repeat much of the detailed discussion of galaxies and clusters, or the large scale structure and dynamics of the Universe. These topics are, however, central to many of the topics in this book and so summaries of the most important topics needed to understand the astronomical context of high energy astrophysics are provided in Part I.
r There was a strong emphasis upon the origin of cosmic rays in the first two editions. I still consider this to be excellent material, particularly in the area of ultra-high energy cosmic rays, but it has been somewhat abbreviated in the new edition.
r There was also a considerable amount of material on detectors and telescopes in the earlier edition. I believe this material is of the greatest interest and importance in un- derstanding our ability of make observations in different wavebands. This aspect of the subject has been strongly moderated in the new edition. These are fascinating topics, but modern telescopes and detectors have become increasingly complex and sophisticated.
Summaries of a number of important topics in the physics of astronomical detectors and telescopes are included as an appendix.
r In the second edition, I devoted some space to high energy astrophysics in the Solar System. This material has been abbreviated, but important topics such as the diffusion of energetic charged particles in the Solar Wind and the acceleration of charged particles in solar flares have been preserved.
r The opportunity has been taken to rationalise the presentation of the physical and astro- physical processes so that duplication of material is avoided.
r The writing has been very considerably tightened up so that the discussion is less dis- cursive than in the earlier editions. Again, I regret the necessity of doing this since often these asides provide valuable physical insights for reader new to the subject.
The aims of the present edition are the same as the earlier editions. A very wide range of physical processes relevant for high energy astrophysics is discussed, the emphasis being strongly upon the understanding of the underlying physics. I aim to maintain the informal style of the earlier editions and have no hesitation about using the first person singular or expressing my personal opinion about the material under discussion. The emphasis is strongly upon physical principles and the discussion of general results rather than particular models which may have only ephemeral appeal.
As I learned during the writing of The Cosmic Century, physics and astrophysics have a symbiotic relation. On the one hand, the astrophysical sciences are concerned with the application of the laws of physics to phenomena on a large scale in the Universe. On the other hand, new laws of physics are discovered and tested through astronomical obser- vations and their astrophysical interpretation. In these ways, the new astrophysics, of which
xv Preface
high energy astrophysics is one of the most important ingredients, is just as much a part of modern physics as laboratory physics.
Although there is limited scope for deviation from the central theme in this new edition, one of my original aims was to give the reader a feeling of what it is like to undertake research at the limits of present understanding. Astrophysics is fortunate in that many of the fundamental problems can be understood without a great deal of new physics or new physical concepts. Thus, the text may also be considered as an introduction to the way in which research is carried out in the astrophysical context.
Above all, however, this material is not only mind-stretching, but also great fun. I have no intention of inhibiting my enthusiasm and enormous enjoyment of the physics and astrophysics for its own sake.
Malcolm Longair Cambridge and Venice January 2010
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Acknowledgements
There are many people whom it is a pleasure to thank for help and advice during the prepa- ration of this volume. Just as the first edition was begun during a visit to the Osservatorio Astronomico di Arcetri in Florence in April 1980, so the second edition could not have been completed without the Regents’ Fellowship of the Smithsonian Institution which I held at the Harvard-Smithsonian Astrophysical Observatory during the period April–June 1990. I am particularly grateful to Professors Irwin Shapiro and Giovanni Fazio for spon- soring this visit to Harvard during which time the final drafts of Chapters 1–10 of the first volume of the second edition were completed. During that period, I had particularly helpful discussions with Drs Eugene Avrett, George Rybicki, Giovanni Fazio, Margaret Geller and many others. I am particularly grateful to them for their advice.
Much of the preliminary rewriting was completed while I was at the Royal Observatory, Edinburgh. Among the many colleagues with whom I discussed the contents of this vol- ume, I must single out Dr John Peacock who provided deep insights into many topics. In completing the final chapter on the high energy astrophysics of the Solar System, I greatly benefitted from the advice of Professors John Brown, Carole Jordan and Eric Priest. Not only did they point me in the correct directions but they also reviewed my first drafts of that chapter. I am especially grateful to them for this laborious task. Many colleagues made helpful suggestions about corrections and additions to the first edition, among whom Dr Roger Chevalier provided an especially useful list.
Coincidentally, the writing of the third edition began while I was a visitor at the Osservatorio Astronomico di Arcetri in Florence during the period April–June 2007. I thank Professor Francesco Palla and his colleagues for their hospitality during that visit.
The catalogue of friends and colleagues who have continued to contribute to my under- standing of high energy astrophysics and astrophysical cosmology since the publication of the second edition is enormous. Many of them are acknowledged in my recent books, but the list is so long that I would be bound to miss someone out. I acknowledge particular insights from my colleagues in the course of the book. Special thanks are due to Dr. David Green for his expert advice, not only on supernova remnants, but also on the more arcane idiosyncracies of LaTeX.
To all of these friends and colleagues I make the usual disclaimer that any misrepresen- tation of the material presented in this book is entirely my responsibility and not theirs.
Finally, I acknowledge the unfailing support and love of my family, Deborah, Mark and Sarah who have contributed much more than they will ever know to the completion of this book.
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PART I
ASTRONOMICAL BACKGROUND
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1 High energy astrophysics – an introduction
1.1 High energy astrophysics and modern physics and astronomy
The revolution in astronomy, astrophysics and cosmology since the end of the Second World War in 1945 has been driven by the opening up of the whole of the electromagnetic spectrum for astronomical observations. This revolution would not have been possible without the development of new techniques and technologies for making astronomical observation from the ground and from space. Hand in hand with these developments have been major advances in laboratory physics and the development of high speed computers. It is the combination of all these factors which has led to dramatic advances in the astrophysical and cosmological sciences.
Among the most important of the new disciplines is high energy astrophysics. I take this term to mean the astrophysics of high energy processes and their application in astrophysical and cosmological contexts. These processes, their application in astrophysics and how they lead to some of the most challenging problems of contemporary physics, are the subjects of this book. For example, we need to explain how the massive black holes present in the nuclei of active galaxies can be studied, how charged particles are accelerated to extremely high energies in astronomical environments, the origins of enormous fluxes of high energy particles and magnetic fields in active galaxies, the physical processes in the interiors and environments of neutron stars, the nature of the dark matter, the expected fluxes of gravitational waves in extreme astronomical environments, and so on. Thus, high energy astrophysics makes feasible the study of the properties of matter under physical conditions which cannot yet be reproduced in the laboratory. Indeed, in many cases, the problems can only be addressed in the astrophysical environment. The aim of this book is to set out the logical sequence of steps by which astrophysicists tackle these problems.
The aim of the astrophysical sciences is two-fold – the application of the laws of physics in the extreme physical conditions encountered in astronomical systems, and the discovery of new laws of physics from observation. This second aspect has a long and distinguished pedigree, as I have recounted in my book The Cosmic Century (Longair, 2006). We will encounter many new and exciting examples in the course of this exposition. Throughout the text, the emphasis will be upon those aspects of high energy astrophysics in which the astrophysical understanding is reasonably secure, and indicative of those areas where the astrophysics is still poorly understood.
The amount of material to be covered is enormous and so, to put some order into the presentation, the book is divided into four parts.
3
4 High energy astrophysics – an introduction
Part I The first part concerns the essential astronomical background needed to under- stand the context within which high energy astrophysical studies are carried out.
If you already have a good grounding in astronomy and astrophysics, you may pass on to the subsequent parts. The first chapter introduces all the accessible astronomical wavebands and outlines the distinctive features of the astrophysical objects observed. There then follow chapters which summarise the essential fea- tures of stellar evolution, galaxies and clusters of galaxies, in order to understand the contexts within which high energy astrophysical phenomena are observed. Even studies such as the properties of galaxies have undergone a significant change of emphasis in the light of the evidence provided by very large surveys of galaxies, such as the Anglo-Australian Telescope 2dF Galaxy Survey and the Sloan Digital Sky Survey.
Part II Chapters 5–11 are principally concerned with the physical processes involved in the interactions and radiation of charged particles. The emphasis is upon a clear description of the physics of these processes. Generally, the simplest physical approach to understanding the processes is given first and then some of the more important of these are studied in more detail. Processes which dominate much of high energy astrophysics, such as bremsstrahlung, synchrotron radiation and inverse Compton scattering, merit such a more detailed treatment.
Part III Chapters 12–17 are principally concerned with high energy astrophysical processes in our Galaxy. A large suite of exotic objects is introduced, including white dwarfs, neutron stars, black holes and supernova explosions. The study of the origin of cosmic ray particles fits naturally into this discussion since these are the only samples of high energy particles originating in extreme astronomical environments which we can study directly within the Solar System. The acceleration of charged particles to high energies in Galactic environments provides clues to the much more extreme events which must take place in active galaxies.
Part IV Chapters 18–23 are devoted to extragalactic high energy astrophysics and involve some of the most extreme energetic phenomena in the Universe – the quasars, radio galaxies, TeV γ -ray sources, γ -ray bursts, and so on. The most extreme objects must involve physical processes originating close to supermassive black holes and what we observed is strongly influenced by relativistic aberration effects.
In Chapter 23, some cosmological aspects of high energy astrophysics and the role that supermassive black holes may play in galaxy formation are described.
This is a very large programme and readers are encouraged to be selective in their use of the material and to customise it to their own requirements.
1.2 The sky in different astronomical wavebands
The dramatic change in perspective of astrophysical research over the last half century is conveniently illustrated by images of the celestial sphere in the different astronomical
5 1.3 Optical waveband
wavebands now accessible to observation. These can be thought of as providing different temperature maps of the Universe according to Wien’s displacement law,
νmax= 1011(T/K) Hz ; λmaxT = 3 × 106nm K, (1.1) where the relations refer to the maximum intensity of a black-body, or Planck, spectrum, expressed either in frequency or wavelength units, of a body in thermodynamic equilibrium at temperature T . These relations are shown in Fig. 1.1a which includes the conventional labels of the different astronomical wavebands. In the optical waveband, for example, the typical temperatures of thermal sources of radiation are about 3000–10 000 K. Thermal sources in the X-ray waveband typically have temperatures of at least 107–108K, while far- infrared observations provide images of the cold Universe, typical temperatures being about 30–100 K. Objects with a wider range of temperatures are observable in any given waveband because of the broad-band nature of the thermal radiation spectrum. Thermodynamically speaking, the above figures are only lower limits to the temperatures of sources which are observable in these wavebands. In the case of non-thermal sources of radiation, by which we mean radiation emitted by sources which do not possess a Maxwellian energy distribution of radiating particles, the effective temperature of the emitting particles can far exceed the above temperatures. This is particularly important for non-thermal sources such as Galactic and extragalactic radio sources, quasars and X- andγ -ray sources in which the continuum radiation is associated with the emission of ultra-relativistic electrons.
Astronomical observations can be made from ground-based observatories in the optical, near-infrared, millimetre and radio wavebands. Once space was opened up for astronomical observations in the late 1950s, it became possible to observe the sky in the mid- and far- infrared, ultraviolet and X- andγ -ray wavebands. The observability of the sky in different astronomical wavebands is illustrated in Fig. 1.1b, which shows the transparency of the atmosphere as a function of wavelength. In this representation, the solid line indicates how high a telescope has to be located above the surface of the Earth for the atmosphere to become transparent to radiation of different wavelengths. Let us first summarise the observational challenges and the nature of the objects which dominate all-sky images of these wavebands.1
1.3 Optical waveband 3 × 10
14ν 10
15Hz;
1 μ m λ 300 nm
1.3.1 Observing in the optical waveband
Until 1945, astronomy meant optical astronomy and Fig. 1.1a shows that this corresponds to studying the Universe in the rather narrow wavelength interval 300–800 nm, and hence to black-body temperatures in the range 3000–10 000 K. The wavelength range to which
1Many more details of the history of the different types of astronomy discussed in the succeeding sections of this chapter are included in my book The Cosmic Century: A History of Astrophysics and Cosmology (Longair, 2006).
6 High energy astrophysics – an introduction
0
1 000 000 000 K
Optical
10 000 K
3 K
Radio Infrared
X-ray γ-ray
Millimetre Ultraviolet
2
6 8 10 12 14 16 18 20 22 24
12
9
6
3
0
–3
–1 0 –2 –3 –4 –5 –6 –7 –8 –9
–2 –4 –6 –8
log (wavelength, m)
log (photon energy, eV) log (frequency, Hz)
6 8 10 12 14 16 18 20 22 24
log (frequency, Hz)
log (temperature, K)log (fraction of atmosphere) Altitude, km
–10 –12 –14
0 2 4 6 8 10
150140 130120 110 100 90 80 70 60 50 40 30 20 10 0 –2
–4 –6
(a)
(b)
Fig. 1.1 (a) The relation between the temperature of a black-body and the frequencyν (or wavelength λ) at which most of the energy is emitted (solid red line). The frequency (or wavelength) plotted is that corresponding to the maximum of a black-body at temperature T. Convenient expressions for this relation are:νmax= 1011(T/K) Hz;
λmaxT= 3 × 106nm K. The ranges of wavelength corresponding to the different wavebands – radio, millimetre, infrared, optical, ultraviolet, X- andγ -ray – are shown. (b) The transparency of the atmosphere for radiation of different wavelengths. The solid line shows the height above sea-level at which the atmosphere becomes transparent for radiation of different wavelengths (Giacconi et al., 1968; Longair, 1988).
7 1.3 Optical waveband
our eyes are sensitive is roughly 400–700 nm, corresponding to the blue and red ends of the optical spectrum, respectively. At the short wavelength end of the waveband, the atmosphere becomes opaque because of absorption by ozone in the upper atmosphere. The absorption sets in rather abruptly with decreasing wavelength so that observations from ground-based observatories at wavelengths less than about 320 nm are generally impossible. This has the beneficial effect of protecting us from the Sun’s hard ultraviolet radiation. Most people derive their intuitive picture of the Universe from observations in the optical waveband.
For most types of observation, photographic plates have now been replaced by electronic detectors such as charge-coupled devices (CCD) which have quantum efficiencies of about 80% at the red end of the optical spectrum (500–1000 nm). The band-gap of silicon corresponds to a limiting maximum wavelength of about 1 μm and so optical CCDs are more or less limited to the classical optical waveband. Nowadays, it is routine to observe with CCD arrays of, say, 2000× 2000 picture elements (pixels) and greater. Mosaics of CCD arrays can be used to provide coverage of large areas of sky, as has been achieved in the Sloan Digital Sky Survey. The result has been a huge increase in the quantity and quality of the data which can be analysed astrophysically.
When it was commissioned in the late 1940s, the Palomar 200-inch telescope was an outstanding feat of optical-mechanical engineering and it dominated much of astrophysical and cosmological research for the subsequent 30 years. Five metres was regarded as the maximum feasible aperture because the telescope had to have sufficient stiffness to track and guide accurately over the entire celestial hemisphere. By the 1980s, it was realised that the route to larger aperture was to use the increasing power of computers to build lighter telescopes and then to restore the stiffness electronically by multiply-embedded computer control systems. In so doing, much improved performance has been achieved for telescopes in the 8–10 metre class. The incorporation of adaptive optics into the optical train of these telescopes has meant that they can now operate close to the diffraction limit. There are now plans for even larger telescopes, the challenge being to build them at affordable cost.
1.3.2 Optical all-sky images
Images of the northern and southern celestial hemispheres are shown in Fig. 1.2a. These are plotted in equidistant azimuthal polar or zenith equidistant projections and were re- constructed by Mellinger from 51 wide-angle photographs (Mellinger, 2007). The image of the northern celestial hemisphere on the left has the north celestial pole at declination δ = 90◦in the centre, while the celestial equator,δ = 0◦, is the bounding circle around the edge of the picture.2Close inspection of the image shows a number of clearly recognisable constellations, for example, the Plough or Great Bear pointing towards the North Pole star, which is close to the centre of the image. The right-hand image shows the southern celes- tial hemisphere, centred on the southern celestial pole atδ = −90◦. Because two images have been used to span the whole sky, the distortions are not too great, as shown in Fig.
A.3a of Appendix A.1. In both diagrams, the Milky Way is clearly seen as a broad band of emission spanning both hemispheres. The Galactic Centre region lies in the southern
2For details of the coordinate systems and projections used in astronomy, see Appendix A.1.
8 High energy astrophysics – an introduction
(a)
(b)
Fig. 1.2 All-sky images of the celestial sphere in the optical waveband created by Dr. Axel Mellinger from 51 wide-angle images. The photographs were taken at observing sites in California, South Africa and Germany, image processed and joined together digitally. How these images were created is explained in his web site at http://home.arcor-online.
de/axel.mellinger/. (a) The northern (left) and southern (right) celestial hemispheres are plotted in equidistant azimuthal polar or zenith equidistant projections. The Milky Way is the broad band of emission seen in both images and is much more prominent in the southern than in the northern skies. (b) The optical image of the whole sky in Galactic coordinates in a Hammer–Aitoff projection. The nearby dwarf companion galaxies to our own Galaxy, the Large and Small Magellanic Clouds, are seen in the southern Galactic hemisphere at about Galactic longitudes 290◦ and 310◦, respectively. (Courtesy of Dr. Axel Mellinger.)
celestial hemisphere atδ ≈ −29◦ and much more of the Galactic plane can be observed from that hemisphere as compared with the northern hemisphere. The two bright galaxies close to the centre of the image of the Southern Galactic Hemisphere are the Large and Small Magellanic Clouds, our nearest neighbouring galaxies.
A Hammer–Aitoff projection of Mellinger’s observations enables the complete 4π stera- dians of the celestial sphere to be projected onto a two-dimensional flat surface (Fig. 1.2b).
This projection adopts a reasonable compromise between shape and scale distortions, the
9 1.4 Infrared waveband
magnitude of these being indicated in Fig. A.3b of Appendix A.1. Although equal areas are preserved, the geometric distortions become large towards the northern and southern Galactic poles. The north and south Galactic poles (b=±90◦) are at the top and bottom of the image. The scale of Galactic longitude runs from 0◦at the centre which is the direction of the Galactic Centre, through+180◦at the left of the image, the anti-Centre direction, and then from+180◦at the right of the image to 360◦(or 0◦) at the Centre. The Hammer–Aitoff projection is commonly used in the astronomical literature to display images of the whole sky, and the all-sky images in other astronomical wavebands discussed later in this chapter are presented in this projection.
The light seen in Fig. 1.2 is almost entirely the integrated light of stars. Some of the light of the Galaxy is due to hot diffuse gas, particularly the ionised gas observed in the vicinity of regions of star formation. One of the disadvantages of observing in the optical waveband is immediately apparent from Fig. 1.2. There are patchy dark features present in the image of the Milky Way and these are associated with extinction by interstellar dust grains. Tiny dust particles, typically about 1 μm in diameter, strongly scatter and absorb light rays, resulting in the patchy obscuration seen in Fig. 1.2b. Dust extinction complicates the interpretation of optical observations and corrections need to be made for it.
Optical observations are fundamental for astronomy because a significant fraction of the baryonic matter in the Universe is locked up in stars with masses within a factor of about 10 of that of the Sun and these emit a large fraction of their energy in the optical waveband. Since they have long lifetimes, they are the most readily observable objects in the Universe. The stars are assembled into galaxies and these are the basic building blocks of the Universe.
Many different types of high energy astrophysical object are present in our Galaxy, in- cluding supernovae, supernova remnants, white dwarfs, neutron stars, stellar-mass black holes and the supermassive black hole in the Galactic Centre. These are, however, often difficult to observe in the optical waveband, partly because they are intrinsically rather faint optically and also because of interstellar extinction. Optical observations are, how- ever, crucial in identifying the sources of the radiation and understanding their roles in stellar evolution. A number of these compact stars are members of binary systems and the companion star can often be identified optically. This is of great importance in determining their distances and masses.3
1.4 Infrared waveband 3 × 10
12ν 3 × 10
14Hz;
100 λ 1 μ m
1.4.1 Observing in the infrared waveband
The problem of dust extinction is a strong function of wavelength, the extinction coefficient α being roughly proportional to λ−1, whereα is defined by I = I0e−αrand r is the distance
3More details of methods of determining distances and masses are given in Appendices A.2 and A.3.
10 High energy astrophysics – an introduction
1 0 0.5 1
1.2 1.65 2.2 3.5 5 10 20 35 350 450 650 850
10
Wavelength (μm)
Obscured by Earth’s atmosphere
Relative transmission
100 1000
Fig. 1.3 The transmission of the atmosphere as a function of wavelength in the infrared (1 λ 100 μm) and submillimetre (100 λ 1000 μm) wavebands. The central wavelengths of the observable windows in these wavebands in microns are indicated by the numbers at the top of the diagram. The precipitable water vapour content of the atmosphere is assumed to be 1 mm. (After diagram, courtesy of the Royal Observatory, Edinburgh.)
of the source. Thus, the effects of extinction become rapidly much less important in the infrared as compared with the optical waveband. Infrared radiation suffers, however, from molecular absorption and scattering in the Earth’s atmosphere, what is often referred to as telluric absorption, so that the sky can only be observed in certain wavelength windows.
Figure 1.3 shows the transmission of the atmosphere in the waveband interval 1 λ 1000 μm. The centres of the infrared windows in the wavelength range 1 λ 100 μm are at wavelengths of 1.2, 1.65, 2.2, 3.5, 5, 10, 20 and 35 μm and they are conventionally labelled the J, H, K, L, M, N, Q and Z infrared wavebands, respectively. The last two windows are only accessible from very high, dry sites and even observations at 10 μm are often difficult, except under the best observing conditions. Observations outside these windows have to be undertaken from balloons, high-flying aircraft or satellite observatories. There is thus a complementarity between the types of observation attempted from the ground and from above the Earth’s atmosphere.
A distinctive problem to be overcome in infrared astronomy is that the telescope and the Earth’s atmosphere are strong thermal emitters of infrared radiation. For example, the radiation of a black-body at room temperature, say 300 K, peaks at a wavelength of about 10 μm. Therefore, normally, the strength of the signal from an astronomical source is very much weaker than the background due to the telescope and the atmosphere at these wavelengths. For this reason, telescopes dedicated to thermal infrared observations, such as IRAS and the Spitzer Space Telescope, incorporate cooling of the telescope and the focal plane instrumentation to minimise the thermal background.
The infrared waveband is conveniently divided into near and thermal infrared wave- lengths. The distinction is related to those parts of the waveband at which the observations are detector-noise limited (the near-infrared) and those in which the thermal background
11 1.4 Infrared waveband
radiation from the sky and the telescope are the dominant source of noise (the thermal infrared). The distinction thus depends upon the type of observation being undertaken.
Broad-band observations at wavelengths longer than 3 μm are thermal background limited, whereas those at shorter wavelengths are normally detector-noise limited. In making obser- vations in the thermal infrared waveband, the observer is almost always searching for very faint signals against an enormous thermal background. Detector technology for infrared wavelengths has made enormous strides over the last 20 years. Infrared detector arrays almost as large as the CCD detector arrays used in optical astronomy are now available and these have revolutionised essentially all areas of astronomy.
The observing strategy is therefore to observe the sky in those wavelength windows in which there is good atmospheric transparency from ground-based telescopes. This has the advantage that the observations can be made with 8–10 metre aperture telescopes and com- plex instrumentation. The wavebands which are inaccessible from the ground have to be observed from above the Earth’s atmosphere, preferably from satellite observatories. Neces- sarily, these are generally smaller than the ground-based telescopes and massive instrumen- tation cannot be accommodated. The Spitzer Infrared Space Telescope is a splendid example of the state-of-the-art in infrared space technology. In due course it will be superseded by the James Webb Space Telescope, which will be a 6.5 metre infrared-optimised space telescope.
1.4.2 Infrared all-sky images
Images of the whole sky in the near-infrared waveband have much reduced interstellar extinction by interstellar dust grains and the structure of our Galaxy can be clearly seen.
Figures. 1.4a and b provide excellent examples of the structure of the Galaxy as observed in the 1.2–2.2 μm wavebands. Figure 1.4a is an all-sky image obtained by the DIRBE instrument of the Cosmic Background Explorer (COBE). This instrument scanned the sky in the J, H and K wavebands and these were combined to create the colour image seen in Fig. 1.4a. The disc and bulge of the Galaxy are clearly seen, as well as a thin dust absorption layer lying in the Galactic plane.
Figure 1.4b shows another approach to mapping the Galaxy using observations from the ground-based Two Micron All-Sky Survey (2MASS). This survey was carried out using two 1.3 metre dedicated infrared telescopes, one located at Mount Hopkins in Arizona and the other at the Cerro Tololo InterAmerican Observatory in Chile. Almost 300 million stars were catalogued. The image shown in Fig. 1.4b was created by plotting the positions of almost 100 million stars brighter than K=13.5 from the 2MASS catalogue. This approach provides an even clearer image of the stellar distribution in the Galaxy. The Large and Small Magellanic Clouds are clearly visible in the southern Galactic hemisphere, as is the elongated central bulge of the Galaxy which has been interpreted as a bar in the central regions of the Galaxy. These images make the important point that, in the infrared waveband, interstellar dust becomes transparent and so it is possible to observe deep inside regions which are obscured at optical wavelengths. Among the most important of these are regions of star formation which are enshrouded in interstellar dust, and the very central regions of our own Galaxy. Observations of infrared stars very close to the Galactic Centre have provided wholly convincing evidence for a supermassive black hole with mass M≈ 2.6 × 106 M.
12 High energy astrophysics – an introduction
(a)
(b)
Fig. 1.4 Images of the celestial sphere in the near-infrared waveband. (a) A false-colour image of the near-infrared sky as observed by the DIRBE instrument of the Cosmic Background Explorer (COBE). Data at 1.25, 2.2 and 3.5 μm are colour-coded blue, green and red, respectively, in a Hammer–Aitoff projection. (Courtesy of NASA and the COBE Science Team.) (b) The structure of the Galaxy determined by the distribution of almost 100 million stars detected in the 2MASS sky survey. (Courtesy of the 2MASS Science Team and IPAC.)
Inspection of Fig. 1.2a shows that the typical temperatures of the objects which radiate in the 1–100 μm waveband are 1000> T > 10 K and so Fig. 1.4 provides images of the cold Universe. Thus, cool stars, cool red giant envelopes and cold objects such as brown dwarfs can be observed directly in these wavebands. One of the most distinctive features of these wavebands is, however, the fact that, at wavelengths longer than about 3 μm, dust grains become strong emitters rather than absorbers of radiation. They emit more or less like little black-bodies at the temperature to which they are heated by the radiation they absorb. They do not radiate at shorter wavelengths because, if the grains were heated to temperatures greater than about 1000 K, they would evaporate.
13 1.4 Infrared waveband
Fig. 1.5 A composite image of the celestial sphere in the far-infrared waveband in a Hammer–Aitoff projection. The observations were made with the DIRBE instrument of the COBE satellite and were made at 60 μm (blue), 100 μm (green) and 240 μm (red). Zodiacal light due to sunlight scattered by interplanetary dust has been removed from this image. (Courtesy of Edward Wright and the COBE Science Team.)
The first complete survey of the far-infrared sky was carried out in 1983–4 by the Infrared Astronomical Satellite (IRAS) in four broad wavelength bands centred on 12, 25, 60 and 100 μm. It revealed intense far-infrared emission from regions of star formation in our own Galaxy and nearby galaxies as well as a host of new detections of stars, galaxies, active galaxies and quasars. Among the most important discoveries was a class of starburst galaxies which emit most of their radiation in the far-infrared waveband.
A more recent image of the far-infrared sky has been created from observations with the DIRBE instrument of COBE from all-sky maps made at 60, 100 and 240 μm (Fig. 1.5). The emission seen in this image is almost entirely the radiation of heated dust grains. Regions of star formation are particularly prominent features of the image. They can be seen forming a thin disc in the Galactic plane, as well as being present in the Magellanic Clouds, which are well known to be sites of active star formation. Intense emission associated with the Orion Molecular Cloud can be seen towards the right-hand edge of the image in the southern Galactic hemisphere. The Orion Nebula is of particular importance for studies of star formation since it is the region of massive star formation closest to the Earth. The colour coding of Fig. 1.5 is such that hot and cold dust have blue and red tinges, respectively. The bluish regions are mostly associated with discrete regions of active star formation, while the reddish clouds appear all over the image and extend to high Galactic latitudes. The latter clouds are often referred to as infrared cirrus.
The importance of these observations for high energy astrophysics is that they indicate where active regions of star formation are located. These are always associated with regions in which the interstellar gas densities are high and this is particularly important in studies
14 High energy astrophysics – an introduction
of the nuclei of active galaxies. The relation between star formation and high energy astrophysical activity is one of the more important and intriguing features of this study.
1.5 Millimetre and submillimetre waveband 30 GHz ν 3 THz; 10 λ 0 .1 mm
1.5.1 Observing in the millimetre and submillimetre waveband
The millimetre and submilimetre wavebands are particularly rich astronomically. In addition to the extension of radio astronomical phenomena to shorter wavelengths, distinct features of these wavebands are the presence of a wealth of molecular lines in cool sources and the Cosmic Microwave Background Radiation. The transparency of the atmosphere varies dramatically with wavelength in this waveband (Fig. 1.3). At wavelengths less than about 1 mm, there are very strong absorption bands due to water vapour, carbon dioxide and other molecules in the Earth’s atmosphere. The transparency of the atmosphere is particularly sensitive to the amount of water vapour in the atmosphere. To have a reasonable chance of making observations in the atmospheric windows at 850, 650, 450 and 350 μm, it is essential to observe from a high, dry site. Examples of such sites include: the Mauna Kea Observatory in Hawaii at 4200 m, where the James Clerk Maxwell Telescope (JCMT), the CalTech Submillimetre Observatory (CSO) and the Smithsonian SubMillimetre Array (SMA) are located; the Chajnantor plateau in the Atacama desert in Chile at 5100 m, the site of the Atacama Large Millimetre Array (ALMA); and the South Pole where the sub-zero temperatures ensure that there is very low precipitable atmospheric water vapour. At these sites, there is less than 1 mm of precipitable water vapour for considerable fractions of the time, enabling observations to be made in the shortest wavelength windows. To make observations in the other parts of the waveband, it is necessary to make observations from above the Earth’s atmosphere, either from high-flying aircraft, such as the Kuiper Airborne Observatory, or from satellite observatories.
The receivers and detectors for the millimetre and submillimetre wavebands have devel- oped dramatically over the last 10 years. Before that time, observations were made using single element detectors which were either heterodyne receivers, similar to those familiar in the radio waveband, or bolometers which measured the total incident power within a given waveband. In 1997, the first submillimetre camera, the SCUBA submillimetre bolometer array, was commissioned on the JCMT and has revolutionised studies in these wavebands.
Arrays of heterodyne receivers are also now available which enable the spectral mapping of extended astronomical objects to be carried out.
1.5.2 Millimetre and submillimetre all-sky images
Millimetre and submillimetre all-sky images are dominated by the Cosmic Microwave Background Radiation which was discovered, more or less by chance, by Penzias and Wilson in 1965 (Penzias and Wilson, 1965). Figure 1.6a illustrates the stunning result that
15 1.5 Millimetre and submillimetre waveband
ΔT = 3.353 mK
ΔT = 18 μK T = 2.728 K (a)
(b)
(c)
Fig. 1.6 Maps of the whole sky in Hammer–Aitoff projections in Galactic coordinates as observed at a wavelength of 5.7 mm (53 GHz) by the COBE satellite at different sensitivity levels. (a) The distribution of total intensity over the sky. (b) Once the uniform component is removed, a dipole component associated with the motion of the Earth through the isotropic background radiation is observed, as well as a weak signal from the Galactic plane. (c) Once the dipole component is removed, radiation from the plane of the Galaxy is seen as a bright band across the centre of the picture. The fluctuations seen at high Galactic latitudes are a combination of noise from the telescope and the instruments and a genuine cosmological signal. At high latitudes, an excess sky noise signal of cosmological origin amounts to 30± 5μK (Bennett et al., 1996).
the Cosmic Microwave Background Radiation is extraordinarily uniform over the whole sky with a perfect black-body spectrum at a radiation temperature of 2.728 K. It is wholly convincing that this radiation is the cooled remnant of the hot early phases of the Big Bang.
At a sensitivity level of about one part in 1000 of the total intensity, large scale anisotropy of dipolar form is observed over the whole sky (Fig.1.6b). The plane of our Galaxy can also be observed as a faint band of emission along the Galactic equator. The global dipole anisotropy is naturally attributed to aberration effects associated with the
16 High energy astrophysics – an introduction
Fig. 1.7 A map of the whole sky in Galactic coordinates as observed by the WMAP satellite at millimetre wavelengths (Bennett et al., 2003). The angular resolution of the map is about 20 times higher than that of Fig. 1.6c. The emissions due to Galactic dust and synchrotron radiation have been subtracted from this map.
Earth’s motion through an isotropic radiation field. Excluding regions close to the Galactic plane, the temperature distribution was found to have precisely the expected dipolar form, T = T0[1+ (v/c) cos θ], where θ is the angle with respect to the direction of maximum intensity andv is the Earth’s velocity through the isotropic background radiation. The am- plitude of the cosmic microwave dipole was 3.353 ± 0.024 mK (Bennett et al., 1996). It was inferred that the Solar System is moving at about 350 km s−1with respect to the frame of reference in which the radiation would be 100% isotropic.
On angular scales of 7◦ and greater, Bennett and his colleagues achieved sensitivity levels better than one part in 100 000 of the total intensity from analyses of the complete microwave dataset obtained over the four years of the COBE mission (Fig. 1.6c). At this sensitivity level, the radiation from the plane of the Galaxy is intense, but is confined to a broad strip lying along the Galactic equator. Away from the plane, there are significant intensity fluctuations of cosmological origin from beamwidth to beamwidth over the sky.
These fluctuations are present at the level of only about 1 part in 100 000 of the total intensity.
The detection of these fluctuations is a crucial result for understanding the origin of large scale structures in the Universe.4 It is interesting to compare the COBE map (Fig. 1.6c) with the more recent Wilkinson Microwave Anisotropy Probe (WMAP) observations made with about 20 times higher angular resolution (Bennett et al., 2003) (Fig. 1.7). It can be seen that the same large scale features are present on both maps.
There is however, much more to the millimetre and submillimetre wavebands than just the Cosmic Microwave Background Radiation. The dust emission seen in Fig. 1.5 has a continuum spectrum with a strongly inverted spectrum, roughly Iν∝ ν3−4, and so con- tributes to the background radiation at submillimetre wavelengths. In addition, line emis- sion of interstellar molecules is observed from the plane of the Galaxy and is particularly intense in regions of star formation. The commonest interstellar molecule is molecular
4 I have dealt in extenso with these observations and their interpretation in my book Galaxy Formation (Longair, 2008).
17 1.6 Radio waveband
Fig. 1.8 A map of the whole sky in Galactic coordinates in the carbon dioxide molecule CO. (Courtesy of the LAMBDA progam of GSFC of NASA.)
hydrogen but it has zero electric dipole moment and so is not observed in emission. The next most common molecule is carbon monoxide CO which has a strong electric dipole moment and is observed throughout the plane of the Galaxy, as can be seen in Fig. 1.8. The radiation is narrowly confined to the Galaxy plane in the hemisphere towards the Galactic Centre, but in the anti-Centre direction the distribution is somewhat broader. To the right of the image in the southern Galactic hemisphere, the giant molecular cloud associated with the Orion Complex can be seen, centred on the Orion Nebula.
In addition, the continuum radiation of radio sources observed in the metre and centimetre radio wavebands is also present at millimetre wavelengths. These include the diffuse radio synchrotron and bremsstrahlung emission of the interstellar medium of our Galaxy and dis- crete Galactic and extragalactic radio sources, which are described in the next section. From the perspective of studies of the Cosmic Microwave Background Radiation, the dust and ra- dio background components are regarded as interfering foregrounds which need to be care- fully subtracted from the millimetre sky maps to reveal the underlying cosmological signals.
Observations in the millimetre and submillimetre wavebands impact high energy astro- physics in many different ways. Perhaps most significantly, the Cosmic Microwave Back- ground provides an omnipresent radiation background from which high energy particles cannot escape.
1.6 Radio waveband 3 MHz ν 30 GHz;
100 m λ 1 cm
1.6.1 Radio astronomy and the origin of high energy astrophysics
Radio waves of extraterrestrial origin were discovered by Jansky in the early 1930s but this caused little stir in the astronomical community. After the Second World War,
18 High energy astrophysics – an introduction
Fig. 1.9 Images of the celestial sphere at a radio frequency of 408 MHz in a Hammer–Aitoff projection. This image is dominated by the radio emission of relativistic electrons gyrating in the interstellar magnetic field, the process known as synchrotron radiation. The radiation is most intense in the plane of the Galaxy but it can be seen that there are extensive ‘loops’ and filaments of radio emission extending far out of the plane. (Courtesy of Max-Planck-Institut f¨ur Radioastrotiomie, Bonn.)
radio astronomy developed very rapidly as major advances were made in electronics, radio techniques and digital computers. Radio emission was discovered from a wide range of dif- ferent astronomical objects. Some of the radio emission processes could be associated with phenomena observed at optical wavelengths, for example, the free–free or bremsstrahlung emission of hot electrons in regions of ionised hydrogen, but others were quite new. It was soon established that the radio emission of most sources was the synchrotron radiation of ultra-relativistic electrons spiralling in magnetic fields. Contrary to what might have been expected from Fig. 1.1a, the radio observations provided information about some of the very hottest, relativistic, plasmas in the Universe.
Two features of the radio observations were of particular significance. First, a number of the most massive galaxies known were found to be extremely powerful sources of radio waves. They were so powerful that it was easy to detect them as radio sources at cosmological distances. Estimates of the amount of energy necessary to power these radio sources showed that they must contain an energy in relativistic matter equivalent to a rest mass energy of about 100 million solar masses, that is, 108Mc2≈ 2 × 1055 J. These galaxies had to be able to convert mass of this order into relativistic particle energy. The second key fact was that the radio emission did not generally originate from the galaxy itself but from huge radio lobes which extended far beyond the confines of the parent galaxy. In the 1960s and 1970s it was established that the sources of these vast energies were the active nuclei of the host galaxies and that the extended structures resulted from the expulsion of this energy from the nuclei in the form of jets of relativistic plasma. These discoveries revealed the presence of two major new components of the Universe, relativistic plasma and magnetic fields. These discoveries were the touchstone for the explosive growth of high energy and relativistic astrophysics over subsequent years.
The study of these radio sources led to further discoveries. Amongst the earliest of these was the fact that supernova remnants are very powerful sources of synchrotron radio
19 1.6 Radio waveband
Fig. 1.10 A Hubble Space Telescope image of the quasar 3C 273, showing the optical jet ejected from the quasar nucleus. The images at the bottom of the picture are galaxies at the same distance as the quasar. 3C 273 was the first quasar for which a redshift was measured, thanks to the presence of the redshifted Balmer series of hydrogen in emission in its optical spectrum (Schmidt, 1963). (Courtesy of NASA and the Space Telescope Science Institute.)
emission and so must be capable of accelerating charged particles to ultra-relativistic energies and creating strong magnetic fields. A remarkable outcome of the study of the extragalactic radio sources was the discovery of the quasi-stellar radio sources, or quasars, in the early 1960s. In these objects, the starlight of the galaxy is completely overwhelmed by the intense non-thermal optical radiation from the nucleus, in some cases, the optical luminosity being more than 1000 times greater than that of the parent galaxy (Fig. 1.10).
These objects and their close relatives, the BL-Lacertae or BL-Lac objects, which were discovered in 1968, are among the most powerful energy sources known in the Universe.
But more was to follow. In 1967 Hewish and Bell constructed a low frequency radio array to study very short time-scale fluctuations imposed upon the intensities of compact radio sources by density fluctuations in the interplanetary plasma streaming out from the Sun, what is known as the Solar Wind. During the commissioning phase of the array, sources consisting entirely of pulsed radio emission with very stable periods of about 1 s were discovered, the radio pulsars. They were soon identified conclusively as rotating, magnetised neutron stars and thus provided the first definite proof of the existence of these highly compact stars in which the central densities are as high as 1018 kg m−3. A key point from the perspective of relativistic astrophysics was the fact that solar mass objects had been discovered with radii only about a factor of 4 or 5 times greater than the Schwarzschild radius of solar mass black holes. Thus, in these compact objects, general
