Gamma-Ray Astrophysics New Insight Into the Universe Second Edition
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NASA Reference Publication 1386 September 1997 Gamma-Ray Astrophysics New Insight Into the Universe Second Edition Carl E. Fichtel Jacob I. Trombka Goddald Space Flight Center Greenbelt, Maryland National Aeronautics and Space Administration Geddard Space Flight Center Greenbelt, Maryland 20771 1996 This publication is available from the NASA Center for AeroSpace Information, 800 Elkridge Landing Road, Linthicum Heights, MD 21090-2934, (301) 621-0390. PREFACE During the 15 years that have passed since the first edition of this book was published, there has been a major increase in our knowledge of gamma-ray astronomy. Much of this advance arises from the extensive results that have been forthcoming from the Compton Gamma-Ray Observatory. There has been the discovery of a new class of gamma-ray objects, namely high-energy gamma- ray-emitting blazars, a special class of Active Galactic Nuclei, whose basic high-energy properties now seem to be understood. A much improved picture of our galaxy now exists in the frequency range of gamma rays. The question of whether cosmic rays are galactic or metagalactic now seems settled with certainty. Significant new information exists on the gamma-ray properties of neutron star pulsars, Seyfert galaxies, and gamma-ray bursts. Substantial new insight has been obtained on solar phenomena through gamma-ray observations. Hence, this seemed to be an appropriate time to write a new edition of this book to add the important scientific implications of these many new findings. The special importance of gamma-ray astrophysics had long been recognized by many physicists and astronomers, and theorists had pursued many aspects of the subject well before the experimental results began to become available. The slower development of the experimental side was not because of a lack of incentive, but due to the substantial experimental difficulties that had to be overcome. Thus, as the gamma-ray results became available in much greater number and detail, it was possible to build upon the theoretical work that already existed and to make substantial progress in the study of many of the phenomena involved. Consequently, a much better understanding of many of the astrophysical phenomena mentioned here and others is now possible. Our principal aims in writing this book are the same as they were for the first edition: to provide a text which describes the significance of gamma-ray astrophysics and to assemble in one place a treatment of gamma rays emitted from bodies in the solar system,from objectsin our galaxy,as well as from interactions betweencosmicraysandtheinterstellarmedium,andfrom beyond our galaxy. Thus,this book is intendedfor thosein astrophysics who wishto havetheopportunity to learnmore abouttheevolving field of gamma-rayastronomyand its relationshipto the high- energy,evolutionaryprocessesoccurringin theuniverse. Wehaveassumedthatthereaderswill havehadbasicundergraduate coursesin physics, includingelectromagnetic,atomic,and nuclear theory,andwill be familiar with therudimentsof astronomy. We haveattemptedto write thoseportionswherereferenceto thecontent of more advancedastronomyandphysicscoursesis appropriatein suchaway thatthegeneralflow of thoughtwill not be interrupted in a significantway for readersunfamiliarwith thedetailsof these particulartheories,yetthosewith afull understandingof the physics involvedwill seethevalidity andqualificationsclearly. After consideringwhetherto discussthe gamma-rayproduction processestogethernearthe beginningof the book or to describe eachas it occurredin the descriptionof individual astrophysical subjects,we chosethelatterapproach.Thisdecisionresultedin part from whatappearedto be a morenaturaldevelopmentandpartially from thestudents',andour own, enthusiasmfor the study of the astrophysicalphenomenaandtheir desireto learnabouteachsubject in depthasit waspresentedto them. Thelastthreechaptersof thebook providea generaldiscussionof the experimentalaspectsof the field that seemedbest treated together,separatelyfrom the astrophysicalaspectsof gamma-ray astronomythat are discussedin the first ten chapters. A list of symbols and a subjectindex are provided at the end for your reference. Thebookwould nothavebeenpossiblewithoutthecooperationof a very largenumberof individuals. We gratefullyacknowledgethe help of many scientistswith whom we had discussionsover the years,who provided us with material--some of which had not appearedin the literatureat thetime-- andwho were alwaysready to answerquestionsabouttheirwork. ii CONTENTS INTRODUCTION TO GAMMA-RAY ASTRONOMY 2 GAMMA-RAY OBSERVATIONS OF THE SOLAR SYSTEM 2.1 Introduction I1 2.2 Models of Solar System Evolution 11 2.3 Solar System Gamma-Ray Astronomical Observations 14 3 PLANETS, COMETS, AND ASTEROIDS 3. I Introduction 18 3.2 Interaction Processes 19 3.3 Solar X-Ray Fluorescence 20 3.4 Charged-Particle X-Ray Fluorescence 27 3.5 Natural Radioactivity 27 3.6 Primary and Secondary Galactic Cosmic-Ray Interactions 30 3.7 Solar Proton Interactions 45 3.8 Experimental Results 46 3.9 Apollo X-Ray Spectrometer Results 48 3.10 The Apollo Gamma-Ray Spectrometer 53 SOLAR OBSERVATIONS 4.1 Introduction 63 4.2 Interaction Processes 65 4.3 Charged- and Neutral-Particle Interactions 65 4.4 Continuum Emission 75 iii 4.5 Decay ofrt ° Mesons 76 4.6 Positron Annihilation and Formation of Positronium 77 4.7 Experimental Results 79 4.8 Solar-Flare Observation in the Energy Region 0.1 to l0 MEV 79 4.9 High-Energy Continuum Radiation 84 5 DIFFUSE GALACTIC EMISSION, COSMIC RAYS, AND THE INTERSTELLAR MEDIUM 5.1 Introduction 90 5.2 Cosmic Rays, Interstellar Matter, Photons, and Magnetic Fields 91 5.3 Interaction Processes for Continuum Emission 98 5.4 Gamma-Ray Lines 103 5.5 Galactic Dynamic Balance and the Cosmic-Ray Distribution 105 5.6 Current High-Energy Gamma-Ray Results and Their Interpretation 107 5.7 Current Low-Energy Gamma-Ray Results and Their Interpretation 118 5.8 Summary 121 GALACTIC COMPACT OBJECTS 6.1 Introduction 124 6.2 Individual Neutron Stars and Pulsars 124 6.3 Gamma-Ray Pulsar Overview 146 6.4 Accretion-Driven Pulsars 150 6.5 Supernovae 153 6.6 Black Holes 156 6.7 Mini Black Holes 158 6.8 Other High-Energy Gamma-Ray Sources 160 7 NORMAL GALAXIES 7.1 Introduction 167 7.2 The Small Magellanic Cloud 168 7.3 The Large Magellanic Cloud 170 iv 7.4 Supernovae in Other Galaxies 171 7.5 Some Other Relatively Close Galaxies 173 ACTIVE GALAXIES 8.1 Introduction 176 8.2 Seyfert and Radio Galaxies 177 8.3 Blazars 184 8.4 The Evolution of Gamma-Ray-Emitting Blazars 198 8.5 Two Types of Blazars 203 8.6 The Origin of the Gamma-Ray Emission From Blazars and the Requirements for the Parent Relativistic Particles 208 8.7 Particle Acceleration 211 8.8 Summary 214 9 DIFFUSE RADIATION 9.1 Introduction 219 9.2 Observations 220 9.3 Cosmology 224 9.4 The Origin of the Diffuse Radiation 230 9.5 Summary 235 10 GAMMA-RAY BURSTS 10.1 Introduction 238 10.2 Distribution of Sources in Direction and Intensity 238 10.3 Energy and Temporal Distributions in the Low- Energy Gamma-Ray Region 242 10.4 High-Energy Gamma-Ray Emulsion 244 10.5 Possible Sources and Mechanisms 246 11 GAMMA-RAY INTERACTION PROCESSES 11.1 Introduction 250 11.2 Photoelectric Effect and Fluorescence Radialion 253 11.3 Pair Production and Annihilation Radiation 254 11.4 Compton Scattering 262 11.5 Electron Leakage and Bremsstrahlung 265 V 12 DETECTORS FOR ENERGIES LESS THAN 25 MeV 12.1 Introduction 267 12.2 Properties of Detectors 268 12.3 Detector Response to Monoenergetic Radiation 286 12.4 Detector Efficiency 296 12.5 Radiation Damage, Annealing 299 12.6 Sources of Gamrna-Ray Background in the Space Environment 301 12.7 Detector Flight Systems 322 13 DETECTORS FOR ENERGIES GREATER THAN 25 MeV 13.1 Introduction and History 343 13.2 The High-Energy Gamma-Ray Interaction 348 13.3 The 20 MeV to 50 GeV Region 35 I 13.4 The Greater Than 20 GeV Region 359 13.5 Angular Resolution 36 l 13.6 Energy Measurement and Resolution 364 SUBJECT INDEX 368 vi CHAPTER 1 GAMMA-RAY ASTRONOMY IN PERSPECTIVE As we begin the study of gamma-ray astronomy and the astrophysical phenomena whose secrets these highest-energy photons best reveal, it is important to remember that, throughout most of history, our view of the heavens was restricted to the visible portion of the electromagnetic spectrum. Our concept of the Universe even now is strongly influenced by knowledge gained from this very important, but relatively quite small, wavelength band. Many major advances in our understanding of the Universe have come in this century as optical telescopes have grown larger. For example, in the 1920s it became clear that many galaxies beyond our own exist and that the Sun is not even at the center of our galaxy, but is, in fact, far from the Galactic center. These findings destroyed the prevalent belief that our solar system was the center of the Universe. Soon after the discovery of the existence of other galaxies, of which there are now known to be over a hundred billion, it was found that they are systematically receding from each other. This discovery lead to the present "big bang" theory of the Universe. It was further realized that our Sun is a rather ordinary star and that many galaxies release far more energy than our own. Still, our present knowledge of the Universe would be much more restricted if the visible wavelength range had remained the sole source of information. Many phenomena can be understood or even revealed only by the investigation of emission in other parts of the electromagnetic spectrum. The first region of the spectrum beyond the optical to be explored was the radio range. Although radio emission from the Galaxy was detected in the 1930s, it was not until the 1950s that radio astronomy began to have a major impact. Studies of the 21 cm line, resulting from the hyperfine splitting of the ground state of atomic hydrogen, showed the full spiral structure of our galaxy. The 21-cm line emission, and the continuum radio emission generally accepted to be synchrotron radiation, which indicates the presence and distribution of relativistic electrons, are just two examples of astrophysical information to come from a wavelength region outside the familiar, visible one.