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16950 results

Page 354 of 848

  • 18 - Active galaxies

  • from Part IV - Extragalactic high energy astrophysics
  • Malcolm S. Longair, University of Cambridge
  • Book:
    High Energy Astrophysics
    Published online:
    05 June 2012
    Print publication:
    03 February 2011, pp 585-609

  • Summary

    Introduction

    The objective of this chapter is to set the scene for the later chapters, in which the physics of many different aspects of active galactic nuclei and their interactions with their surroundings are studied in some detail. Much of the lore and terminology of the active galactic nuclei are the product of the historical development of the subject. The history of the discovery of different types of active galaxy and the techniques used to find them are described briefly in this chapter. A key feature of active galactic nuclei is that they are intrinsically broad-band, indeed multi-waveband, objects, each waveband providing complementary information, as well as possessing their own terminology and astrophysical infrastructure. There are several excellent books on different aspects of active galaxies. Active Galactic Nuclei by Robson (1999), An Introduction to Active Galactic Nuclei by Peterson (1997), Quasars and Active Galactic Nuclei – an Introduction by Kembhavi and Narlikar (1999), and Active Galactic Nuclei by Krolik (1999) can be recommended as providing a range of varied approaches to putting some order into their study.

    Radio galaxies and high energy astrophysics

    Cosmic rays, discovered by Hess in 1913, provided the first evidence for the existence of relativistic matter originating from extraterrestrial sources (Sect. 1.10). It was, however, only after the Second World War and the development of the new astronomies that the astrophysical role of high energy particles and cosmic magnetic fields could be addressed on the basis of astronomical observation.

  • 5 - Ionisation losses

  • from Part II - Physical processes
  • Malcolm S. Longair, University of Cambridge
  • Book:
    High Energy Astrophysics
    Published online:
    05 June 2012
    Print publication:
    03 February 2011, pp 131-153

  • Summary

    Introduction

    When high energy particles pass through a solid, liquid or gas, they can cause considerable wreckage to the constituent atoms, molecules and nuclei. Specifically, they cause:

  • (i) the ionisation and excitation of the atoms and molecules of the material. In the process of ionisation, electrons are torn off atoms by the electrostatic forces between the charged high energy particle and the electrons. This is not only a source of ionisation but also a source of heating of the material because of the transfer of kinetic energy to the electrons;

  • (ii) the destruction of crystal structures and molecular chains;

  • (iii) nuclear interactions between the high energy particles and the nuclei of the atoms of the material.

    • In this chapter we will be principally concerned with the first of these processes, ionisation losses, which are important in a number of different contexts. They influence the propagation of high energy particles under cosmic conditions and the associated energy losses provide an effective mechanism for heating the interstellar gas, for example, in giant molecular clouds. Equally important is the use of the ionisation losses of high energy particles in particle detectors – these provide a means of identifying the properties of the particles as well as providing a measure of their incident fluxes upon the detector.

    There is a pedagogical reason for beginning with ionisation losses. From the astrophysical perspective, ionisation losses provide an example of the procedures which have to be followed in working out the various ways in which high energy particles interact with matter.

  • 23 - Cosmological aspects of high energy astrophysics

  • from Part IV - Extragalactic high energy astrophysics
  • Malcolm S. Longair, University of Cambridge
  • Book:
    High Energy Astrophysics
    Published online:
    05 June 2012
    Print publication:
    03 February 2011, pp 714-752

  • Summary

    The cosmic evolution of galaxies and active galaxies

    Evidence for strong evolutionary changes of the populations of extragalactic objects with cosmic epoch was first found in surveys of extragalactic radio sources and quasars in the 1950s and 1960s. An excess of faint sources was discovered in radio source and quasar surveys as compared with the expectations of uniform world models. The inference was that these classes of object were much more common at earlier cosmic epochs than they are at the present time. During the 1980s, the first deep counts of galaxies to very faint magnitudes became available thanks to the CCD revolution in optical detector technology. An excess of faint blue galaxies was discovered and these studies were extended to extremely faint apparent magnitudes by Hubble Space Telescope observations of the Hubble Deep Field and the Hubble Ultra-Deep Field.

    This pattern of the discovery of excess numbers of faint objects at early cosmic epochs has been repeated in essentially all wavebands as deep surveys have become feasible. In the 1990s, surveys of the X-ray sky carried out by the ROSAT X-ray Observatory provided evidence for an excess of faint X-ray sources, similar to that found for the extragalactic radio sources and quasars. These studies were extended to much fainter X-ray sources by observations with the Chandra and XMM-Newton X-ray Observatories. The IRAS survey of the mid-and far-infrared sky, although not extending to as large redshifts as the radio and X-ray surveys, found evidence for an excess of faint sources.

  • 15 - Cosmic rays

  • from Part III - High energy astrophysics in our Galaxy
  • Malcolm S. Longair, University of Cambridge
  • Book:
    High Energy Astrophysics
    Published online:
    05 June 2012
    Print publication:
    03 February 2011, pp 493-535

  • Summary

    Cosmic ray protons, nuclei and electrons are the only particles which have been detected from sources outside the Solar System. As observed at the top of the atmosphere, about 98% of the particles are protons and nuclei whilst about 2% are electrons. Of the protons and nuclei, about 87% are protons, 12% are helium nuclei and the remaining 1% are heavier nuclei.

    Figure 15.1 provides a quick overview of the complete cosmic ray spectrum. A very wide range of energies is observed and the spectrum can be described by power-law distributions over many decades in energy. There are, however, important features in the spectrum, including the ‘knee’ at 1015 eV and the ‘ankle’ at 1018 eV. It is convenient to consider first cosmic rays with energies in the range 109−1015 eV and then those with higher energies. This division corresponds to the different techniques which are used to detect the cosmic rays, particle detectors in space observatories in the lower energy range and the cosmic ray air-shower technique at the higher energies. These are distinguished by the change of symbols in Fig. 15.1, which shows the results of a large number of experiments.

    The energy spectra of cosmic ray protons and nuclei

    The energy spectra of cosmic rays can be well represented by power-law energy distributions as illustrated in Fig. 1.16, which shows the differential energy spectra for protons, helium, carbon and iron nuclei as a function of the kinetic energy per nucleon of the particles (Simpson, 1983).

  • Preface

  • Malcolm S. Longair, University of Cambridge
  • Book:
    High Energy Astrophysics
    Published online:
    05 June 2012
    Print publication:
    03 February 2011, pp xiii-xvi

  • Summary

    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). Volume 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.

  • 8 - Synchrotron radiation

  • from Part II - Physical processes
  • Malcolm S. Longair, University of Cambridge
  • Book:
    High Energy Astrophysics
    Published online:
    05 June 2012
    Print publication:
    03 February 2011, pp 193-227

  • Summary

    The synchrotron radiation of ultra-relativistic electrons dominates much of high energy astrophysics. The radiation, which was first observed in early betatron experiments, is the emission of high energy electrons gyrating in a magnetic field and is the process responsible for the radio emission of our Galaxy, of supernova remnants and extragalactic radio sources. It is also the origin of the non-thermal continuum optical emission of the Crab Nebula and quite possibly of the optical and X-ray continuum emission of quasars. The term non-thermal emission is frequently used in high energy astrophysics and is conventionally taken to mean the continuum radiation of a distribution of particles with a non-Maxwellian energy spectrum. Continuum emission is often referred to as ‘non-thermal’ if its spectrum cannot be accounted for by the spectrum of thermal bremsstrahlung or black-body radiation.

    It is a major undertaking to work out all the detailed properties of synchrotron radiation. For more complete treatments, the enthusiast is referred to the books by Bekefi (1966), by Pacholczyk (1970) and by Rybicki and Lightman (1979), and to the review articles by Ginzburg and Syrovatskii (1965, 1969). Many of the most important results can, however, be derived by simple physical arguments (Scheuer, 1966). First of all, let us work out the total energy loss rate.

  • 1 - High energy astrophysics – an introduction

  • from Part I - Astronomical background
  • Malcolm S. Longair, University of Cambridge
  • Book:
    High Energy Astrophysics
    Published online:
    05 June 2012
    Print publication:
    03 February 2011, pp 3-34

  • Summary

    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 observations 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.

  • 22 - Compact extragalactic sources and superluminal motions

  • from Part IV - Extragalactic high energy astrophysics
  • Malcolm S. Longair, University of Cambridge
  • Book:
    High Energy Astrophysics
    Published online:
    05 June 2012
    Print publication:
    03 February 2011, pp 681-713

  • Summary

    The evidence of Chap. 21 shows that the huge fluxes of relativistic material needed to power extended extragalactic radio sources originate close to the active galactic nuclei of the host galaxies. Direct evidence for extreme events in active galactic nuclei is provided by the superluminal motions observed in compact radio sources, by the properties of variable extragalactic γ-ray sources and by the γ-ray bursts. The extreme properties of these sources require them to be moving at highly relativistic velocities.

    Compact radio sources

    Direct evidence for the presence of ultra-relativistic electrons in the nuclei of active galaxies is provided by very long baseline interferometric (VLBI) studies of radio quasars and BL-Lac objects at centimetre wavelengths. Combining the angular sizes of these ultra-compact radio sources with their flux densities Sv, the brightness temperature Tb = (λ2/2k)(Sv/Ω) of the source region can be determined, where Ω is the solid angle subtended by the radio source. Observations of large samples of strong compact radio sources with structures on the scale of 1 milliarcsecond have shown that the maximum brightness temperatures are of the order of 1011–1012 K, none of them exceeding the limit of 1012 K at which catastrophic synchrotron self-Compton radiation would take place, as described in Sect. 9.6 (Kellermann et al., 1998).

  • 20 - The vicinity of the black hole

  • from Part IV - Extragalactic high energy astrophysics
  • Malcolm S. Longair, University of Cambridge
  • Book:
    High Energy Astrophysics
    Published online:
    05 June 2012
    Print publication:
    03 February 2011, pp 637-660

  • Summary

    The prime ingredients of active galactic nuclei

    It is convenient to divide the necessary ingredients of active galactic nuclei into two types – the primary ingredients, which originate close to the black hole and its associated accretion disc, and secondary phenomena, which result from the interaction of the primary ingredients with the environment of the black hole. Figure 20.1 is a schematic diagram showing some of the components of typical models. The primary ingredients are intense non-thermal continuum radiation and fluxes of relativistic material in the form of highly collimated jets. The secondary phenomena result from the interaction of the primary components with the surrounding medium, in particular, gas clouds in the vicinity of the nucleus and the ambient interstellar and intergalactic gas. The former gives rise to the strong emission line spectrum observed at optical, ultraviolet and infrared wavelengths whilst the interactions of the relativistic jets with the interstellar and intergalactic gas give rise to the structures observed in extragalactic radio sources and in intense γ-ray emission. We study the physics of high energy particles in extragalactic radio sources and galactic nuclei and the role of relativistic beaming in the following chapters.

    The continuum spectrum

    As discussed in Chap. 18, active galactic nuclei contain intense continuum emission with non-thermal spectra. The examples illustrated in that chapter include typical spectra of Types 1 and 2 Seyfert galaxies (Fig. 18.5a and b), a composite quasar spectrum (Fig. 18.1) and a multi-waveband spectrum of the BL-Lac object OJ287 (Fig. 18.7).

  • 21 - Extragalactic radio sources

  • from Part IV - Extragalactic high energy astrophysics
  • Malcolm S. Longair, University of Cambridge
  • Book:
    High Energy Astrophysics
    Published online:
    05 June 2012
    Print publication:
    03 February 2011, pp 661-680

  • Summary

    Extended radio sources – Fanaroff–Riley types

    All galaxies are sources of radio emission – high energy electrons are accelerated in supernova remnants and these are dispersed throughout the interstellar medium where they radiate radio waves by synchrotron radiation. These are, however, very weak radio emitters indeed compared with what are conventionally referred to by the terms radio galaxy or radio quasar in which the radio luminosity can exceed that of our own Galaxy by factors of 108 or more. The big surprise was the discovery that the most luminous radio sources contain jets of relativistic material which give rise to a wide variety of large scale radio structures. The example of the brightest extragalactic radio source in the northern sky, Cygnus A, as observed by the Very Large Array, illustrates a number of the characteristic features of these sources (Fig. 21.1). Many more details are contained in the review by Carilli and Barthel (1996).

  • The radio spectra of all regions of the radio structure have non-thermal spectra and the radiation is linearly polarised. These features make the identification of the radio emission as synchrotron radiation wholly convincing.

  • The huge radio lobes are symmetrically disposed on either side of the active galactic nucleus but they extend far beyond the confines of the host galaxy as can be seen in Fig. 21.1a and b.

  • […]

  • 6 - Radiation of accelerated charged particles and bremsstrahlung of electrons

  • from Part II - Physical processes
  • Malcolm S. Longair, University of Cambridge
  • Book:
    High Energy Astrophysics
    Published online:
    05 June 2012
    Print publication:
    03 February 2011, pp 154-177

  • Summary

    Introduction

    Bremsstrahlung, or free–free emission, appears in many different guises in astrophysics. Applications include the radio emission of compact regions of ionised hydrogen at temperature T ≈ 104 K, the X-ray emission of binary X-ray sources at T ≈ 107 K and the diffuse X-ray emission of intergalactic gas in clusters of galaxies, which may be as hot as T ≈ 108 K. It is also an important loss mechanism for relativistic cosmic ray electrons. Before proceeding to the analysis of the bremsstrahlung of electrons, we need to establish a number of general results concerning the electromagnetic radiation of accelerated charged particles and its spectrum. These results will be of wide applicability to the many radiation processes studied in this book.

    The radiation of accelerated charged particles

    Relativistic invariants

    Gould has provided an excellent introduction to the use of relativistic invariants in the study of electromagnetic processes (Gould, 2005). We will develop a number of these in the course of this exposition. The first of these is the transformation of the energy loss rate by electromagnetic radiation as observed in different inertial frames of reference, that is, how dE/dt changes from one inertial frame of reference to another.

    In fact, dE/dt is a Lorentz invariant between inertial frames of reference.

Page 354 of 848