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28213 results in Astrophysics

Page 797 of 1411

  • 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.

  • 14 - Accretion power in astrophysics

  • 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 443-492

  • Summary

    Introduction

    Accretion means the accumulation of diffuse gas or matter onto some object under the influence of gravity. Accretion from the interstellar medium onto stationary and moving stars was the subject of a number of pioneering papers by Bondi, Lyttelton and Hoyle and, in the light of subsequent studies, these have proved to provide quite accurate predictions for the rate of accretion (Hoyle and Lyttleton, 1939; Bondi and Hoyle, 1944; Bondi, 1952). The subject was reinvigorated in the 1960s by the realisation that accretion of matter onto supermassive black holes is a remarkably effective means of accounting for the extreme properties of active galactic nuclei and, even more, by the discovery of intense X-ray sources associated with binary systems in our Galaxy. The discovery of these objects and the ensuing flourishing of theory ushered in a new epoch in high energy astrophysics. Accretion was also applied to binary systems involving white dwarf stars and these processes could account for the properties of cataclysmic variables.

    Let us begin our analysis by deriving some of the simple relations which show how naturally accretion can account, in principle, for many of the key features of galactic X-ray sources and active galactic nuclei.

    Accretion – general considerations

    The efficiency of the accretion process

    Consider the accretion of matter onto a star of mass M and radius R. If the matter falls onto the star in free-fall from infinity, it acquires kinetic energy as its gravitational potential energy becomes more negative.

  • 13 - Dead stars

  • 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 378-442

  • Summary

    The stars described in Chap. 3 are held up by the thermal pressure of hot gas, the source of energy being nuclear energy generation in their central regions. As evolution proceeds from the main sequence, up the giant branch and towards the final phases when the outer layers of the giant star are ejected, nuclear processing continues until the available nuclear energy resources of the star are exhausted. The more massive the star, the more rapidly it evolves and the further it can proceed along the path to the synthesis of iron, the most stable of the chemical elements. In the most massive stars, M ≥ 8 M⊙, it is likely that the nuclear burning can proceed all the way through to iron whereas in less massive stars, the oxygen flash, which occurs when core burning of oxygen begins, may be sufficient to disrupt the star. In any case, at the end of these phases of stellar evolution, the core of the star runs out of nuclear fuel and collapses until some other form of pressure support enables a new equilibrium configuration to be attained.

    Possible equilibrium configurations which can exist when the nuclear fuel runs out are as white dwarfs, neutron stars or black holes. In white dwarfs and neutron stars, the star is supported by degeneracy pressure associated with the fact that electrons, protons and neutrons are fermions and so only one particle can occupy any single quantum mechanical state.

  • 2 - The stars and stellar evolution

  • 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 35-76

  • Summary

    Introduction

    The theory of stellar structure and evolution is one of the most exact of the astrophysical sciences. It is inextricably involved in many of the topics needed to understand the role which high energy astrophysical processes play in the origin and evolution of stars and galaxies, providing, for example, evidence on their chemical abundances, the ages of the systems, and so on. The objective of this chapter is to provide a succinct summary of a number of the key results needed in the subsequent development of the story. Many of the equations and concepts will recur in different guises in the course of the exposition. There are many excellent books on these vast topics, my personal favourites being the books by Tayler, Karttunen and his colleagues, and by Kippenhahn and Weigert (Tayler, 1994; Karttunen et al., 2007; Kippenhahn and Weigert, 1990). The last volume is a classic and is particularly strong on the physics of the stars.

    Basic observations

    It is necessary to become familiar with some of the vocabulary of the study of the stars and the basic results of observation. These studies begin with measurements of the total amount of radiation emitted by a star, its luminosity L, and its surface temperature T. The spectra of stars are not black-bodies and so the effective temperature Teff is introduced. It is defined to be the temperature of a black-body of the same radius as the star which would emit the same luminosity.

  • 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 129-130

  • Summary

    The second part of this book is concerned with elementary physical processes involved in studies of high energy phenomena in the Universe. There are many excellent books which discuss this material at various levels of sophistication. Those which I have found most helpful are Jackson's Classical Electrodynamics (Jackson, 1999), Radiation Processes in Astrophysics by Rybicki and Lightman (1979) and Electromagnetic Processes by Gould (2005). Zombeck's Handbook of Space Astronomy and Astrophysics (Zombeck, 2006) contains a very useful compendium of relevant data.

    My intention is to emphasise the underlying physical principles involved in these processes so that the functional forms of the equations have an intuitive significance. I will build up each discussion gently, often deriving approximate results which give physical insight before deriving, or quoting, the results of more complete calculations. I will treat the key processes of synchrotron radiation and inverse Compton scattering in some detail.

    In the various calculations and derivations, I use Système International (SI) units, which have been officially adopted by almost all countries in the world. According to the Wikipedia web site (2008), ‘Three nations have not officially adopted the International System of Units as their primary or sole system of measurement: Liberia, the Union of Myanmar (Burma) and the United States.’

  • Appendix: Astronomical conventions and nomenclature

  • Malcolm S. Longair, University of Cambridge
  • Book:
    High Energy Astrophysics
    Published online:
    05 June 2012
    Print publication:
    03 February 2011, pp 753-782

  • Summary

    Galactic coordinates and projections of the celestial sphere onto a plane

    The complexities of defining the celestial system of coordinates go far beyond what is needed in this text. These arise because the Earth does not move in a perfectly elliptical orbit about the Sun but is subject to wobbles and precessions because of the perturbing influence of the Moon and planets. These issues are dealt with in the textbooks by Smart and Murray (Smart, 1977; Murray, 1983).

    The positions of celestial objects are described by a fixed set of spherical polar coordinates on the sky known as right ascension (RA or α) and declination (Dec or δ). The north celestial pole (NCP) is defined to be the mean direction of the rotation axis of the Earth and declination is the polar angle measured from the equator (δ = 0°) towards the north celestial pole (δ = 90°) (Fig. A.1). The south celestial pole (SCP) has declination δ = −90°. In the year 2000.0, the Earth's rotation axis was tilted at an angle of 23° 26′ 21.448″ with respect to the direction perpendicular to the plane of the ecliptic, which is the plane of the Earth's orbit about the Sun (Fig. A.1). The coordinates of right ascension and declination are referred to the reference epoch 2000.0 which is known as the 2000.0 coordinate system. The Earth's rotation axis points more or less in the same direction as the Earth moves round the Sun and this gives rise to the seasonal changes of climate.

  • 3 - The galaxies

  • 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 77-98

  • Summary

    Introduction

    Galaxies are complex, many-body systems. Typically, a galaxy can consist of hundreds of millions or billions of stars, it can contain considerable quantities of interstellar gas and dust and can be subject to environmental influences through interactions with other galaxies and with the intergalactic gas. Star formation takes place in dense regions of the interstellar gas. To complicate matters further, it is certain that dark matter is present in galaxies and in clusters of galaxies and that its mass is considerably greater than the mass in baryonic matter. Consequently, the dynamics of galaxies are dominated by this invisible dark component, the nature of which is unknown.

    Traditionally, galaxies have been classified by meticulous morphological studies of samples of bright galaxies. These morphological classification schemes had to encompass a vast amount of detail and this was reflected in Hubble's pioneering studies, as elaborated by de Vaucouleurs, Kormendy, Sandage, van den Bergh and others. The Hubble sequence of galaxies has real astrophysical significance because a number of physical properties are correlated with Hubble type. While the detailed study of individual galaxies was feasible for reasonably large samples, a different approach had to be adopted for massive surveys of galaxies such as the Anglo-Australian 2dF survey (AAT 2dF) and the Sloan Digital Sky Survey (SDSS) which have provided enormous quantitative databases for the studies of galaxies.

  • 12 - Interstellar gas and magnetic fields

  • 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 333-377

  • Summary

    The interstellar medium in the life cycle of stars

    The understanding of the nature and physical properties of the interstellar medium is of the first importance astrophysically since new stars are formed in dense regions of the interstellar gas and the medium is continually replenished by mass loss from stars and by metal-rich material processed in supernova explosions. Thus, the interstellar medium plays a key role in the birth-to-death cycle of stars. The same diagnostic tools are applicable to the study of diffuse gas and magnetic fields anywhere in the Universe, be they galaxies, the intergalactic gas or the environs of active galactic nuclei. Furthermore, interstellar gas will prove to be an essential ingredient in the fuelling of active galactic nuclei.

    The mass of the interstellar gas amounts to about 5% of the visible mass of our Galaxy. In the Galactic plane close to the Sun, the overall gas density is to about 106 particles m−3, but there are very wide variations in density and temperature from place to place throughout the interstellar medium.

    Diagnostic tools – neutral interstellar gas

    Neutral hydrogen: 21-cm line emission and absorption

    Neutral hydrogen emits line radiation at a frequency ν0 = 1420.4058 MHz (λ0 = 21.1 cm) through an almost totally forbidden hyperfine transition in which the spins of the electron and proton change from being parallel to antiparallel.

  • 4 - Clusters of galaxies

  • 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 99-128

  • Summary

    Associations of galaxies range from pairs and small groups, through giant clusters containing over a thousand galaxies, to the vast structures on scales much greater than clusters such as the vast ‘walls’ and voids observed in the distribution of galaxies. Clustering occurs on all scales and very few galaxies can be considered truly isolated. Rich clusters of galaxies are of particular interest because they are the largest gravitationally bound systems in the Universe. The gravitational potential of the cluster is defined by the distribution of dark matter, the mass of which greatly exceeds that of the baryonic matter, such as that contained in the stars in galaxies and the associated interstellar gas and the intracluster gas. The deep gravitational potential wells of clusters can be observed directly through the bremsstrahlung X-ray emission of hot intracluster gas which forms a hydrostatic atmosphere within the cluster. The hot gas can also be detected through the decrements which it causes in the Cosmic Microwave Background Radiation as a result of the Sunyaev–Zeldovich effect. Gravitational lensing has proved to be a very powerful tool for defining the large scale distribution of dark matter in clusters, as well as in individual galaxies within them. Interactions of galaxies with each other and with the intergalactic medium in the cluster can be studied and radio source events can strongly perturb the distribution of hot gas.

  • 16 - The origin of cosmic rays in our Galaxy

  • 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 536-560

  • Summary

    Introduction

    In Sect. 8.9, a convincing case was made that the high energy electrons observed at the top of the atmosphere are a representative sample of those present throughout the interstellar medium and are responsible for the diffuse Galactic synchrotron radio emission. In Sect. 15.4, a similar exercise was carried out for cosmic ray protons. The spectrum and properties of the γ-ray emission of the Galaxy provide compelling evidence that a flux of cosmic ray protons, of similar properties to those observed in our vicinity in the Galaxy, permeates the plane of the Galaxy. In this chapter, these observations are interpreted in terms of the propagation of these particles from their sources through the interstellar medium and the energetics of potential sources in the Galaxy. Key diagnostic tools are provided by the aging processes which can result in observable features in the synchrotron spectra of relativistic electrons and by the energy requirements of sources of synchrotron radiation. The TeV γ-ray emission of supernova remnants is direct evidence for the presence of large fluxes of particles with cosmic ray energies in supernova remnants, although it is not yet clear if these are associated with high energy electrons or protons (Sect. 16.4.2). The tools are developed in the context of the origin of cosmic rays in supernovae explosions and are of applicability to the whole of high energy astrophysics.

Page 797 of 1411