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Author

Robert J. Gould

Other affiliations: University of Sydney
Bio: Robert J. Gould is an academic researcher from University of California, San Diego. The author has contributed to research in topics: Photon & Cosmic ray. The author has an hindex of 16, co-authored 28 publications receiving 2867 citations. Previous affiliations of Robert J. Gould include University of Sydney.

Papers
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Journal ArticleDOI
TL;DR: In this paper, the authors derived the total energy loss and photon-production spectrum by the processes of Compton scattering, bremsstrahlung, and synchrotron radiation from highly relativistic electrons.
Abstract: Expressions are derived for the total energy loss and photon-production spectrum by the processes of Compton scattering, bremsstrahlung, and synchrotron radiation from highly relativistic electrons. For Compton scattering, the general case, the Thomson limit, and the extreme Klein-Nishina limit are considered. Bremsstrahlung is treated for the cases where the electron is scattered by a pure Coulomb field and by an atom. For the latter case the effects of shielding are discussed extensively. The synchrotron spectrum is derived for an electron moving in a circular orbit perpendicular to the magnetic field and also for the general case where the electron's motion is helical. The total photon-production spectrum is derived for each process when there is a power-law distribution of electron energies. The problems of the effects of the three processes on the electron distribution itself are considered. It is shown that if the electron loses a small fraction of its energy in a single occurrence of a process, the electron distribution function satisfies a continuity equation which is a differential equation in energy space. For the more general case where the electron can lose energy in discrete amounts (as in bremsstrahlung and extreme Klein-Nishina Compton losses), the electron distribution function satisfies an integro-differential equation. Some approximate solutions to this equation are derived for certain special cases.

1,504 citations

Journal ArticleDOI
TL;DR: In this article, the absorption probability of high-energy photons traversing isotropic photon gases having various spectra was derived in graphical form for photon spectra having blackbody and power-law form.
Abstract: General formulas for the absorption probability from the process $\ensuremath{\gamma}+{\ensuremath{\gamma}}^{\ensuremath{'}}\ensuremath{\rightarrow}{e}^{+}+{e}^{\ensuremath{-}}$ are given for high-energy photons traversing isotropic photon gases having various spectra. Asymptotic formulas are derived and general results are given in graphical form for photon spectra having blackbody and power-law form. The results are applied in a following paper to calculate the absorption probability for high-energy photons traversing cosmic distances.

426 citations

Journal ArticleDOI
TL;DR: In this paper, high energy photon absorption in space due to electron-positron pair production in photon-photon collision was investigated. But the photon absorption rate was not shown.
Abstract: High energy photon absorption in space due to electron-positron pair production in photon-photon collision

284 citations

Journal ArticleDOI
TL;DR: In this paper, the absorption probability per unit path length by the process of pair production in photon-photon collisions is computed as a function of energy for high-energy photons traversing this photon gas, using the results of the previous paper.
Abstract: Based on observational data, the spectra of cosmic radio, microwave, infrared, optical, and x-ray photons are estimated. The absorption probability per unit path length by the process of pair production in photon-photon collisions is then computed as a function of energy for high-energy photons traversing this photon gas, using the results of the previous paper. These calculations show that there should be a dip in the intensity of the high-energy cosmic photon spectrum by about a factor of 10 between ${10}^{12}$ and ${10}^{13}$ eV due to absorption by optical (\ensuremath{\sim} a few eV) photons. Above ${10}^{14}$ eV, the high-energy cosmic photon spectrum should essentially cut off because of the strong absorption by the cosmic 3\ifmmode^\circ\else\textdegree\fi{}K blackbody photons, and at higher energies by the cosmic radio photons.

278 citations

Journal ArticleDOI
TL;DR: In this article, the total photo-ionization cross section of atomic helium was analyzed and a lower abundance (10.92 based on 12.00 for hydrogen) was adopted for helium, based on recent radio determinations.
Abstract: Improved results on the total photo-ionization cross section of atomic helium are given. The total cross section includes contributions from simultaneous ionization and excitation of ${\mathrm{He}}^{+}$ and double ionization; the combined effect of these two processes adds about 10% to the normal photo-ionization process where ${\mathrm{He}}^{+}$ is left in the ground state. A lower abundance (10.92 based on 12.00 for hydrogen) is adopted for helium, based on recent radio determinations. In calculating the opacity due to $K$-shell photo-ionization of heavy elements, a lower abundance (8.00) is also adopted for neon. Brief mention is made of the effects of irregularities in the density distribution of the interstellar gas on the problems of both x-ray and radio-wave absorption.

93 citations


Cited by
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Journal ArticleDOI
TL;DR: In this paper, an improved model for the absorption of X-rays in the interstellar medium (ISM) is presented for use with data from future X-ray missions with larger effective areas and increased energy resolution such as Chandra and the X-Ray Multiple Mirror mission.
Abstract: We present an improved model for the absorption of X-rays in the interstellar medium (ISM) intended for use with data from future X-ray missions with larger effective areas and increased energy resolution such as Chandra and the X-Ray Multiple Mirror mission, in the energy range 100 eV. Compared with previous work, our formalism includes recent updates to the photoionization cross section and revised abundances of the interstellar medium, as well as a treatment of interstellar grains and the H2 molecule. We review the theoretical and observational motivations behind these updates and provide a subroutine for the X-ray spectral analysis program XSPEC that incorporates our model.

3,239 citations

Journal ArticleDOI
TL;DR: A review of the current theoretical understanding of the physical processes believed to take place in GRB's can be found in this article, where the authors focus on the afterglow itself, the jet break in the light curve, and the optical flash that accompanies the GRB.
Abstract: Gamma-ray bursts (GRB's), short and intense pulses of low-energy $\ensuremath{\gamma}$ rays, have fascinated astronomers and astrophysicists since their unexpected discovery in the late sixties. During the last decade, several space missions---BATSE (Burst and Transient Source Experiment) on the Compton Gamma-Ray Observatory, BeppoSAX and now HETE II (High-Energy Transient Explorer)---together with ground-based optical, infrared, and radio observatories have revolutionized our understanding of GRB's, showing that they are cosmological, that they are accompanied by long-lasting afterglows, and that they are associated with core-collapse supernovae. At the same time a theoretical understanding has emerged in the form of the fireball internal-external shocks model. According to this model GRB's are produced when the kinetic energy of an ultrarelativistic flow is dissipated in internal collisions. The afterglow arises when the flow is slowed down by shocks with the surrounding circumburst matter. This model has had numerous successful predictions, like the predictions of the afterglow itself, of jet breaks in the afterglow light curve, and of the optical flash that accompanies the GRB's. This review focuses on the current theoretical understanding of the physical processes believed to take place in GRB's.

1,800 citations

Journal ArticleDOI
TL;DR: In this paper, the authors derived the total energy loss and photon-production spectrum by the processes of Compton scattering, bremsstrahlung, and synchrotron radiation from highly relativistic electrons.
Abstract: Expressions are derived for the total energy loss and photon-production spectrum by the processes of Compton scattering, bremsstrahlung, and synchrotron radiation from highly relativistic electrons. For Compton scattering, the general case, the Thomson limit, and the extreme Klein-Nishina limit are considered. Bremsstrahlung is treated for the cases where the electron is scattered by a pure Coulomb field and by an atom. For the latter case the effects of shielding are discussed extensively. The synchrotron spectrum is derived for an electron moving in a circular orbit perpendicular to the magnetic field and also for the general case where the electron's motion is helical. The total photon-production spectrum is derived for each process when there is a power-law distribution of electron energies. The problems of the effects of the three processes on the electron distribution itself are considered. It is shown that if the electron loses a small fraction of its energy in a single occurrence of a process, the electron distribution function satisfies a continuity equation which is a differential equation in energy space. For the more general case where the electron can lose energy in discrete amounts (as in bremsstrahlung and extreme Klein-Nishina Compton losses), the electron distribution function satisfies an integro-differential equation. Some approximate solutions to this equation are derived for certain special cases.

1,504 citations

Book
01 Jan 1981
TL;DR: In this article, the authors introduce high energy astrophysics in the context of galaxies and the origin of cosmic rays in our galaxy, as well as the acceleration of high energy particles in magnetic fields.
Abstract: Part I. Astronomical Background: 1. High energy astrophysics - an introduction 2. The stars and stellar evolution 3. The galaxies 4. Clusters of galaxies Part II. Physical Processes: 5. Ionisation losses 6. Radiation of accelerated charged particles and bremsstrahlung of electrons 7. The dynamics of charged particles in magnetic fields 8. Synchrotron radiation 9. Interactions of high energy photons 10. Nuclear interactions 11. Aspects of plasma physics and magnetohydrodynamics Part III. High Energy Astrophysics in our Galaxy: 12. Interstellar gas and magnetic fields 13. Dead stars 14. Accretion power in astrophysics 15. Cosmic rays 16. The origin of cosmic rays in our galaxy 17. The acceleration of high energy particles Part IV. Extragalactic High Energy Astrophysics: 18. Active galaxies 19. Black holes in the nuclei of galaxies 20. The vicinity of the black hole 21. Extragalactic radio sources 22. Compact extragalactic sources and superluminal motions 23. Cosmological aspects of high energy astrophysics Appendix References Index.

1,280 citations

Journal ArticleDOI
TL;DR: In this paper, the authors present a self-consistent model of accretion flows around black holes that unifies all of these states except the very high state, which is an extension of the following paradigm applied successfully to the quiescent state.
Abstract: Black hole X-ray binaries (BHXBs) are known to display five distinct spectral states. In order of increasing luminosity these are the quiescent state, low state, intermediate state, high state, and very high state. We present a self-consistent model of accretion flows around black holes that unifies all of these states except the very high state. The model is an extension of the following paradigm, which has been applied successfully to the quiescent state. The accretion flow consists of two zones, an inner advection-dominated accretion flow (ADAF) that extends from the black hole horizon to a transition radius rtr, and an outer thin accretion disk that is present beyond rtr. Above the disk is a hot corona, which is a continuation of the inner ADAF. The model consistently treats the dynamics of the accreting gas, the thermal balance of the ions and electrons in the two-temperature ADAF and corona, and the radiation processes that produce the observed spectrum. At low mass accretion rates, 0.01 (in Eddington units), the inner ADAF zone in the model radiates extremely inefficiently, and the outer thin disk is restricted to large radii (rtr ~ 102-104, in Schwarzschild units). The luminosity therefore is low, and this configuration is identified with the quiescent state. For 0.01 and up to a critical value crit${r crit}$ --> ~0.08, the radiative efficiency of the ADAF increases rapidly and the system becomes fairly luminous. The spectrum is very hard and peaks around 100 keV. This is the low state. The exact value of crit${r crit}$ --> depends on the viscosity parameter α (crit${r crit}$ --> ~1.3α -->2; the paper assumes α = 0.25). For values of > crit${r crit}$ --> and up to a second critical value about 10% higher, the ADAF progressively shrinks in size, the transition radius decreases, and the X-ray spectrum changes continuously from hard to soft. We identify this stage with the intermediate state. Finally, when is sufficiently large, the inner ADAF zone disappears altogether and the thin accretion disk extends down to the marginally stable orbit. The spectrum is dominated by an ultrasoft component with a weak hard tail. This is the high state. Model spectra calculated with this unified scenario agree well with observations of the quiescent, low, intermediate, and high states. Moreover, the model provides a natural explanation for the low state to high state transition in BHXBs. We also make a tentative proposal for the very high state, but this aspect of the model is less secure. An important feature of the model is that it is essentially parameter free. We test the model against observations of the soft X-ray transient Nova Muscae during its 1991 outburst. The model reproduces the observed light curves and spectra surprisingly well and makes a number of predictions that can be tested by observations of other BHXBs.

1,257 citations