Part I. Talks

Section E. Physical Processes in Relativistic Astrophysical Plasmas



Title: Collisionless Halos around Black Holes
Author: A. Gruzinov
Affil: Institute for Advanced Study, School of Natural Sciences, Princeton, NJ 08540, U.S.A.
Abstract: When a black hole accretes slowly, the radiative cooling of the infalling gas is weak and the accretion disk does not form. A hot, collisionless, quasi-spherical halo is formed instead. The properties of such halos are discussed. The rate of accretion, the radiative efficiency, and the temperature and density near the hole are evaluated.


Title: Gamma-ray Bursts and Afterglow
Author: R. Sari
Affil: Theoretical Astrophysics 130-33, California institute of Technology, Pasadena CA 91125, U.S.A.
Abstract: The origin of GRBs has been a mystery for almost 30 years. Their sources emit a huge amount of energy on short time scales, and the process involves extreme relativistic motion with a bulk Lorentz factor of at least a few hundred. In the last two years, ``afterglow'' emission in X-ray, optical, IR, and radio was detected. The afterglow can be measured up to months and even years after the few-seconds GRB. We review the theories for the gamma-ray emission and its afterglow, and show that these are strongly supported by observations. A recent detection of optical emission simultaneous with the GRB agrees well with theoretical predictions and further constrains the free parameters of the models. We discuss the evidence that some of the bursts are jets and discuss the prospects of polarization measurements.


Title: Population Synthesis of GRB Progenitors: Problems With Kicks
Author: C. L. Fryer
Affil: UCO/Lick Observatory, UC Santa Cruz, Santa Cruz, CA 95064, U.S.A.
Abstract: Accretion disks around stellar-mass black holes are now thought to be the engines which power classical gamma-ray bursts (GRBs). These disks are formed almost exclusively in binaries, and to study the characteristics of the progenitors of these black-hole accretion disk (BHAD) GRBs, we must understand the uncertainties in binary population synthesis calculations. Kicks imparted onto nascent neutron stars and black holes are among the most misunderstood concepts of binary population synthesis. In this paper, we outline the current understanding (or lack of understanding) of these kicks and discuss their effect on BHAD GRBs and binary population synthesis as a whole.


Title: Hypernovae: SNe 1997ef, 1998bw, and 1997cy
Authors: T. Nakamura(1), K. Maeda(1), K. Iwamoto(2), T. Suzuki(2), K. Nomoto(2), P. A. Mazzali(2), M. Turatto(3), I. J. Danziger(4), and F. Patat(5)
Affil: (1) Department of Astronomy and Research Center for the Early Universe, University of Tokyo, Japan (2) Department of Physics, College of Science and Technology, Nihon University, Japan (3) Osservatorio Astronomico di Padova, vicolo dell'Osservatorio, Padova, Italy (4) Osservatorio Astronomico di Trieste, via G. B. Tiepolo, Trieste, Italy (5) European Southern Observatory, Garching, Germany
Abstract: We discuss the properties of the very energetic Type Ic supernovae (SNe Ic) 1998bw and 1997ef, and of Type IIn supernova (SN IIn) 1997cy. SNe Ic 1998bw and 1997ef are characterized by their large luminosity and very broad spectral features. Their observed properties can be explained if they are very energetic SN explosions (E_K > (or approximately) 1x10^[52] erg), originating probably from the core collapse of the bare C+O cores of massive stars (~30--40 M_[Sun]). At late times, both the light curve and the spectra suggest that the explosion may have been asymmetric; this may help us understand the claimed connection with GRBs. Type IIn SN 1997cy is even more luminous than SN 998bw, and the light curve declines more slowly than the [^56]Co decay. We model such a light curve with circumstellar interaction, which requires the explosion energy of ~5x10^[52] erg. Because these kinetic energies of explosion are much larger than in normal core-collapse SNe, we call objects like these SNe ``hypernovae''.


Title: Compton Scattering, Pair Annihilation, and Pair Production in a Plasma
Author: V. Krishan
Affil: Indian Institute of Astrophysics, Bangalore 560034, India
Abstract: The square of the four-momentum of a photon in vacuum is zero. However, in an unmagnetized plasma, it is equal to the square of the plasma frequency. Further, the electron-photon coupling vertex is modified in a plasma to include the effect of the plasma medium. I calculate the cross sections of three processes in a plasma---Compton scattering and electron-positron pair annihilation and production. At high plasma densities, the cross sections are found to change significantly. Such high plasma densities exist in several astrophysical sources.