Part I. Talks

Section B. High-Energy Phenomena in and Emission from Astrophysical Plasmas



Title: Neutron Stars in Supernova Remnants
Author: F. Pacini
Affil: Arcetri Astrophysical Obs. and Dept. of Astronomy & Space Sciences, University of Florence, Largo E. Fermi 5, I-50125, Firenze, Italy
Abstract: In the following, we shall briefly summarize some facts and ideas concerning the presence of neutron stars in supernova remnants. While sources similar to the Crab Nebula require the presence of a central energetic object, shell-type remnants such as Cas A are compatible with the presence of neutron stars releasing a weak relativistic wind.


Title: Multiwavelength Diagnostics of Pulsar Plasmas
Author: R. W. Romani
Affil: Dept. of Physics, Stanford University, CA 94305-4060, U.S.A.
Abstract: The phase-resolved broadband (optical to gamma-ray) spectral energy distributions of spin-powered pulsars contain important diagnostics of the emission zone plasma and its acceleration. I summarize some recent observations of young pulsars, describe models for the various spectral components, and argue that the next generation of telescopes and spacecraft will allow a true mapping of the positron/electron plasma flow in the pulsar magnetosphere. Of course, spectral lines and breaks are more powerful tracers than the spectral energy distribution, and since we believe that synchrotron emission dominates in the near-IR to UV, features are expected that will allow even more detailed probes of the plasma physics. Most young pulsars are, however, quite faint, and new technology is needed to extract these diagnostics.


Title: Thermal Radiation from Isolated Neutron Stars
Authors: G. G. Pavlov(1) and V. E. Zavlin(2)
Affil: (1) The Pennsylvania State University, Department of Astronomy and Astrophysics, 525 Davey Lab, University Park, PA 16802, U.S.A. (2) Max-Planck Institut fur Extraterrestrische Physik, 85740 Garching, Germany
Abstract: The analysis of thermal radiation emitted from the atmospheres of isolated neutron stars allows one to measure their surface temperatures, magnetic fields, masses, and radii, as well as the chemical composition of their atmospheres. Thus, multiwavelength observations of this radiation provide an important tool for studying the structure and evolution of neutron stars and for elucidating properties of the superdense matter in their interiors. We describe recent theoretical and observational results on thermal radiation from radio pulsars and radio-quiet neutron stars and discuss their astrophysical implications.


Title: Jets from Compact Objects
Author: H. C. Spruit
Affil: Max-Planck-Institut fur Astrophysik, Postfach 1523, D-85740 Garching, Germany
Abstract: Some topics in the theory of jets are reviewed. These include jet precession, unconfined jets, the origin of knots, the internal shock model as a unifying theme from protostellar jets to gamma-ray bursts, relations between the Blandford-Znajek and MHD disk-wind models, and jet collimation in magnetic acceleration models.


Title: Gamma-rays from Solar Flares
Author: R. Ramaty(1) and N. Mandzhavidze(2)
Affil: (1) NASA/GSFC, Greenbelt, MD 20771, U.S.A. (2) NASA/GSFC and USRA, Greenbelt, MD 20771, U.S.A.
Abstract: Gamma-ray emission is the most direct diagnostic of energetic ions and relativistic electrons in solar flares. Analysis of solar flare gamma-ray data has shown: (i) ion acceleration is a major consequence of flare energy release, as the total flare energy in accelerated particles appears to be equipartitioned between > (or approximately) 1 MeV/nucleon ions and > (or approximately) 20 keV electrons, and amounts to an important fraction of the total energy release; (ii) there are flares for which over 50% of the energy is in alpha particles and heavier ions; (iii) in both impulsive and gradual flares, the particles that interact at the Sun and produce gamma rays are essentially always accelerated by the same mechanism that operates in impulsive flares, probably stochastic acceleration through gyroresonant wave particle interaction; and (iv) gamma-ray spectroscopy can provide new information on solar abundances, for example the site of the FIP-bias onset and the photospheric [^3]He abundance. We propose a new technique for the investigation of mass motion and mixing in the solar atmosphere: the observations of gamma-ray lines from long-term radioactivity produced by flare accelerated particles.


Title: The Scaling of Solar Flare Hard X-ray Emission to Other Flaring Objects in the Universe
Author: P. C. H. Martens
Affil: Department of Physics, Montana State University---Bozeman, Bozeman, MT 59717-3840, U.S.A.
Abstract: Fletcher & Martens have successfully modeled solar hard X-ray sources observed at the top and footpoints of flaring magnetic loops with a Fokker-Planck type particle transport code. I show here that there are invariances in the Fokker-Planck equations that make these results applicable to environments with vastly different physical parameters, such as hard X-ray flares in accretion disks in active galactic nuclei, and in RS CVn and ALGOL type binaries.


Title: High-energy Emission from Supernovae and Remnants
Author: R. A. Chevalier
Affil: Department of Astronomy, University of Virginia, P.O. Box 3818, Charlottesville, VA 22903, U.S.A.
Abstract: An early burst of energetic radiation is expected from a supernova at the time of shock breakout. This emission has not been directly observed but has been inferred from the photoionization around SN 1987A. X-ray emission has been detected from core-collapse supernovae in the days to years after the explosion as they interact with their circumstellar winds. Young Galactic supernova remnants provide the possibility of determining the composition structure of the ejecta through X-ray spectroscopy. An exciting finding for older remnants is that a number of remnants that appear to be interacting with molecular gas may be sources of high-energy gamma-ray emission. The clumpy structure of molecular clouds has implications for the structure expected in high-energy emission. Finally, the field of gamma-ray--line spectroscopy is beginning to yield results relevant to the explosive nucleosynthesis of radionuclides in supernovae.


Title: X/gamma-rays from Active Galactic Nuclei
Author: R. Svensson
Affil: Stockholm Observatory, SE-133 36 Saltsjobaden, Sweden
Abstract: Recent progress using RXTE and BeppoSAX to study the X/gamma emission from radio-quiet active galactic nuclei, i.e., Seyfert galaxies, is reviewed. These satellites allow simultaneous broad-band spectra extending from 0.1--200 keV to be observed for the first time and allow the various spectral components to be determined with some certainty. In particular, the new observations support the unified model of Seyfert galaxies. Most importantly, it has been found that a large fraction of Seyfert 2 galaxies have Compton-thick tori surrounding their nuclei. Spectral transitions in Seyfert galaxies are discussed, as well as recent efforts trying to synthesize the cosmic X/gamma-ray background. Finally, thermal Comptonization in these sources are discussed.


Title: Radiative Processes and Geometry of Spectral States of Black-hole Binaries
Author: A. A. Zdziarski
Affil: N. Copernicus Astronomical Center, Bartycka 18, 00-716 Warszawa, Poland
Abstract: I review radiative processes responsible for X-ray emission in the hard (low) and soft (high) spectral states of black-hole binaries. The main process in the hard state appears to be thermal Comptonization (in a hot plasma) of blackbody photons emitted by a cold disk. This is supported by correlations between the spectral index, the strength of Compton reflection, and the peak frequencies in the power-density spectrum, as well as by the frequency-dependence of Fourier-resolved spectra. Spectral variability may then be driven by the variable truncation radius of the disk. The soft state appears to correspond to the smallest truncation radii. However, the lack of high-energy cutoffs observed in the soft state implies that its main radiative process is Compton scattering of disk photons by nonthermal electrons. The bulk-motion Comptonization model for the soft state is shown to be ruled out by the data.


Title: Gamma-ray Emission from Pulsar Outer Magnetospheres
Author: K. Hirotani
Affil: National Astronomical Observatory, Mitaka, Tokyo 181-8588, Japan
Abstract: We study the gamma-ray emission from an outer-gap accelerator around a rotating neutron star. Assuming the existence of global currents in the magnetosphere, the charge depletion causes a large electric field along the magnetic field lines. This electric field accelerates migratory electrons and positrons which radiate gamma-rays via curvature radiation. These gamma-rays, in turn, produce yet more radiating particles by colliding with the X-rays, leading to a pair-production cascade. Imposing a gap-closure condition that a single pair produces one pair in the gap, on average, we explicitly solve the strength of the acceleration field and demonstrate how the peak energy and the luminosity of the curvature-radiated GeV photons and the cutoff energy and luminosity of Compton-scattered TeV photons depend on such parameters as the surface temperature, the rotational frequency, and the magnetic moment. It is demonstrated that both the GeV and TeV emissions of Geminga will be harder than those of B1055-52, B0656+14, and Vela, and that the TeV fluxes are too small to be observed by current ground-based telescopes.


Title: Population Synthesis of Old Neutron Stars in the Galaxy
Authors: S. B. Popov(1), M. Colpi(2), A. Treves(3), R. Turolla(4), V. M. Lipunov(1), and M. E. Prokhorov(1)
Affil: (1) Sternberg Astronomical Institute, Universiteskii Pr. 13, 119899, Moscow, Russia (2) Dept. of Physics, Univ. of Milan, Via Celoria 16, 20133 Milan, Italy (3) Dipartimento di Scienze, Univ. dell'Insubria, Via Lucini 3, 22100, Como, Italy (4) Dept. of Physics, Univ. of Padova, Via Marzolo 8, 35131 Padova, Italy
Abstract: The paucity of old, isolated accreting neutron stars in ROSAT observations is used to derive a lower limit on the mean velocity of neutron stars at birth. The secular evolution of the population is simulated following the paths of a statistical sample of stars for different values of the initial kick velocity, drawn from an isotropic, Gaussian distribution with mean velocity 0 LE <V> LE 550 km/s. The spin-down, induced by dipole losses and the interaction with the ambient medium, is tracked together with the dynamical evolution in the Galactic potential, allowing for the determination of the fraction of stars which are, at present, in each of the four possible stages: Ejector, Propeller, Accretor, and Georotator. Taking from the ROSAT All-Sky Survey an upper limit of ~10 accreting neutron stars within ~140 pc from the Sun, we infer a lower bound for the mean kick velocity, <V> GT (or approximately) 200--300 km/s. The same conclusion is reached for both a constant (B ~10^[12] G) and an exponentially decaying magnetic field with a timescale ~10^9 yr. Present results, moreover, constrain the fraction of low-velocity stars which could have escaped pulsar statistics to < (or approximately) 1%.


Title: Properties of Nonthermal Emission in Plerions
Author: R. Bandiera
Affil: Osservatorio Astrofisico di Arcetri, Largo Fermi 5, 50125 Firenze, Italy
Abstract: The synchrotron emission observed in plerions depends on the characteristics of the magnetic fields and relativistic electrons in these supernova remnants. Therefore, an analysis of the spectral and spatial properties of this emission, combined with models for the evolution and structure of plerions, would allow one to investigate the evolution of the synchrotron nebula, the structure of the magnetic field, and the distribution of the relativistic electrons, as well as to put constraints on the history of the energy input from the associated neutron star.

The identification of a new class of plerionic remnants, with spectral properties different from the Crab Nebula, has been proposed. The spectra of these objects typically show a sharp spectral break at very low frequencies (below 50 GHz), with a steep spectrum right beyond the break. In order to model the properties of their emission, a nonstandard evolution of the pulsar output seems to be required.

X-ray observations of the synchrotron emission from the Crab Nebula are shown. They are compared with previous data, and their implications on the structure of the Crab are discussed. Recent millimetric data of this object are also presented. A spatially resolved analysis, based on radio, millimetric, and X-ray data, will be carried out also for other plerions.


Title: The Impact of Cooling Flows in Clusters of Galaxies
Author: S. W. Allen
Affil: Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, U.K.
Abstract: I discuss the impact of cooling flows on the observable X-ray properties of clusters of galaxies. I show that accounting for the effects of cooling flows and subcluster merger events leads to consistent determinations of the distribution of gravitating matter in clusters from X-ray and gravitational lensing studies. Accounting for the effects of cooling flows significantly reduces the scatter in the L_[Bol] / T_X relation determined for the hottest, most luminous systems and changes the best-fitting slope of the relation to a value close to L_[Bol] ~ T_X^2, in agreement with theoretical models. A clear segregation between the mean metallicities of cooling-flow and non--cooling-flow clusters is observed, which can be explained by the presence of metallicity gradients in the cooling-flow systems.


Title: Ultrasoft Narrow-line Seyfert 1 Galaxies: An Extreme of Accretion onto Supermassive Black Holes
Author: W. N. Brandt
Affil: The Pennsylvania State University, Department of Astronomy & Astrophysics, 525 Davey Lab, University Park, PA 16802, U.S.A.
Abstract: X-ray studies of ultrasoft narrow-line Seyfert 1 galaxies are revealing surprising new phenomena that are expanding our understanding of Seyfert activity, and they may well allow us to observe the effects of high mass accretion rates onto supermassive black holes. I briefly review the basic properties of these galaxies, their importance in a general context, and the prospects for studying them further in the future.