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.