4 M~ ). They have
also made a systematic study of the
surface compositional changes resulting
from the first and second dredge-ups, as
a function of stellar mass and metallicity.
Chiang, together with Michelson
(Stanford) and the EGRET team, has
determined the luminosity function and
luminosity evolution of the gamma-ray
loud AGN discovered by EGRET, and has
calculated their contribution to the diffuse
gamma-ray background. Binette,
collaborating with Fosbury and Parker
(ST-ECF), has analysed the Ha
equivalent width in various classes of
AGN, and finds no trend over 5 orders of
magnitude of continuum power. Martin
and Urry (STScI) have measured the
ultraviolet variability of the BL Lac object
PKS 2155-304. Edelson (NASA/GSFC),
Martin, et al. have detected strong
correlations between different spectral
bands from the radio to the x-rays, and
suggest that these observations are most
consistent with the entire radio to x-ray
continuum arising from direct synchrotron
emission from a relativistic jet. Nowak and Wagoner (Stanford) have
calculated the turbulent excitation of
trapped modes in black hole accretion
disks, and have estimated the resulting
modulation of the UV and optical emission
from AGN. Blaes and Balbus (Virginia)
have analyzed the magnetic shearing
instability in partially ionized accretion
disks, such as protoplanetary disks and
the molecular tori seen in the centers of
galaxies. They find that the instability is
still present if the collision frequency
between ions and neutrals exceeds the
orbital frequency, although if ionization
equilibrium holds in the disk then a much
higher collision frequency is required.
Blaes and Madau (STScI) have
continued their examination of quiescent
and transient UV and X-ray radiation from
isolated old neutron stars that accrete
interstellar material. They have made
detailed predictions for the numbers of
such stars detectable by EUVE and by the
ROSAT-PSPC all sky survey. Nelson and
Wang, in collaboration with Salpeter and
Wasserman (Cornell), predict that low
luminosity accreting neutron stars with
strong magnetic fields should emit a
substantial fraction (0.5 - 5%) of their
total luminosity in a narrow (E/~E ~ 2-4)
cyclotron emission line which peaks in the
energy range
5-20 keV. This nonthermal cyclotron
component should be absorbed much less
than the thermal continuum. It may be
observable in Be/X-ray binaries, and may
be the only detectable radiative signature
from isolated accreting neutron stars.
Wang and Nelson also suggest that the
same cyclotron line would be present in
quiescent emission from gamma-ray burst
sources (if these sources are galactic
neutron stars).
Nowak has developed a detailed
kinematic model of X-ray power spectra of
black hole candidates in their "very high
state", based on the viscous and thermal
instabilities that are believed to be
present in accretion disks. He has made
specific comparisons between the model
and data for GX339-4. Chiang and Romani
(Stanford) have modeled the gamma-ray
light curves and phase-resolved spectra
of young, rapidly spinning pulsars. They
have calculated in detail the pair-photon
cascade triggered by single-photon pair
production off the magnetic field, as well
as by photon-photon collisions.
Thompson and Duncan (U. Texas)
have proposed that the soft-gamma
repeaters are a class of neutron stars with
external magnetic fields much stronger
than those of ordinary pulsars. They
have developed a detailed radiative model
for the SGR bursts that explains their
weak spectral evolution and strongly
super-Eddington luminosities. Malaney
and Holdom (U. Toronto) have proposed
that the neutrino emission from a
supernova-type explosion can be
converted into a gamma-ray burst of total
energy ~ 1050 erg, if the explosion is
situated inside a ball of trapped
neutrinos, which in turn may lie in a
galactic core. Grossman and Nowak have
made a detailed computation of the rate at
which cosmological gamma-ray bursts are
lensed by intervening galaxies, using
improved lensing cross-sections, burst
fluxes and spectra, and taking into
account the duty cycle of the BATSE
detector. They have also developed new
statistical test for distinguishing GRB
light curves, which involves fourier
transforming the burst light curves and
then comparing pairs of bursts only at
frequencies that are signal-dominated.
They find that nearly all the bursts in the
publically available BATSE catalogue are
distinguishable. Nowak has explored the
sensitivity of recent claims of nearest
neighbor correlations in the angular
positions of gamma-ray bursts, to both
the burst subclasses and the burst
positional errors. Syer and Saha have
computed bookmaker's odds for competing
models of the sky distribution of gamma-
ray bursts, and on the basis of the first
BATSE catalogue have concluded that an
isotropic distribution is favored, but only
weakly. Blaes, Clarke and Tremaine have
investigated the possibility that gamma-
ray burst sources lie in the Oort cloud,
and conclude that such models face
serious problems.
Richardson is developing a tree
code optimized for local simulations of
flattened dynamical systems such as the
solar system and planetary rings. Dones
and Tremaine have examined the
statistical properties of the large impacts
that probably occured during the last
stages of planetary accumulation. They
have calculated the mass of the largest
expected impactor, the expected spin,
and so on, and compared their results
with Wetherill's simulations. Tremaine,
Webster (Melbourne) and their
collaborators have searched for satellites
of Uranus and Neptune using automated
measurements of large Schmidt plates. No
new satellites were found, to a limiting
magnitude of BJ = 20.5. Saha and
Tremaine have devised an integration
scheme for planetary orbits which uses a
different timestep for each planet, and
which is symplectic and time-reversible.
Tremaine and Wiegert are investigating
the evolution of the orbits of long-period
comets in the combined gravitational field
of the Sun, the giant planets, and the
Galactic tide.
Arnold Boothroyd Christopher Thompson
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Please e-mail any suggestions/comments to Jack Penfold (jpenfold@mtroyal.ab.ca)