title: Internal Motions in Globular Clusters

abstract:
  Observations of internal motions in globular clusters offer unique insights
into the dynamics of the clusters. We have recently developed methods of
high-precision astrometry with HST's WFPC2 camera, which allow us to measure
internal proper motions of individual stars. These new data open up many new
avenues for study of the clusters. Comparison of the dispersion of proper
motion with that of radial velocity offers what is potentially the best method
of measuring cluster distances, but reliable results will require dynamical
modeling of each cluster. Proper motions are much better able to measure
anisotropy of stellar motions than are radial velocities. In 47 Tucanae we have
measured thousands of proper motions near the center; their velocity
distribution is remarkably Gaussian. In two outer fields we have begun to study
anisotropy, which appears in the high velocities but not in the lower ones,
contrary to the spheroidal velocity distributions that have commonly been
assumed.


title: The anomalous Galactic globular cluster NGC 2808

abstract:
  We present UBVI photometry for about 60,000 stars in a region of 44'x33'
centered on the Galactic globular cluster (GGC) NGC 2808. The central region
has been mapped with the HST WFPC2 camera. We cover a field which extends from
the center to about 1.7 tidal radii. We confirm the anomalously elongated HB
with gaps along it, and we show that the gaps are statistically significant. We
find that: 1) the extended blue tail of HB (EBT) is present beyond
400''(=9r_h), and extends to V=21.2 also in these external regions; 2) also the
gaps on the EBT are present at least out to 400'' from the cluster center, and
possibly beyond that. The location of the gaps in the HB seems to be the same
all over the cluster. 3) there are no significant radial gradients in the
distribution of the stars in the HB. The observational facts presented in this
paper seem to exclude the possibility that the EBTs originate from tidal
stripping of the envelope of the RGB stars due to close encounters in high
density environments or to mass transfer in close binaries. Also, it is not
clear whether the presence of EBTs and gaps on them are the manifestation of
the same physical phenomenon. We have also shown that the jump recently
identified in the HB using Str"omgren-u photometry is clearly visible also in
the U, U-B plane, at T_e~11,600K. Finally, we present the luminosity function
(LF) of the RGB. The LF clearly shows the presence of the usual RGB bump, but
also a second feature, 1.4 magnitudes brighter in V, that we have named RGB
heap. The RGB heap, visible also in other GGCs, is a new feature that, because
of its position on the RGB, we have tentatively associated with the recently
discovered K giant variables.


title:
A CCD Study of the Environment of Seyfert Galaxies: III. Host Galaxies
  and the Nearby Environments

abstract:
  A technique is described that permits the robust decomposition of the bulge
and disk components of a sample of Seyfert galaxies, as well as a (control)
sample of non-active galaxies matched to the Seyferts in the distributions of
redshift, luminosity and morphological classification. The structural
parameters of the host galaxies in both samples are measured. No statistically
significant differences at greater than the 95% level are found in these
parameters according to a K-S test.
  ``Companion galaxies'' --- defined as any galaxy within a projected
separation of 200 h^{-1} kpc from the center of the host --- are identified and
their basic properties measured. A comparison between the active and control
samples in the distributions of apparent R magnitude, absolute R magnitude
(assuming the ``companions'' are at the distance of the host), projected
separation from the host, position angle relative to the host, magnitude
difference between the companion and host, and strength of the tidal parameter,
show no statistically significant differences.
  Similarly, no statistically significant differences are found between the
control and active sample host galaxies in terms of light asymmetries --- bars,
rings, isophotal twisting, etc. The implications for a model in which
interactions and mergers are responsible for inciting activity in galactic
nuclei are discussed briefly.


title: The ages of very cool hydrogen-rich white dwarfs

abstract:
  The evolution of white dwarfs is essentially a cooling process that depends
primarily on the energy stored in their degenerate cores and on the
transparency of their envelopes. In this paper we compute accurate cooling
sequences for carbon-oxygen white dwarfs with hydrogen dominated atmospheres
for the full range of masses of interest. For this purpose we use the most
accurate available physical inputs for both the equation of state and opacities
of the envelope and for the thermodynamic quantities of the degenerate core. We
also investigate the role of the latent heat in the computed cooling sequences.
We present separately cooling sequences in which the effects of phase
separation of the carbon-oxygen binary mixture upon crystallization have been
neglected, and the delay introduced in the cooling times when this mechanism is
properly taken into account, in order to compare our results with other
published cooling sequences which do not include a treatment of this
phenomenon. We find that the cooling ages of very cool white dwarfs with pure
hydrogen atmospheres have been systematically underestimated by roughly 1.5 Gyr
at log(L/Lo)=-4.5 for an otherwise typical 0.6 Mo white dwarf, when phase
separation is neglected. If phase separation of the binary mixture is included
then the cooling ages are further increased by roughly 10%. Cooling tracks and
cooling isochrones in several color-magnitude diagrams are presented as well.


title:
Electron Heating and Cosmic Rays at a Supernova Shock from Chandra X-ray
  Observations of E0102.2-7219

abstract:
  In this Letter we use the unprecedented spatial resolution of the Chandra
X-ray Observatory to carry out, for the first time, a measurement of the
post-shock electron temperature and proper motion of a young SNR, specifically
to address questions about the post-shock partition of energy among electrons,
ions, and cosmic rays. The expansion rate, 0.100 +/- 0.025 percent per yr, and
inferred age, ~1000 yr, of E0102.2-7219, from a comparison of X-ray
observations spanning 20 years, are fully consistent with previous estimates
based on studies of high velocity oxygen-rich optical filaments in the remnant.
With a radius of 6.4 pc for the blast wave estimated from the Chandra image,
our expansion rate implies a blast wave velocity of ~6000 km/s and a range of
electron temperatures 2.5 - 45 keV, dependent on the degree of collisionless
electron heating. Analysis of the Chandra ACIS spectrum of the immediate
post-shock region reveals a thermal plasma with abundances and column density
typical of the Small Magellanic Cloud and an electron temperature of 0.4-1 keV.
The measured electron temperature is significantly lower than the plausible
range above, which can only be reconciled if we assume that a significant
fraction of the shock energy, rather than contributing to the heating of the
post-shock electrons and ions, has gone into generating cosmic rays.


title:
Mining The Local Universe for Data: the QSO Luminosity Function with the
  Asiago-ESO/RASS QSO Survey

abstract:
  We present progress results of a new survey for bright QSOs (V<14.5, R<15.4,
B_J<15.2) covering the whole sky at |b|>30. The surface density of QSOs
brighter than B_J=14.8 turns out to be $2.9 \pm 0.8 \cdot 10^{-3} deg^{-2}$.
The optical Luminosity Function at 0.04 < z < 0.3 shows significant departures
from the standard pure luminosity evolution, providing new insights in the
modelling of the QSO phenomenon.


title:
On the Energy Equation and Efficiency Parameter of the Common Envelope
  Evolution

abstract:
  We have investigated the structure of evolved giant stars with masses 3-10
M_sun in order to evaluate the binding energy of the envelope to the core prior
to mass transfer in close binary systems. This binding energy is expressed by a
parameter lambda which is crucial for determining the outcome of binaries
evolving through a common envelope (CE) and spiral-in phase. We discuss the
lambda-parameter and the efficiency of envelope ejection in the CE-phase, and
show that lambda depends strongly on the evolutionary stage (i.e. stellar
radius) of the donor star at the onset of the mass transfer. The existence of
this relation enables us to introduce a new approach for solving the energy
equation. For a given observed binary system we can derive a unique solution
for the original mass and age of the donor star, as well as the pre-CE orbital
period. We find that the value of lambda is typically between 0.2 and 0.8. But
in some cases, particularly on the asymptotic giant branch of lower-mass stars,
it is possible that lambda > 5. A high value of lambda (rather than assuming a
high efficiency parameter, eta_CE >1) is sufficient to explain the long final
orbital periods observed among those binary millisecond pulsars which are
believed to have evolved through a CE-phase. We also present a tabulation of
lambda as a function of stellar radius and mass, which is useful for a quick
estimation of the orbital decay during a common envelope and spiral-in phase.


title:
The Parker Instability in a Thick Gaseous Disk II: Numerical Simulations
  in 2D

abstract:
  We present 2D, ideal-MHD numerical simulations of the Parker instability in a
multi-component warm disk model. The calculations were done using two numerical
codes with different algorithms, TVD and ZEUS-3D. The outcome of the numerical
experiments performed with both codes is very similar, and confirms the results
of the linear analysis for the undular mode derived by Kim et al. (2000): the
most unstable wavelength is about 3 kpc and its growth timescale is between
30-50 Myr (the growth rate is sensitive to the position of the upper boundary
of the numerical grid). Thus, the time and length scales of this multicomponent
disk model are substantially larger than those derived for thin disk models. We
use three different types of perturbations, random, symmetric, and
antisymmetric, to trigger the instability. The antisymmetric mode is dominant,
and determines the minimum time for the onset of the nonlinear regime. The
instability generates dense condensations and the final peak column density
value in the antisymmetric case, as also derived by Kim et al. (2000), is about
a factor of 3 larger than its initial value. These wavelengths and density
enhancement factors indicate that the instability alone cannot be the main
formation mechanism of giant molecular clouds in the general interstellar
medium. The role of the instability in the formation of large-scale
corrugations along spiral arms is briefly discussed.


title:
Coordinated Monitoring of the Eccentric O-star Binary Iota Orionis: The
  X-ray Analysis

abstract:
  We analyse two ASCA observations of the highly eccentric O9III + B1III binary
Iota Orionis obtained at periastron and apastron. Based on the assumption of a
strong colliding winds shock between the stellar components, we expected to see
significant variation in the X-ray emission between these phases. The
observations proved otherwise: the X-ray luminosities and spectral
distributions were remarkably similar. The only noteworthy feature was the hint
of a proximity effect during periastron passage, supported also in the optical.
We discuss the accuracy of our results, and also analyse archival ROSAT
observations. We investigate why we do not see a clear colliding winds
signature. A simple model shows that the wind attenuation to the expected
position of the shock apex is negligible throughout the orbit, which poses the
puzzling question of why the expected 1/D variation (ie. a factor of 7.5) in
the intrinsic luminosity is not seen in the data. Two scenarios are proposed:
either the colliding winds emission is unexpectedly weak such that intrinsic
shocks in the winds dominate the emission, or, alternatively, that the emission
observed is colliding winds emission but in a more complex form than we would
naively expect. Complex hydrodynamical models are then analyzed. Despite
strongly phase-variable emission from the models, both were consistent with the
observations. We find that if the mass-loss rates of the stars are low then
intrinsic wind shocks could dominate the emission. However, when we assume
higher mass-loss rates of the stars, we find that the observed emission could
also be consistent with a purely colliding winds origin. To distinguish between
the different models X-ray observations with improved phase coverage will be
necessary.


title: Spectral Models of Convection-Dominated Accretion Flows

abstract:
  For small values of the dimensionless viscosity parameter, namely
$\alpha\lesssim 0.1$, the dynamics of non-radiating accretion flows is
dominated by convection; convection strongly suppresses the accretion of matter
onto the central object and transports a luminosity $\sim 10^{-3}-10^{-2} \dot
M c^2$ from small to large radii in the flow. A fraction of this convective
luminosity is likely to be radiated at large radii via thermal bremsstrahlung
emission. We show that this leads to a correlation between the frequency of
maximal bremsstrahlung emission and the luminosity of the source, $\nu_{\rm
peak} \propto L^{2/3}$. Accreting black holes with X-ray luminosities $10^{-4}
L_{Edd}\gtrsim L_X(0.5-10{\rm keV}) \gtrsim 10^{-7}L_{Edd}$ are expected to
have hard X-ray spectra, with photon indices $\Gamma\sim2$, and sources with
$L_X\lesssim 10^{-9}L_{Edd}$ are expected to have soft spectra, with
$\Gamma\sim3.5$. This is testable with {\it Chandra} and {\it XMM}.


title: The evolution of the color gradients of early-type cluster galaxies

abstract:
  We investigate the origin of color gradients in cluster early-type galaxies
to probe whether pure age or pure metallicity gradients can explain the
observed data in local and distant (z approx 0.4) samples. We measure the
surface brightness profiles of the 20 brightest early-type galaxies of
CL0949+44 (hereafter CL0949) at redshift z=0.35-0.38 from HST WF2 frames taken
in the filters F555W, F675W, F814W. We determine the color profiles (V-R)(r),
(V-I)(r), and (R-I)(r) as a function of the radial distance r in arcsec, and
fit logarithmic gradients in the range -0.2 to 0.1 mag per decade. These values
are similar to what is found locally for the colors (U-B), (U-V), (B-V) which
approximately match the (V-R), (V-I), (R-I) at redshift approx 0.4. We analyse
the results with up to date stellar population models. We find that passive
evolution of metallicity gradients (approx 0.2 dex per radial decade) provides
a consistent explanation of the local and distant galaxies' data. Invoking pure
age gradients (with fixed metallicity) to explain local color gradients
produces too steep gradients at redshifts z approx 0.4. Pure age gradients are
consistent with the data only if large present day ages (>=15 Gyr) are assumed
for the galaxy centers.


title: The tetra-lobed planetary nebula NGC 1501

abstract:
  Direct imagery and long-slit, spatially resolved echellograms of the high
excitation planetary nebula NGC 1501 allowed us to study in detail the
expansion velocity field, the physical conditions (electron temperature,
electron density, ionization) and the spatial distribution of the nebular gas.
An electron temperature of 11500 K and a turbulence of 18 km/s are derived by
comparing the Halpha and [OIII] emission line profiles, but large, small scale
fluctuations of both these quantities are present in the ionized gas. The
radial density distribution shows external peaks up to 1400 cm-3; they have
steep outwards profiles and extended inwards tails probably originated by
Rayleigh-Taylor instability and winds interaction. The complexity of the
expanding motions indicates that the main part of NGC 1501 is a thin ellipsoid
of moderate ellipticity, but the presence of a pair of large lobes along both
the major and the intermediate axes and of a multitude of smaller bumps spread
on the whole nebular surface, makes the general 3-D structure of NGC 1501 like
a boiling, tetra-lobed shell. This peculiar morphology can be qualitatively
explained in terms of interaction of the slow nebular material with the intense
and fast wind from the WC4/OVI central star.


title:
Decreasing Density Gradients in Circumnuclear HII Regions of Barred
  Galaxies NGC 1022, 1326 & 4314

abstract:
  A re-analysis of the radio continuum emission from circumnuclear regions of
the barred galaxies NGC 1022, 1326 & 4314 is presented. The spatial
distributions of Halpha and radio continuum are similar and suggest the
existence of several giant HII regions in the circumnuclear zone. The spectral
indices of the HII regions, obtained with similar angular resolution and at
three different wavelengths, indicate that in all three galaxies, the spectral
index, alpha^6_2 >-0.1. This result suggests the existence of giant HII regions
with decreasing density gradients. The average density stratifications of these
HII regions could be approximated by power-laws of the form n_e~r^-w, with
exponents in the range 1.6 < w < 2.4.


title:
Multiwavelength optical observations of chromospherically active binary
  systems. III. High resolution echelle spectra from Ca II H&K to Ca II IRT

abstract:
  This is the third paper of a series aimed at studying the chromosphere of
active binary systems using the information provided for several optical
spectroscopic features. High resolution echelle spectra including all the
optical chromospheric activity indicators from the Ca II H & K to Ca II IRT
lines are analysed here for 16 systems. The chromospheric contribution in these
lines has been determined using the spectral subtraction technique. Very broad
wings have been found in the subtracted H_alpha profile of the very active star
HU Vir. These profiles are well matched using a two-component Gaussian fit
(narrow and broad) and the broad component can be interpreted as arising from
microflaring. Red-shifted absorption features in the H_alpha line have been
detected in several systems and excess emission in the blue wing of FG UMa was
also detected. These features indicate that several dynamical processes, or a
combination of them, may be involved. Using the E(H_alpha)/E(H_beta) ratio as a
diagnostic we have detected prominence-like extended material viewed off the
limb in many stars of the sample, and prominences viewed against the disk at
some orbital phases in the dwarfs OU Gem and BF Lyn. The He I D_3 line has been
detected as an absorption feature in mainly all the giants of the sample. Total
filling-in of the He I D_3, probably due to microflaring activity, is observed
in HU Vir. Self-absorption with red asymmetry is detected in the Ca II H & K
lines of the giants 12 Cam, FG UMa and BM CVn. All the stars analysed show
clear filled-in Ca II IRT lines or even notable emission reversal. The small
values of the E(8542)/E(8498) ratio we have found indicate Ca II IRT emission
arises from plage-like regions.


title: Simulations of accretion flows crossing the last stable orbit

abstract:
  We use three dimensional magnetohydrodynamic simulations, in a
pseudo-Newtonian potential, to study geometrically thin accretion disc flows
crossing the marginally stable circular orbit around black holes. We
concentrate on vertically unstratified and isothermal disk models, but also
consider a model that includes stratification. In all cases, we find that the
sonic point lies just inside the last stable orbit, with a modest increase in
the importance of magnetic field energy, relative to the thermal energy,
observed interior to this radius. The time-averaged gradient of the specific
angular momentum of the flow, (dl/dr), is close to zero within the marginally
stable orbit, despite the presence of large fluctuations and continuing
magnetic stress in the plunging region. The result that the specific angular
momentum is constant within the last stable orbit is in general agreement with
traditional disk models computed using a zero-torque boundary condition at the
last stable orbit.


title:
An Unusual Burst from Soft Gamma Repeater SGR 1900+14: Comparisons with
  Giant Flares and Implications for the Magnetar Model

abstract:
  The Soft Gamma Repeater SGR 1900+14 entered a remarkable phase of activity
during the summer of 1998. This activity peaked on August 27, 1998 when a giant
periodic gamma-ray flare resembling the famous March 5, 1979 event from SGR
0526-66 was recorded. Two days later (August 29), a strong, bright burst was
detected with RXTE and BATSE. This event reveals several similarities to the
giant flares of August 27 and March 5, and shows a number of unique features
not previously seen in SGR bursts. Unlike typically short SGR bursts, this
event features a 3.5 s burst peak that was preceded by an extended (~ 1 s)
complex precursor, and followed by a long (~ 1000 s) periodic tail modulated at
the 5.16 s stellar rotation period. Spectral analysis shows a striking
distinction between the spectral behavior of the precursor, burst peak and
extended tail. While the spectrum during the peak is uniform, a significant
spectral evolution is detected in both the precursor and tail emissions.
Temporal behavior shows a sharp rise (~ 9.8 ms) at the event onset and a rapid
cutoff (~ 17 ms) at the end of the burst peak. The tail pulsations show a
simple pulse profile consisting of one 5.16 s peak that did not evolve with
time. The spectral and temporal signatures of this event imply that the
precursor, main peak, and extended tail are produced by different physical
mechanisms. We discuss these features and their implications in the context of
the magnetar model. The energetics of the August 29 event, and its close
proximity to the August 27 flare, suggest that it is an `aftershock' of the
preceding giant flare. "P.S. This is an abbreviated version of the original
abstract."


title: Suppression of giant planet formation in stellar clusters

abstract:
  Photoevaporation driven by the ultraviolet radiation from massive stars
severely limits the lifetime of protoplanetary discs around stars formed within
stellar clusters. I investigate the resulting influence of clustered
environments on the probability of giant planet formation, and show that for
clusters as rich, or richer than, Orion, the time available for planet
formation is likely to be limited to the length of any delay between low mass
and high mass star formation. Under popular models for the formation of massive
planets, the fraction of stars with giant planets in rich clusters is expected
to be substantially suppressed as compared to less clustered star formation
environments.


title: Detecting Dark Matter in High Velocity Clouds

abstract:
  Many high velocity HI clouds (HVCs) are now believed to be scattered
throughout the Galactic halo on scales of tens of kiloparsecs. Some of these
clouds appear to contain substantial HI masses (>10^6 Msun). It has been
suggested that these structures may be associated with dark matter `mini halos'
accreting onto the Galactic halo. For a compact HVC along the sight line to a
more distant galaxy, we demonstrate that `pixel gravitational lensing' provides
a crucial test for the presence of a dark halo in the form of massive compact
objects. The detection of pixel lensing will provide an independent means to
map the mass distribution within HVCs.


title:
Riemannian collineations in General Relativity and in Einstein-Cartan
  cosmology

abstract:
  Riemannian vectorial collineations along with current Killing conservation
are shown to lead to tensorial collineations for the energy-stress tensor in
general relativity and in Einstein-Cartan Weyssenhoff fluid cosmology.


title: The Cosmological Angular Momentum Problem of Low-Mass Disk Galaxies

abstract:
  The rotational properties of the visible and dark components of low-mass disk
galaxies (vrot<=100 km/s) are investigated using the Swaters sample. The
rotational parameter lambda'=lambda_DM*(j_d/m_d) is determined, where lambda_DM
is the dark halo spin parameter and j_d/m_d is the ratio between the specific
angular momentum of the disk and that of the dark halo. The distribution of
lambda' is in excellent agreement with cosmological predictions of hierarchical
clustering if j_d/m_d=1, that is if the protogalactic gas did not loose a
significant amount of specific angular momentum due to dynamical friction. This
result is in disagreement with current cosmological Nbody/SPH simulations where
the baryonic component looses 90% of its specific angular momentum while
settling into the equatorial plane. The Swaters sample also shows a
surprisingly strong correlation between lambda' and the baryonic mass fraction
m_d: lambda'=0.4 m_d**0.6. This correlation can be explained if the total
amount of gas in protogalaxies is a universal fraction of their dark matter
mass, of order 10%, and if the variation in m_d is a result of the fact that
only the inner parts of the primordial gas distribution managed to form a
visible disk component. In this case the specific angular momentum of the gas
out of which the disk formed is a factor of 2.75 larger than that of the dark
halo, which would require a yet unknown spin-up process for the visible
baryonic component.


title:
Application of Data Compression Methods to the Redshift-space
  distortions of the PSCz galaxy catalogue

abstract:
  We apply a spherical harmonic analysis to the Point Source Redshift Survey
(PSCz), to compute the real-space galaxy power spectrum and the degree of
redshift distortion caused by peculiar velocities. We employ new parameter
eigenvector and hierarchical data compression techniques, allowing a much
larger number of harmonic modes to be included, and correspondingly smaller
error bars. Using 4644 harmonic modes, compressed to 2278, we find that the
IRAS redshift-space distortion parameter is $\beta = 0.39 \pm 0.12$ and the
amplitude of galaxy clustering on a scale of $k=0.1 \Mpch$ is $\Delta_{\rm
gal}(0.1)=0.42 \pm 0.02$. Combining these we find the amplitude of mass
perturbations is $\Delta_m(0.1)=(0.16\pm0.04) \Omega_m^{-0.6}$. A preliminary
model fitting analysis combining the PSCz amplitudes with the CMB and abundance
of clusters yields the cosmological matter density parameter $\Omega_m=0.16\pm
0.03$, the amplitude of primordial perturbations $Q=(8.4\pm 3.8) \times
10^{-5}$, and the IRAS bias parameter $b=0.84\pm 0.28$.


title:
Fluctuations in the Radio Background from Intergalactic Synchrotron
  Emission

abstract:
  The shocks produced in the intergalactic medium during large-scale structure
formation accelerate a population of highly relativistic electrons which emit
synchrotron radiation due to intergalactic magnetic fields. In a previous paper
(Loeb & Waxman 2000) we have shown that these electrons cool primarily by
inverse-Compton scattering of the microwave background photons and can thereby
produce the observed intensity and spectrum of the diffuse gamma-ray
background. Here we calculate the intensity and angular fluctuations of the
radio synchrotron background that results from the same high-energy electrons,
as well as the expected angular fluctuations in the gamma-ray background. On
angular scales smaller than a degree, the predicted fluctuations in the
microwave background temperature are of order 40micro-K*(xi_B/0.01)(nu/10
GHz)^{-3}, where xi_B is the magnetic fraction of the post-shock energy
density. This foreground might have already dominated the anisotropy signal
detected in existing low-frequency CMB experiments, and can be identified with
confidence through multi-frequency observations. Detection of the synchrotron
fluctuations would allow to determine the strength of the intergalactic
magnetic field. We predict a strong correlation between high-resolution maps
taken at low-frequency radio waves and at high-energy gamma-rays. Young X-ray
clusters may also appear as radio or gamma-ray clusters. The detailed study of
this correlation will become easily accessible with the future launch of GLAST.


title: High Resolution Observations of X-ray Absorbers/Emitters

abstract:
  We present photoionization and collisional ionization models, and their
application to three important fields: (a) the Warm Absorbers/Emitters in type
1 AGN, (b) the Warm Reflectors in Type 2 AGN, and (c) X-ray absorption of
background quasars by intergalactic gas. A number of cases are investigated,
and the dependences of the main parameters explored.


title:
Face-on galaxies NGC 524 and NGC 6340: chemically decoupled nuclei and
  inclined circumnuclear disks

abstract:
  Central regions of the early-type disk galaxies NGC 524 and NGC 6340 have
been investigated with the Multi-Pupil Field Spectrograph at the 6m telescope
of the Special Astrophysical Observatory of the Russian Academy of Sciences. I
confirm the existence of chemically distinct stellar nuclei in these galaxies
which have been claimed earlier. The metallicity differences which are found
between the nuclei and the bulges, 0.5 - 1.0 kpc from the centers, reach 0.5 -
0.6 dex. Both nuclei are magnesium overabundant, but the bulges have different
magnesium-to-iron ratios: it is solar in NGC 6340 and the same as the nuclear
one in NGC 524. The kinematical and morphological analyses reveal the existence
of inclined central disks in these galaxies. In NGC 524 the central disk
consists of stars, dust, and ionized gas; its extension may be as large as up
to R=3 kpc, and it is inclined by more than 20 deg to the global galactic
plane. In NGC 6340 only a gaseous polar disk with the radius less than 500 pc
is detected.


title: FGS Astrometry with the Hubble Space Telescope

abstract:
  This article describes astrometric science that can be done with the Hubble
Space Telescope Fine Guidance Sensors and presents some recent science results.
We outline how these data are modeled, acquired, and calibrated. We next show
how the astrometer, FGS 3, works as an interferometer. Finally, we present a
guide to the literature that provides additional detail for each item discussed
in the article.


title:
Influence of the Magnetic Field on the Fermion Scattering off Bubble and
  Kink Walls

abstract:
  We investigate the scattering of fermions off domain walls at the electroweak
phase transition in presence of a magnetic field. We consider both the bubble
wall and the kink domain wall. We derive and solve the Dirac equation for
fermions with momentum perpendicular to the walls, and compute the transmission
and reflection coefficients. In the case of kink domain wall, we briefly
discuss the zero mode solutions localized on the wall. The possibile role of
the magnetic field for the electroweak baryogenesis is also discussed.


title:
High spatial resolution observations of CUDSS14A: a SCUBA-selected
  Ultraluminous galaxy at high redshift

abstract:
  We present a high-resolution millimetre interferometric image of the
brightest SCUBA-selected galaxy from the Canada-UK deep SCUBA survey (CUDSS).
We make a very clear detection at 1.3 mm, but fail to resolve any structure in
the source. The interferometric position is within 1.5 arcsec of the SCUBA 850
$\mu$m centroid, and also within 1.5 arcsec of a 44 $\mu$Jy radio source and a
very faint, extremely red galaxy which we had previously identified as the
submillimetre source. We also present new optical and infrared imaging, and
infrared spectroscopy of this source. We model the overall spectral energy
distribution and conclude that it lies within the redshift range 2$< z < 4.5$.
The submm/FIR luminosity of CUDSS14A is very weakly dependent on redshift
within the constrained range, and is roughly 4$\times10^{12}$ L$_\odot$ (for
H$_0$=75 and an assumed Arp220-like spectrum), which implies a star-formation
rate $\sim$1000 M$_\odot$ yr$^{-1}$. We derive an approximate gas mass of $\sim
10^{10}$ M$_\odot$ which would imply the current star-forming activity cannot
be sustained for longer than about 10 million years. With the present data
however we are unable to rule out a significant AGN contribution to the total
luminosity.


title:
On a Theoretical Interpretation of the Period Gap in Binary Millisecond
  Pulsars

abstract:
  We reexamine evolutionary channels for the formation of binary millisecond
pulsars in order to understand their observed orbital period distribution. The
available paths provide a natural division into systems characterized by long
orbital periods (> 60 d) and short orbital periods (< 30 d). Systems with
initial periods 1 - 2 d ultimately produce low mass He white dwarfs with short
orbital periods (< 1 d). For longer initial periods (> few days), early massive
Case B evolution produces CO white dwarfs with orbital periods < 20 d. Common
envelope evolution result in short period systems (P < 1 d) from unstable low
mass Case B evolution producing He white dwarfs, and from unstable Case C
evolution leading to CO white dwarfs. On the other hand, the long orbital
period group arises from stable low mass Case B evolution with initial periods
> few days producing low mass He white dwarfs and periods > 30 d, and from
stable Case C evolution producing CO white dwarfs. The lack of observed systems
between 23 and 56 days probably reflects the fact that for comparable initial
orbital periods (< few days) low mass Case B and early massive Case B evolution
lead to very discrepant final periods. We show in particular that the lower
limit (~ 23 d) cannot result from common-envelope evolution.


title: Are there real orthogonal polarization modes in pulsar radio emission?

abstract:
  The Orthogonal Polarization Modes (OPM) have been reported observationally
and accepted widely by pulsar researchers. However, no acceptable theory can
show the origin of the OPM, which becomes a mystery in pulsar research field.
Here a possible way to solve this mystery is presented. We ask a question: Does
there exist any real so-called OPM in pulsar radiation? It is proposed in this
paper that the `observed OPM' in individual pulses could be the results of
depolarization of pulsar radiation and the observational uncertainties
originated from polarimeter in observation. A possible way to check this idea
is suggested. If the idea is verified, the pulsar research would be influenced
significantly in theory and in observation.


title: The Host Stars of Extrasolar Planets Have Normal Lithium Abundances

abstract:
  The lithium abundances of planet-harbouring stars have been compared with
those of open clusters and field stars. Young (chromospherically active) and
subgiant stars have been eliminated from the comparison because they are at
different stages of evolution and Li processing to the planet-harbouring stars,
and hence have systematically higher Li abundances. The analysis showed that
the Li abundances of the planet-harbouring stars are indistinguishable from
those of non-planet-harbouring stars of the same age, temperature, and
composition. This conclusion is opposite to that arrived at by Gonzalez & Laws
(2000); it is believed that the field star sample used by them contained too
wide a range of ages, evolutionary types, and temperatures to be accommodated
by the model they adopted to describe the dependence on parameters. Li does not
appear set to provide key insights into the formation and evolution of
planetary systems.


title:
Local Axisymmetric Instability Criterion in the Thin, Rotating,
  Multicomponent Disk

abstract:
  Purely gravitational perturbations are considered in a thin rotating disk
composed of several gas and stellar components. The dispersion relation for the
axisymmetric density waves propagating through the disk is found and the
criterion for the local axisymmetric stability of the whole system is
formulated. In the appropriate limit of two-component gas we confirm the
findings of Jog & Solomon (1984) and extend consideration to the case when one
component is collisionless. Gravitational stability of the Galactic disk in the
Solar neighborhood based on the multicomponent instability condition is
explored using recent measurements of the stellar composition and kinematics in
the local Galactic disk obtained by Hipparcos satellite.


title: The Initial Mass Function of Low-Mass Stars and Brown Dwarfs in Taurus

abstract:
  By combining deep optical imaging and infrared spectroscopy with data from
the Two-Micron All-Sky Survey (2MASS) and from previous studies (e.g., Briceno
et al.), I have measured the Initial Mass Function (IMF) for a
reddening-limited sample in four fields in the Taurus star forming region. This
IMF is representative of the young populations within these fields for masses
above 0.02 Msun. Relative to the similarly derived IMF for the Trapezium
Cluster (Luhman et al.), the IMF for Taurus exhibits a modest deficit of stars
above one solar mass (i.e., steeper slope), the same turnover mass (~0.8 Msun),
and a significant deficit of brown dwarfs. If the IMF in Taurus were the same
as that in the Trapezium, 12.8+/-1.8 brown dwarfs (>0.02 Msun) are expected in
these Taurus fields where only one brown dwarf candidate is found. These
results are used to test theories of the IMF.


title: COBE- and Cluster-Normalized CDM Simulations

abstract:
  We introduce a set of four new publicly available N-body simulations, the
most recent additions to the Texas P^3M Database. Our models probe the less
studied parameter space region of moderate volume (100/h Mpc box) combined with
fine mass resolution (~10^12 M_Sol, roughly comparable to a L_* galaxy), making
these simulations especially suitable for study of major large-scale structure
features such as voids, and for comparison with the largest three-dimensional
redshift surveys currently available. Our cosmological models (LCDM, TOCDM,
OCDM, TCDM) are all COBE-normalized, and when possible (LCDM and TOCDM) also
cluster-normalized, based on the X-ray cluster M--T relation. The COBE- and
cluster-normalized LCDM model reiterates the attractiveness of this currently
favored model which does not require the introduction of tilt in order to fit
the constraints imposed by observations of other cosmological parameters.


title: Synchrotron Radiation as the Source of Gamma-Ray Burst Spectra

abstract:
  We investigate synchrotron emission models as the source of gamma-ray burst
spectra. We show that including the possibility for synchrotron
self-absorption, a ``smooth cutoff'' to the electron energy distribution, and
an anisotropic distribution for the electron pitch angles produces a whole
range of low energy spectral behavior. In addition, we show that the procedure
of spectral fitting to GRB data over a finite bandwidth can introduce a
spurious correlation between spectral parameters - in particular, the value of
the peak of the nu F_nu spectrum, E_p, and the low energy photon spectral index
alpha (the lower E_p is, the lower (softer) the fitted value of alpha will be).
  From this correlation and knowledge of the E_p distribution, we show how to
derive the expected distribution of alpha. We show that optically thin
synchrotron models with an isotropic electron pitch angle distribution can
explain the distribution of alpha below alpha=-2/3. This agreement is achieved
if we relax the unrealistic assumption of the presence of a sharp low energy
cutoff in the spectrum of accelerated electrons, and allow for a more gradual
break. We show that this low energy portion of the electron spectrum can be at
most flat. We also show that optically thin synchrotron models with an
anisotropic electron pitch angle distribution can explain all bursts with -2/3
< alpha <= 0$. The very few bursts with low energy spectral indices that fall
above alpha=0 may be due the presence of a the synchrotron self-absorption
frequency entering the lower end of the BATSE window. Our results also predict
a particular relationship between alpha and E_p during the temporal evolution
of a GRB. We give examples of spectral evolution in GRBs and discuss how the
behavior are consistent with the above models.


title:
HST Observations of Vibrationally-Excited Molecular Hydrogen in Cluster
  Cooling Flow Nebulae

abstract:
  We report new HST NICMOS and WFPC2 imaging of emission-line nebulae in the
central galaxies of 3 clusters of galaxies purported to host massive cooling
flows, NGC1275, A2597, and PKS0745. The spectral signature of vibrationally-
excited molecular hydrogen (VEMH) has been seen in every galaxy searched so far
in a cluster cooling flow with an optical emission line nebula. We have
discovered that the VEMH gas extends several kpc from the centers of A2597 and
PKS0745, while the vibrationally-excited molecular hydrogen in NGC1275 appears
to be mostly confined to its nucleus, with some extended emission <1 kpc from
the center. The VEMH in A2597 and PKS0745-191 seems to be nearly co-spatial
with the optical emission-line filaments in those systems. Candidates for
heating the nebulae are X-ray irradiation by the ICM, UV fluorescence by young
stars, and shocks. UV heating by young stars provides the most satisfactory
explanation for the H2 emission in A2597 and PKS0745; X-ray irradiation is
energetically unlikely and strong shocks (v>40 km/s) are ruled out by the high
H2/H-alpha ratios. If UV heating is the main energy input, a few billion solar
masses of molecular gas is present in A2597 and PKS0745. UV irradiation models
predict a significant amount of 1-2 micron emission from higher excitation H2
transitions and moderate far infrared luminosities (~1e44/h^2 erg/s) for A2597
and PKS0745. Even in the context of UV fluorescence models, the total amount of
H2 gas and star formation inferred from these observations is too small to
account for the cooling flow rates and longevities inferred from the X-rays. We
note an interesting new constraint on cooling flow models: the radio sources do
not provide a significant amount of shock heating, and therefore cannot
counterbalance the cooling of the X-ray gas in the cluster cores.


title: Ages and Metallicities of Globular Clusters in NGC 4472

abstract:
  We have derived ages and metallicities from co-added spectra of 131 globular
clusters associated with the giant elliptical galaxy NGC 4472. Based upon a
calibration with Galactic globular clusters, we find that our sample of
globular clusters in NGC 4472 span a metallicity range of approximately -1.6 <=
[Fe/H] <= 0 dex. There is evidence of a radial metallicity gradient in the
globular cluster system, which is steeper than that seen in the underlying
starlight. Determination of the absolute ages of the globular clusters is
uncertain, but formally, the metal-poor population of globular clusters has an
age of 14.5 +/- 4 Gyr and the metal-rich population is 13.8 +/- 6 Gyr old.
Monte Carlo simulations indicate that the globular cluster populations present
in these data are older than 6 Gyr at the 95% confidence level. We find that
within the uncertainties, the globular clusters are old and coeval, implying
that the bimodality seen in the broadband colours primarily reflects
metallicity and not age differences.


title: Self-Similar Hot Accretion Flow onto a Neutron Star

abstract:
  We consider hot, two-temperature, viscous accretion onto a rotating,
unmagnetized neutron star. We assume Coulomb coupling betweenthe protons and
electrons, and free-free cooling from the electrons. We show that the accretion
flow has an extended settling region which can be described by means of two
analytical self-similar solutions: a two-temperature solution which is valid in
an inner zone, r<10^{2.5}, where r is the radius in Schwarzchild units; and a
one-temperature solution which is valid in an outer zone, r>10^{2.5}. In both
zones the density varies as \rho ~ r^{-2} and the angular velocity as \Omega ~
r^{-3/2}. We solve the flow equations numerically and confirm that the
analytical solutions are accurate. The settling flow radiates the energy
dissipated by viscosity; so it is not advection-dominated. Except for the
radial velocity, all other gas properties - density, angular velocity,
temperature, luminosity, angular momentum flux - are independent of the mass
accretion rate; these quantities do depend sensitively on the spin of the
neutron star. The angular momentum flux is outward under most conditions;
therefore, the central star is nearly always spun-down. The luminosity of the
settling zone arises from the rotational energy that is released as the star is
braked by viscosity, and the contribution from gravity is small; hence the
radiative efficiency can be arbitrarily large at low $\mdot$. For reasonable
values of the gas adiabatic index, the Bernoulli parameter is negative;
therefore, a strong outflow or wind is not expected. The flow is convectively
stable, but may be thermally unstable. The described solution is not
advection-dominated; however, when the spin of the star is small enough, it
transforms smoothly to an advection-dominated branch of solution.


title: Galaxy Mergers: A Search for Chemical Signatures

abstract:
  We have gathered high resolution echelle spectra for more than two dozen
high-velocity metal-poor field stars, including BD+80 245, a star previously
known to have extremely low [alpha/Fe] abundances, as well as G4-36, a new
low-alpha star with unusually large [Ni/ Fe]. In this kinematically selected
sample, other chemically anomalous stars have also been uncovered. In addition
to deriving the alpha-element abundances, we have also analysed iron-group and
s-process elements. Not only does chemical substructure exist in the halo, but
the chemical anomalies are not all the same within all elemental groups.


title:
Spillover and diffraction sidelobe contamination in a double-shielded
  experiment for mapping Galactic synchrotron emission

abstract:
  We have analyzed observations from a radioastronomical experiment to survey
the sky at decimetric wavelengths along with feed pattern measurements in order
to account for the level of ground contamination entering the sidelobes. A
major asset of the experiment is the use of a wire mesh fence around the
rim-halo shielded antenna with the purpose of levelling out and reducing this
source of stray radiation for zenith-centered 1-rpm circular scans. We
investigate the shielding performance of the experiment by means of a geometric
diffraction model in order to predict the level of the spillover and
diffraction sidelobes in the direction of the ground. Using 408 MHz and 1465
MHz feed measurements, the model shows how a weakly-diffracting and unshielded
antenna configuration becomes strongly-diffracting and double-shielded as
far-field diffraction effects give way to near-field ones. Due to the
asymmetric response of the feeds, the orientation of their radiation fields
with respect to the secondary must be known a priori before comparing model
predictions with observational data. By adjusting the attenuation coefficient
of the wire mesh the model is able to reproduce the amount of differential
ground pick-up observed during test measurements at 1465 MHz.


title:
Galaxy-dark matter correlations applied to galaxy-galaxy lensing:
  predictions from the semi-analytic galaxy formation models

abstract:
  We use semi-analytic models of galaxy formation combined with high resolution
N-body simulations to make predictions for galaxy-dark matter correlations and
apply them to galaxy-galaxy lensing. We analyze cross-correlation spectra
between the dark matter and different galaxy samples selected by luminosity,
color or star formation rate. We compare the predictions to the recent
detection by SDSS. We show that the correlation amplitude and the mean
tangential shear depend strongly on the luminosity of the sample on scales
below 1 Mpc, reflecting the correlation between the galaxy luminosity and the
halo mass. The cross-correlation cannot however be used to infer the halo
profile directly because different halo masses dominate on different scales and
because not all galaxies are at the centers of the corresponding halos. We
compute the redshift evolution of the cross-correlation amplitude and compare
it to those of galaxies and dark matter. We also compute the galaxy-dark matter
correlation coefficient and show it is close to unity on scales above r > 1 Mpc
for all considered galaxy types. This would allow one to extract the bias and
the dark matter power spectrum on large scales from the galaxy and galaxy-dark
matter correlations.


title:
The GHz-Peaked Spectrum radio galaxy 2021+614: Detection of slow motion
  in a compact symmetric object

abstract:
  We have analysed VSOP (VLBI Space Observatory Programme) data at 5 GHz and
ground-based VLBI (Very Long Baseline Interferometry) data at 15 GHz for the
GHz-Peaked Spectrum (GPS) radio galaxy 2021+614. Its morphology is consistent
with it being a compact symmetric source extending over 30 h^-1 pc (H_0 = 100 h
km s^-1 Mpc^-1, q_0 = 0.5). From a comparison with earlier observations we have
detected an increase in the separation and a decrease in the size of the two
most prominent components. We determine the projected speed with which these
two components recede from each other to be 0.12+/-0.02 h^-1 c. Given the
projected separation of the two components of 16.1 h^-1 pc, the infered
kinematic age is 440+/-80 years, measured in the source reference frame. These
results provide additional support for the contention that compact symmetric
radio objects are young and the precursors of the classical FR I or FR II radio
sources. The sizes of individual components appear to contract with time which
is not consistent with the self-similar evolution model for peaked spectrum
sources.
  In order to overcome problems related to the estimation of uncertainties for
separation measurements between source components, we have developed and
applied a method that compares two uv-data sets obtained at different epochs.
This method parametrizes the most important structural change, the increase in
separation between components, by rescaling the u and v axis of the amplitude
interference pattern. It provides best-fit values for the parameters and uses a
bootstrap method to estimate the errors in the parameters.


title: Convective instability in proto-neutron stars

abstract:
  The linear hydrodynamic stability of proto-neutron stars (PNSs) is considered
taking into account dissipative processes such as neutrino transport and
viscosity. We obtain the general instability criteria which differ essentially
from the well-known Ledoux criterion used in previous studies. We apply the
criteria to evolutive models of PNSs that, in general, can be subject to the
various known regimes such as neutron fingers and convective instabilities. Our
results indicate that the fingers instability arises in a more extended region
of the stellar volume and lasts a longer time than expected.


title:
On the effect of coronal outflow on spectra formation in galactic black
  hole systems

abstract:
  We present the results of both analytical and numerical calculations of the
amplitude of the reflection component in X-ray spectra of galactic black hole
systems. We take into account the anisotropy of Compton scattering and the
systematic relativistic bulk motion of the hot plasma. In the case of single
scattering approximation the reflection from the disc surface is significantly
enhanced due to the anisotropy of Compton scattering. On the other hand the
calculations of multiple scattering obtained using the Monte Carlo method show
that the anisotropy effect is much weaker in that case. Therefore, the enhanced
backscattered flux may affect the observed spectra only if the disc surface is
highly ionized, which reduces the absorption in the energy band corresponding
to the first Compton scattering.


title: Low and High Frequency Quasi-Periodic Oscillations in 4U1915-05

abstract:
  The X-ray burster and dipper 4U1915-05 (also known as XB1916-053) was
observed 19 times for a total exposure of roughly 140 ks between 1996 February
and October with the proportional counter array (PCA) aboard Rossi X-ray Timing
Explorer (RXTE). We present here the detailed timing study of its X-ray
emission. The source was observed in both a high intensity/soft spectral and a
low intensity/hard spectral state. The 2-20 keV luminosities are about
1.4x10^37 and 3.2x10^36 ergs/s in the two regimes respectively (assuming a
source distance of 9.3 kpc). We confirm that 4U1915-05 is an atoll source based
on its spectral behaviour and its aperiodic variability in the frequency range
4x10^{-3}-128 Hz. 4U1915-05 displays red noise below 1 Hz, but is especially
remarkable for its quasi-periodic variability detected for the first time at
several timescales. We detect low frequency quasi-periodic oscillations
(LFQPOs) between 5 and 80 Hz, as well as high frequency QPOs (HFQPOs) between
200 and 1300 Hz. Both LFQPOs and HFQPOs frequencies positively correlate with
the mass accretion rate inferred from the position of the source on the
color-color diagram. In a narrow range of luminosity, we also detect twin
simultaneous HFQPOs separated by 348 +/- 12 Hz.


title:
Galaxy Formation and Chemical Evolution in Hierarchical Hydrodynamical
  Simulations

abstract:
  We report first results of an implementation of a chemical model in a
cosmological code, based on the Smoothed Particle Hydrodynamics (SPH)
technique. We show that chemical SPH simulations are a promising tool to
provide clues for the understanding of the chemical properties of galaxies in
relation to their formation and evolution in a cosmological framework.


title:
A study of the dipping low mass X-ray binary X\th 1624-490 from the
  broadband BeppoSAX observation

abstract:
  A study of the luminous dipping LMXB X1624-490 was made using BeppoSAX. From
modelling the MECS radial profile in several energy bands, the optical depth to
dust scattering at 1 keV was found to be 2.4+/-0.4. The non-dip and dip spectra
of X1624-490 in the energy band 1-100 keV are well-described by emission
consisting of a point-like blackbody from the neutron star plus extended
Comptonized emission from an ADC. The blackbody temperature was 1.31+/-0.07 keV
and the Comptonized emission had photon power law index 2.0+0.5/-0.7 and
cut-off energy ~12 keV. Good fits to the dip spectra were obtained by allowing
the Comptonized emission to be progressively covered by an extended absorber
while the blackbody was rapidly absorbed and a constant halo component
accounted for dust scattering into the line-of-sight. From the dip ingress
time, a diameter of the extended Comptonized emission region of 5.3+/-0.8x10^10
cm was derived, consistent with a hot, X-ray emitting corona extending to ~50%
of the accretion disk radius. The source luminosity for a distance of 15 kpc is
7.3x10^37 erg/s, an appreciable fraction of the Eddington limit making
X1624-490 the most luminous dipping LMXB. The half-height of the blackbody
emitting region on the neutron star of 6.8+/-1.8 km agrees well with the
half-height of the radiatively supported inner accretion disk of 6.3+/-2.9 km.
Together with similar agreement recently obtained for 13 other LMXB in an ASCA
survey (Church & Balucinska-Church 2000), this strongly supports the
identification of the neutron star as the origin of the blackbody emission in
LMXB and suggests a mechanism in which the disk height is the main factor
determining the level of blackbody emission. Finally, from RXTE ASM data, we
derive an improved orbital period of 20.87+/-0.01 hr.


title:
Chemical evolution using SPH cosmological simulations. I:
  implementation, tests and first results

abstract:
  We develop a model to implement metal enrichment in a cosmological context
based on the hydrodynamical AP3MSPH code described by Tissera, Lambas and Abadi
(1997).The star formation model is based on the Schmidt law and has been
modified in order to describe the transformation of gas into stars in more
detail. The enrichment of the interstellar medium due to supernovae I and II
explosions is taken into account by assuming a Salpeter Initial Mass Function
and different nucleosynthesis models.The different chemical elements are mixed
within the gaseous medium according to the Smooth Particle Hydrodynamics
technique. We present tests of the code that assess the effects of resolution
and model parameters on the results. scenario and we present results of the
analysis of the star formation and chemical properties of the interstellar
medium and stellar population of the simulated galactic objects. We show that
these systems reproduce abundance ratios for primary and secondary elements of
the interstellar medium, and the correlation between the (O/H) abundance and
the gas fraction of galaxies. The numerical simulations performed provide a
detailed description of the chemical properties of galactic objects formed in
hierarchical clustering scenarios and proved to be useful tools to deepen our
understanding of galaxy formation and evolution.


title: Magnetic Reconnection: Sweet-Parker Versus Petschek

abstract:
  The two theories for magnetic reconnection, one of Sweet and Parker, and the
other of Petschek, are reconciled by exhibiting an extra condition in that of
Petschek which reduces his theory to that of Sweet and Parker, provided that
the resistivity is constant in space. On the other hand, if the resistivity is
enhanced by instabilities, then the reconnection rate of both theories is
increased substantially, but Petschek's rate can be faster. A different formula
from the usual one is presented for enhanced Petschek reconnection.


title:
A Possible X-ray and Radio Counterpart of the High-Energy Gamma-ray
  Source 3EG J2227+6122

abstract:
  The identity of the persistent EGRET sources in the Galactic plane is largely
a mystery. For one of these, 3EG J2227+6122, our complete census of X-ray and
radio sources in its error circle reveals a remarkable superposition of an
incomplete radio shell with a flat radio spectrum, and a compact, power-law
X-ray source with photon index Gamma = 1.5 and with no obvious optical
counterpart. The radio shell is polarized at a level of 25%. The anomalous
properties of the radio source prevent us from deriving a completely
satisfactory theory as to its nature. Nevertheless, using data from ROSAT,
ASCA, the VLA, and optical imaging and spectroscopy, we argue that the X-ray
source may be a young pulsar with an associated wind-blown bubble or bow shock
nebula, and an example of the class of radio-quiet pulsars which are
hypothesized to comprise the majority of EGRET sources in the Galaxy. The
distance to this source can be estimated from its X-ray absorption as 3 kpc. At
this distance, the X-ray and gamma-ray luminosities would be 1.7x10^{33} and
3.7x10^{35} erg s^{-1}, respectively, which would require an energetic pulsar
to power them.
  If, on the contrary, this X-ray source is not the counterpart of 3EG
J2227+6122, then by process of elimination the X-ray luminosity of the latter
must be less than 10^{-4} of its gamma-ray luminosity, a condition not
satisfied by any established class of gamma-ray source counterpart. This would
require the existence of at least a quantitatively new type of EGRET source, as
has been suggested in studies of other EGRET fields.


title: UV Observations of the Powering Source of the Supergiant Shell in IC2574

abstract:
  A multi-band analysis of the region containing the supergiant HI shell in the
nearby dwarf irregular galaxy IC2574 presents evidence of a causal relationship
between a central star cluster, the surrounding expanding HI shell, and
secondary star formation sites on the rim of the HI shell. Comparisons of the
far-UV (FUV, 1521 A), optical broad-band, H-alpha, X-ray, and HI morphologies
suggest that the region is in an auspicious moment of star formation triggered
by the central stellar cluster. The derived properties of the HI shell, the
central stellar cluster, and the star forming regions on the rim support this
scenario: The kinematic age of the HI shell is <14 Myr and in agreement with
the age of the central stellar cluster derived from the FUV observations (sim
11 Myr). An estimate for the mechanical energy input from SN and stellar winds
of the central stellar cluster made from FUV photometry and the derived cluster
age is 4.1 x 10^52 erg, roughly a few times higher than the kinetic energy of
the HI shell. The requisite energy input needed to create the HI shell, derived
in the `standard' fashion from the HI observations (using the numerical models
of Chevalier), is 2.6 x 10^53 erg which is almost an order of magnitude higher
than the estimated energy input as derived from the FUV data. Given the
overwhelming observational evidence that the central cluster is responsible for
the expanding HI shell, this discrepancy suggests that the required energy
input is overestimated using the `standard' method. This may explain why some
other searches for remnant stellar clusters in giant HI holes have been
unsuccessful so far. Our observations also show that stellar clusters are
indeed able to create supergiant HI shells, even at large galactocentric radii,
a scenario which has recently been questioned by a number of authors.


title: Metal Abundances in the Magellanic Stream

abstract:
  We report on the first metallicity determination for gas in the Magellanic
Stream, using archival HST GHRS data for the background targets Fairall 9, III
Zw 2, and NGC 7469. For Fairall 9, using two subsequent HST revisits and new
Parkes Multibeam Narrowband observations, we have unequivocally detected the
MSI HI component of the Stream (near its head) in SII1250,1253 yielding a
metallicity of [SII/H]=-0.55+/-0.06(r)+/-0.2(s), consistent with either an SMC
or LMC origin and with the earlier upper limit set by Lu et al. (1994). We also
detect the saturated SiII1260 line, but set only a lower limit of
[SiII/H]>-1.5. We present serendipitous detections of the Stream, seen in
MgII2796,2803 absorption with column densities of (0.5-1)x10^13 cm^-2 toward
the Seyfert galaxies III Zw 2 and NGC 7469. These latter sightlines probe gas
near the tip of the Stream (80 deg down-Stream of Fairall 9). For III Zw 2, the
lack of an accurate HI column density and the uncertain MgIII ionization
correction limits the degree to which we can constrain [Mg/H]; a lower limit of
[MgII/HI]>-1.3 was found. For NGC 7469, an accurate HI column density
determination exists, but the extant FOS spectrum limits the quality of the
MgII column density determination, and we conclude that [MgII/HI]>-1.5.
Ionization corrections associated with MgIII and HII suggest that the
corresponding [Mg/H] may range lower by 0.3-1.0 dex. However, an upward
revision of 0.5-1.0 dex would be expected under the assumption that the Stream
exhibits a dust depletion pattern similar to that seen in the Magellanic
Clouds. Remaining uncertainties do not allow us to differentiate between an LMC
versus SMC origin to the Stream gas.


title:
Constraints over Cosmological Constant and Quintessence Fields in an
  Accelerating Universe

abstract:
  A brief account of the current cosmological observations is given and their
implications for QCDM and $\Lambda$CDM cosmologies are discussed. The
nucleosynthesis and the galaxy formation constraints have been used to put
limits on $\Omega_\phi$ during cosmic evolution, and develop a realistic
approach to the tracking behaviour of quintessence fields. The astrophysical
constraints are applied to interpolate the value of the tracking parameter
$\epsilon \simeq 0.75$ at the present epoch and also to find the lower and the
upper limits for $\Lambda$ in the accelerating universe. It is shown that the
transition from deceleration to acceleration in the cosmic expansion occurs
earlier in $\Lambda$CDM cosmology compared to QCDM cosmology.


title: To Probe a Black Hole

abstract:
  We show how an idealized high-frequency VLBI can probe the metric of nearby
supermassive black holes. No quantitative assumptions about plasma dynamics are
used in the suggested diagnostic, which is based on strong-field lensing.


title: On the Determination of the He Abundance in Extragalactic H II Regions

abstract:
  Of the light element abundances capable of testing standard big bang
nucleosynthesis, only He4 is measured with an accuracy of a few percent. Thus,
it is imperative to establish a comprehensive technique for determining He4
abundances and reliable estimates of the true systematic uncertainties. Helium
abundance determinations in H II regions are made from the observations of
several distinct He I emission lines and their strengths relative to H I
emission lines. With the general availability of large format, linear CCD
detectors, the accuracy of relative emission line ratios has improved to the
point where several terms in the error budget which were assumed to be
negligible may now be important. Here we investigate the estimation of errors
in deriving and reporting nebular helium abundances from optical emission line
spectra.


title: Good Views of the Galaxy

abstract:
  Fitting Galactic structure models to star counts only provides useful
information about the Galaxy in some directions. In this paper, we investigate
the use of $\chi^2$ goodness-of-fit tests to discriminate between degenerate
Galactic structure models, and the implications of this technique for the
Galactic spheroid and thick disk components. The axis ratio of the Galactic
spheroid and the normalization of spheroid stars with respect to disk stars
introduce a degenerate effect which means that Galactic structure models with
certain combinations of these parameters are indistinguishable from each other
in most directions. We present an analysis of the optimal directions in which
these degeneracies can be lifted. Poisson and magnitude errors are taken into
account, and an attempt is made to place an upper limit on the systematic error
due to separation of spheroid stars from thick/old disk stars. We find that the
magnitude range $20 < V < 21$ is the best for lifting most degeneracies, and
present the optimal combinations of directions using which this can be
achieved. We also give directions in which the signature of the presence of a
Galactic thick disk can be most readily identified, and the directions in which
contamination from a thick disk can be minimized. It is hoped that forthcoming
data from large-scale sky surveys would reveal much about the structure of our
Galaxy using star count techniques.


title: ASCA Observations of Three Shakhbazyan's Compact Groups of Galaxies

abstract:
  X-ray observations of three Shakhbazyan's Compact Groups of Galaxies, SCGG
202, SCGG 205, and SCGG 223, are presented for the first time. Extended X-ray
emission was detected from SCGG 202 and SCGG 223 with 0.5-2 keV luminosity of
1E42 erg/s and 3E42 erg/s (for H_0 = 75 km/s/Mpc), respectively, while no
significant emission was detected from SCGG 205 with an upper limit of 0.7E42
erg/s. The X-ray spectra of SCGG 202 and SCGG 223 can be described with
thin-thermal plasma emission with temperature about 1 keV. X-ray properties of
SCGG 202 and SCGG 223 are in good agreement with those of other known groups of
galaxies, proving the physical nature of their grouping.


title: Direct Simulation of Dense Stellar Systems with GRAPE-6

abstract:
  In this paper we describe the current status of the GRAPE-6 project to
develop a special-purpose computer with a peak speed exceeding 100 Tflops for
the simulation of astrophysical N-body problems. One of the main targets of the
GRAPE-6 project is the simulation of dense stellar systems. In this paper,
therefore, we overview the basic algorithms we use for the simulation of dense
stellar systems and their characteristics. We then describe how we designed
GRAPE hardwares to meet the requirements of these algorithms. GRAPE-6 will be
completed by the year 2001. As an example of what science can be done on
GRAPE-6, we describe our work on the galactic center with massive black holes
performed on GRAPE-4, the predecessor of GRAPE-6.


title: MID-IR Surveys - The Next Generations

abstract:
  Using new cosmological models for galaxy evolution, predictions for the next
generation of deep mid-IR surveys are presented for both the ASTRO-F space
telescope (due for launch 2003) and the proposed HII/L2 mission (2010).
Although ASTRO-F will be able to detect between 20,000-40,000 sources in the
mid-IR, deep surveys will be severely constrained by confusion due to faint
sources (due to the relatively small aperture of all IR space telescopes to
date). However, HII/L2 with its larger 3.5m mirror will be able to observe to
fluxes 10-100 times deeper below the present confusion limits.


title:
Avalanching and Self Organised Criticality, a paradigm for geomagnetic
  activity?

abstract:
  The characterization of global energy storage and release in the coupled
solar wind-magnetosphere system remains one of the fundamental problems of
space physics.Recently, it has been realised that a new paradigm in physics,
that of Self Organised Criticality (SOC) may encapsulate the mixing and merging
of flux on many scales in the magnetotail prompting bursty energy release and
reconfiguration. SOC is consistent with qualitative measures such as power law
power spectra and bursty bulk flows and with more quantitative tests such as
power law burst distributions in auroral indices and auroral optical activity.
Here, we present a careful classification of the broad range of systems that
fall under the general description of "SOC". We argue that some, but not all,
of these are consistent with our current understanding of the magnetosphere. We
discuss the observed low dimensionality of the dynamic magnetosphere in terms
of both SOC model properties, and observables. Observations of burst statistics
are highlighted; we show that these are currently suggestive but not sufficient
to confirm SOC and in particular we find that auroral indices are not effective
at distinguishing the internal dynamics of the magnetosphere from that of the
intermittent solar wind driver. This may also elucidate the paradox of
predictability and complexity of the coupled solar wind-magnetosphere system.


title: The transition to nonlinearity and new constraints on biasing

abstract:
  We present two new dynamical tests of the biasing hypothesis. The first is
based on the amplitude and the shape of the galaxy-galaxy correlation function,
$\xi_g(r)$, where $r$ is the separation of the galaxy pair. The second test
uses the mean relative peculiar velocity for galaxy pairs, $\vs(r)$. This
quantity is a measure of the rate of growth of clustering and it is related to
the two-point correlation function for the matter density fluctuations,
$\xi(r)$. Under the assumption that galaxies trace the mass ($\xi_g = \xi$),
the expected relative velocity can be calculated directly from the observed
galaxy clustering. The above assumption can be tested by confronting the
expected $\vs$ with direct measurements from velocity-distance surveys. Both
our methods are checked against N-body experiments and then compared with the
$\xi_g(r)$ and $\vs$ estimated from the {\sc APM} galaxy survey and the Mark
III catalogue, respectively. Our results suggest that cosmological density
parameter is low, $\Omega_m \approx 0.3$, and that the {\sc APM} galaxies trace
the mass at separations $r \ga 5 \Mlu$, where $h$ is the Hubble constant in
units of 100 km s$^{-1}$Mpc. The present results agree with earlier studies,
based on comparing higher order correlations in the {\sc APM} with weakly
non-linear perturbation theory. Both approaches constrain the linear bias
factor to be within 20% of unity. If the existence of the feature we identified
in the {\sc APM} $\xi_g(r)$ -- the inflection point near $\xi_g = 1$ -- is
confirmed by more accurate surveys, we may have discovered gravity's smoking
gun: the long awaited ``shoulder'' in $\xi$, generated by gravitational
dynamics and predicted by Gott and Rees 25 years ago.


title: Study of FK Comae Berenices. II. Spot evolution from 1994 to 1997

abstract:
  We present new surface (Doppler) images of the late-type single giant FK Com
for June-July 1996, July-August 1996, April 1997 and June 1997. These images
are compared with the previously published images from 1994 and 1995. The
consecutive maps are cross-correlated to see the possible migration of the
spots and the effects of differential rotation. The cross-correlation confirms
an average longitudinal spot migration of 0.22+/-0.03 in phase within a year.
This movement is probably an artifact caused by a difference between the
accepted rotation period and the real photometric period for these years. If
this is true, then the photometric rotation period for these years is
2.4037+/-0.0005 days. Measurements from these four years and six maps limit the
surface differential rotation to alpha=0.0001+/-0.0002, where alpha is the
difference between polar and equatorial angular velocities relative to the
equatorial angular velocity.


title: New results from the HRX BL Lac sample

abstract:
  We present results for the Hamburg BL Lac sample, based on data provided by
the RASS-BSC. By fitting a single power law to the X-ray data we find, in a
number of objects, an additional absorbing component to the galactic value of
NH, which might be attributed to intrinsic absorption. A more probable cause
seems however to be a curvature in the X-ray spectra in the sense that they are
more curved for steeper slopes. The known relation between the X-ray spectral
slope and the ratio between optical and X-ray flux alpha_ox also applies to
this BL Lac sample, even though less significant than in previous works. We
also find a dependence of X-ray luminosity on alpha_ox.


title: Hard synchrotron BL Lacs: the case of 1ES 1101-232

abstract:
  The bright X-ray selected BL Lac object 1ES1101--232 shows a flat X-ray
spectrum, making it detectable with high statistics over the wide BeppoSAX
energy range. We have observed it in two different epochs with BeppoSAX, and
found a variation of the flux of about 30% that can be explained by a change in
the spectral index above the synchrotron peak. We present here the data and
infer limits on the strength of the magnetic field based on models of emission
for High-frequency peaked BL Lacs.


title: Evaporation: The change from accretion via a thin disk to a coronal flow

abstract:
  We present a model for a corona above a geometrically thin standard disk
around a black hole. This corona is fed by matter from the thin disk which
evaporates from the cool layers underneath. An equilibrium establishes between
the cool accretion stream and the hot flow. In the inner region evaporation
becomes so efficient that at low accretion rates all matter flows via the
corona and proceeds towards the black hole as an advection-dominated accretion
flow (ADAF). We investigate this transition for accretion disks around stellar
black holes. We derive a set of four ordinary differential equations which
describe the relevant processes and solve them numerically. The evaporation
efficiency has a maximum value where the advection-dominated structure of the
corona (at large distances from the black hole) changes to a
radiation-dominated structure (at small distances). This maximum can be related
to the observed spectral transitions in black hole X-ray binaries. Our
solutions of coronal structure are valid for both, accretion in galactic X-ray
transients and in AGN.


title:
High frequencies in the power spectrum of Cyg X-1 in the hard and soft
  spectral states

abstract:
  We analyzed a large number of RXTE/PCA observations of Cyg X-1 in the hard
and soft spectral states with total exposure time of ~190 and ~10 ksec
respectively and time resolution better than ~250us in order to investigate its
variability down to a few milliseconds time scales. The model of modifications
of the power density spectra due to the dead time effect was tested using RXTE
observations of an extremely bright source - Sco X-1. The results of these
tests demonstrated, that although some problems still remain, for sources like
Cyg X-1 (which is an order of magnitude less bright than Sco X-1) present
knowledge of the instrument is sufficient to reliably analyze the power spectra
in the kHz frequency range.
  In both spectral states of Cyg X-1 we detected statistically significant
variability up to ~150-300 Hz with a fractional rms amplitude above 100 Hz at
the level of ~2-3%. The power spectrum in the hard state shows a steepening at
frequency of ~40-80 Hz with the power law slope changing from ~1.7 to ~2.3-2.4.
In the soft state the slope of power spectrum changes from ~1 to ~2 at the
frequency of ~15-20 Hz without any evidence of further steepening up to
\~100-150 Hz. These break frequencies represent the highest characteristic
frequencies detected in the power density spectrum of Cyg X-1 so far.


title: The highly polarized open cluster Trumpler 27

abstract:
  We have carried out multicolor linear polarimetry (UBVRI) of the brightest
stars in the area of the open cluster Trumpler 27. Our data show a high level
of polarization in the stellar light with a considerable dispersion, from $P =
4%$ to $P = 9.5%$. The polarization vectors of the cluster members appear to be
aligned. Foreground polarization was estimated from the data of some non-member
objects, for which two different components were resolved: the first one
associated with a dust cloud close to the Sun producing $P_{\lambda max}=1.3%$
and $\theta=146$ degrees, and a second component, the main source of
polarization for the cluster members, originated in another dust cloud, which
polarizes the light in the direction of $\theta= 29.5$ degrees. From a detailed
analysis, we found that the two components have associated values $E_{B-V} <
0.45 $ for the first one, and $E_{B-V} > 0.75$ for the other. Due the
difference in the orientation of both polarization vectors, almost 90 degrees
(180 degrees at the Stokes representation), the first cloud ($\theta \sim 146$
degrees) depolarize the light strongly polarized by the second one ($\theta
\sim 29.5$ degrees).


title: Non-LTE effects in neutron star atmospheres

abstract:
  We present the first non-LTE model atmospheres for neutron stars (NS). We
study their structure and NLTE effects on the emergent thermal radiation of old
isolated NS.


title: HNCO in massive galactic dense cores

abstract:
  We surveyed 81 dense molecular cores associated with regions of massive star
formation and Sgr A in the 5_{05}-4_{04} and 10_{010}-9_{09} lines of HNCO.
Line emission was detected towards 57 objects. Selected subsamples were also
observed in other HNCO lines covering a frequency range from 22 to 461 GHz.
HNCO lines from the K_{-1} = 2,3 ladders were detected in several sources.
Towards Orion-KL, K_{-1} = 5 transitions with upper state energies E_u/k ~ 1100
and 1300 K could be observed. Five HNCO cores were mapped. The sources remain
spatially unresolved at 220 and 461 GHz with beam sizes of 24" and 18",
respectively. Typical HNCO abundances relative to H_2 as derived from a
population diagram analysis are ~ 10^{-9}. The rotational temperatures reach ~
500 K. The gas densities in regions of HNCO $K_{-1}=0$ emission should be n >
10^6 cm^{-3} and in regions of K_{-1}>0 emission about an order of magnitude
higher even for radiative excitation. HNCO abundances are found to be enhanced
in high-velocity gas. HNCO integrated line intensities correlate well with
those of thermal SiO emission. This indicates a spatial coexistence of the two
species and may hint at a common production mechanism, presumably based on
shock chemistry.


title: Acceleration of UHE Cosmic Rays in Gamma-Ray Bursts

abstract:
  Gamma-Ray Bursts are good candidates of the ``bottom up'' scenario for the
generation of the UHE Cosmic Rays. In the most discussed model of GRBs, namely
the ``fireball'' model, a highly relativistic shock forms and seems capable of
accelerating the cosmic rays up to the EeV range. However, only the first Fermi
cycle produces a large energy gain to particles coming from the external
medium. Thus, a complementary acceleration is proposed, downstream of the
external shock, in the relativistic plasma of the GRBs, where crossings of
relativistic fronts are likely to occur. Both forward and backward fronts are
necessary for the internal Fermi acceleration to work and the physical process
that generates them is presented. We found that there exists a relevant
physical process similar to Brillouin backscattering that redistributes the
incoming energy in the plasma shell. This redistribution occurs through the
generation of sound waves that heat the plasma shell and also through the
generation of both forward and backward relativistic Alfven fronts that
accelerate cosmic rays by the Fermi process. We show that this ensemble of
processes is able to account for the generation of UHE cosmic rays.
  There are two opportunities for these combined processes, first during the
``primary'' Gamma-Ray Burst where baryons are entrained by the relativistic
pair wind, second during the predeceleration and deceleration stages when the
fireball interacts with the interstellar medium. However, because of the
synchrotron losses, only the second stage can produce the UHE cosmic rays.


title:
Coherent radio emission from the magnetic chemically peculiar star CU
  Virginis

abstract:
  Radio observations of the magnetic chemically peculiar star CU Vir, carried
out with the VLA in three different days, show that the radio emission at 20 cm
is characterized by a strong enhancement at particular rotational phases. This
radio emission is found to be right hand polarized with a degree of
polarization close to 100 %. As common for this class of stars, the magnetic
axis of CU Vir is oblique with respect to the rotational axis. By comparing the
20 cm radio light curve with the effective magnetic field available from the
literature, a coincidence of the main peaks of the radio emission with the
magnetic nulls has been found. This happens when the magnetic axis lies in the
plane of the sky. We suggest that the high degree of polarization, together
with the high directivity of the radiation, can be explained in terms of
coherent radio emission. The data have been interpreted on the basis of the
Electron Cyclotron Maser Emission from electrons accelerated in current sheets
out of the Alfven radius toward the stellar surface and eventually reflected
outward by magnetic mirroring.


title: Extreme Scattering Events: An Observational Summary

abstract:
  We review observational constraints on the structures responsible for extreme
scattering events, focussing on a series of observations of the quasar PKS
1741-038. VLA observations were conducted to search for changes in the rotation
measure and H I absorption during the ESE, while VLBI observations sought
ESE-induced changes in the source's image. No RM changes were found implying
B_|| < 12 mG, and no H I opacity changes were found implying N(H I) < 6.4 x
10^{17} cm^{-2}. No multiple imaging was observed, but the diameter of the
source increased by 0.7 mas, contrary to what is predicted by simple refractive
lens modeling of ESEs. We summarize what these limits imply about the structure
responsible for this ESE.


title: Near-IR Imaging of LSB Galaxies: the High and Low HI Content Cases

abstract:
  Recently acquired near-IR imaging (J and Ks) of low surface brightness
galaxies is presented. The data includes 52 galaxies with log[MHI/Msolar] < 9.0
and 58 galaxies with log[MHI/Msolar] > 9.5. These galaxies have been selected
from the catalogue of Impey et al. (1996), and all of them are observable from
both hemispheres. The principal goal of this research is to investigate the
poorly understood giant and old stellar content of these galaxies. Current work
includes total and isophotal photometry, and comparison with spectrophotometric
models of galaxy evolution, including the role of age and metallicity. Already
allocated observing time at Las Campanas Observatory will provide high S/N B
and I imaging.


title: A Cyclic Universe without Missing Mass: Implications of R->alpha'/R

abstract:
  Results from string theory conclude that a spatial dimension R is equivalent
to *'/R. If R is considered as a four-space dimension, several interesting
results emerge. In this paradigm, *'/R exists in a time-reversed, "anti-
parallel" universe. Cosmological distances in our real-time universe are
equivalent to the compact dimensions at the instant of a Big Bang in a time-
inverted universe. This model agrees with standard black hole theory in that
particles become frozen in time when they reach the 'singularity' (of dimension
alpha'). A particle which enters a black hole in real time exits from the Big
Bang or "white hole" in the time-inverted universe, leaving behind a trace of
annihilation radiation. A diagram of this phenomenon is constructed, consistent
with existing knowledge of the early universe. The primordeal black holes
predicted by Hawking are required. A cyclic universe is described by M^2\p,n +
T^2\n = 1. The "missing mass" proscribed by standard closed- universe theories
is not required. This result is useful if experimental evidence in the search
for dark matter continues to leave a significant "missing mass" - and
particularly in light of more recent observations indicating that, in fact,
inflation continues at a much reduced rate.


title:
Correlations in the Lyman alpha forest: testing the gravitational
  instability paradigm

abstract:
  We investigate correlations between the long wavelength fluctuations and the
small scale power in the Lyman alpha forest. We show that such correlations can
be used to discriminate between fluctuations induced by large scale structure
and those produced by non-gravitational processes such as fluctuations in the
continuum of the quasar or in the temperature of the gas. The correlations
observed in Q1422+231 are in agreement with the predictions of numerical
simulations, indicating that non-gravitational fluctuations on large scales
have to be small compared those induced by the large scale structure of the
universe, contributing less than 10 % (95 % confidence) of the observed power.
We also show the sensitivity of such statistics to the equation of state of the
gas and its mean temperature.


title: Multi-GeV Neutrinos from Internal Dissipation in GRB Fireballs

abstract:
  Sub-photospheric internal shocks and transverse differences of the bulk
Lorentz factor in relativistic fireball models of GRB lead to neutron diffusion
relative to protons, resulting in inelastic nuclear collisions. This produces
significant fluxes of ~3 GeV muon neutrinos (antineutrinos) and ~2 GeV electron
neutrinos (antineutrinos), scaling with the Lorentz factor eta < 400. This
extends significantly the parameter space for which neutrinos from inelastic
collision are expected, which in the absence of the above effects requires eta
> 400. A model with sideways diffusion of neutrons from a slower wind into a
fast jet can lead to production of muon and electron neutrinos (antineutrinos)
in the 2-25 GeV or higher range, depending on the value of eta. The emission
from either of these mechanisms at z~1 may be detectable in suitably densely
spaced detectors.


title:
The Influence of Interactions and Minor Mergers on the Structure of
  Galactic Disks: I.Observations and Disk Models

abstract:
  This paper is the first part in our series on the influence of tidal
interactions and minor mergers on the radial and vertical disk structure of
spiral galaxies. We report on the sample selection, our observations, and data
reduction. Surface photometry of the optical and near infrared data of a sample
of 110 highly-inclined/edge-on disk galaxies are presented. This sample
consists of two subsamples of 61 non-interacting galaxies (control sample) and
of 49 interacting galaxies/minor merging candidates. Additionally, 41 of these
galaxies were observed in the near infrared. We show that the distribution of
morphological types of both subsamples is almost indistinguishable, covering
the range between 0 <= T <= 9. An improved, 3-dimensional disk modelling- and
fitting procedure is described in order to analyze and to compare the disk
structure of our sample galaxies by using characteristic parameters. We find
that the vertical brightness profiles of galactic disks respond very sensitive
even to small deviations from the perfect edge-on orientation. Hence,
projection effects of slightly inclined disks may cause substantial changes in
the value of the disk scale height and must therefore be considered in the
subsequent study.


title:
The Influence of Interactions and Minor Mergers on the Structure of
  Galactic Disks: II. Results and Interpretations

abstract:
  We present the second part of a detailed statistical study focussed on the
effects of tidal interactions and minor mergers on the radial and vertical disk
structure of spiral galaxies. In the first part we reported on the sample
selection, observations, and applied disk models. In this paper the results are
presented, based on disk parameters derived from a sample of 110
highly-inclined/edge-on galaxies. This sample consists of two subsamples of 49
interacting/merging and 61 non-interacting galaxies. Additionally, 41 of these
galaxies were observed in the NIR. We find significant changes of the disk
structure in vertical direction, resulting in ~1.5 times larger scale heights
and thus vertical velocity dispersions. The radial disk structure,
characterized by the cut-off radius and the scale length, shows no
statistically significant changes. Thus, the ratio of radial to vertical scale
parameters, h/z0, is ~1.7 times smaller for the sample of interacting/merging
galaxies. The total lack of typical flat disk ratios (h/z0)>7 in the latter
sample implies that vertical disk heating is most efficient for (extremely)
thin disks. Statistically nearly all galactic disks in the sample (93%) possess
non-isothermal vertical luminosity profiles like the sech (60%) and exp (33%)
distribution, independent of the sample and passband investigated. This
indicates that, in spite of tidal perturbations and disk thickening, the
initial vertical distribution of disk stars is not destroyed by interactions or
minor mergers.


title: The broad band spectrum and variability of Seyfert 1

abstract:
  Recent results on the X-ray spectrum and variability of Seyfert 1 galaxies
are reviewed. New spectral results from BeppoSAX observations are also
presented and briefly discussed.


title: Resolving the 10-40 keV Cosmic X-ray Background with Constellation-X

abstract:
  The energy density of the Cosmic X-ray background (XRB) peaks around 30 keV
(see Figure 1), an energy not yet probed by focussing imaging instruments. The
first hard X-ray telescope due to fly on a space mission will be that on board
Constellation-X. The imaging capability, besides providing an improvement of
several orders of magnitude in sensitivity over current passively collimated
detectors, will permit for the first time to resolve a fraction of the XRB at
this most crucial energy. Synthesis models of the XRB based on obscured AGN
predict that at least 40% of the 10-40 keV XRB will be resolved by
Constellation-X.


title: Two BeppoSAX observations of BL Lac

abstract:
  We present the results of two ToO BeppoSAX observations of BL Lac. During the
first observation we detected both the synchrotron and the Compton components.
Fast time variability was present, but only for the synchrotron component.
During the second observation the spectrum was flatter and only the Compton
component was present. Four different SED are presented and can be described by
a blob moving along a jet and responsible for the SSC emission. The seed
photons for the Compton scattering are the synchrotron photons themself plus
possibly external photons coming from the broad line region.


title: Charge conservation and time-varying speed of light

abstract:
  It has been recently claimed that cosmologies with time dependent speed of
light might solve some of the problems of the standard cosmological scenario,
as well as inflationary scenarios. In this letter we show that most of these
models, when analyzed in a consistent way, lead to large violations of charge
conservation. Thus, they are severly constrained by experiment, including those
where $c$ is a power of the scale factor and those whose source term is the
trace of the energy-momentum tensor. In addition, early Universe scenarios with
a sudden change of $c$ related to baryogenesis are discarded.


title: A Note on TeV Cerenkov Events as Bose-Einstein Gamma Condensations

abstract:
  The idea that the TeV air showers, thought to be produced by >10 TeV gamma
rays from Mrk 501, can be mimicked by coherent bunches of sub-TeV photons is
reexamined, focusing on fundamental considerations. In particular, it is shown
that the minimum spot size of the beam of pulsed TeV photons arriving at Earth
is on the order of a few kilometers, unless a lens with certain characteristics
is placed between the TeV laser and Earth. The viability of the laser
production mechanism proposed by Harwit et al. (2000) is also reassessed.


title: Bizarre Hard X-ray Outbursts of Cygnus X-1

abstract:
  A very high activity of Cygnus X-1 on 1999 April 19-21 was recorded by BATSE
Large Area Detectors onboard the Compton Gamma-Ray Observatory. The peak
luminosity was one order of magnitude higher than the normal luminosity of Cyg
X-1. This fact can be critical for models of the hard state of Cyg X-1. The
longest outburst lasted ~1000 s and demonstrated very unusual temporal and
spectral behavior which indicates the presence of two emission components. One
component is relatively soft (with a cutoff below 100 keV) and highly variable,
and the other one is hard (extending above 100 keV), with much slower
variability.


title: H-alpha Photometry of Abell 2390

abstract:
  We present the results of a search for strong H-alpha emission line galaxies
(rest frame equivalent widths greater than 50 A) in the z=0.23 cluster Abell
2390. The survey contains 1189 galaxies over 270 square arcminutes, and is 50%
complete at M_r=-17.5+5 log h. The fraction of galaxies in which H-alpha is
detected at the 2sigma level rises from 0.0 in the central regions (excluding
the cD galaxy) to 12.5+/-8% at R200. For 165 of the galaxies in our catalogue,
we compare the H-alpha equivalent widths with their [OII] equivalent widths,
from the CNOC1 spectra. The fraction of strong H-alpha emission line galaxies
is consistent with the fraction of strong [OII] emission galaxies in the CNOC1
sample: only 2+/-1% have no detectable [OII] emission and yet significant
(>2sigma) H-alpha equivalent widths. Dust obscuration, non-thermal ionization,
and aperture effects are all likely to contribute to this non-correspondence of
emission lines. We identify six spectroscopically 'secure' k+a galaxies
(W(OII)<5 A and W(Hdelta)>5 A); at least two of these show strong signs in
H-alpha of star formation in regions that are covered by the slit from which
the spectra were obtained. Thus, some fraction of galaxies classified k+a based
on spectra shortward of 6000 A are likely to be undergoing significant star
formation. These results are consistent with a 'strangulation' model for
cluster galaxy evolution, in which star formation in cluster galaxies is
gradually decreased, and is neither enhanced nor abruptly terminated by the
cluster environment.


title:
BeppoSAX Observations of the Galactic Center Region: Soft x-rays from
  the radio halo of SgrAEast

abstract:
  The BeppoSAX satellite performed a survey of the Galactic Center Region in
the 1-10 keV energy band with its Narrow Field Instruments. Several bright
X-ray sources containing neutron stars and black holes have been observed and
studied, including the possible counterpart of SgrA*. Here we report the
results on the diffuse emission coming from the SgrA Complex. The emission from
within 8 arcmin from SgrA* has a double-temperature thermal spectrum (kT1~0.6
keV and kT2~8 keV) and an energy-dependent morphology: the hard emission (5-10
keV) is elongated along the galactic plane, while the soft one (2-5 keV) shows
a triangular shape, very similar to the radio halo of SgrAEast. This spatial
correlation and the physical parameters of the lower temperature component
support the interpretation of the radio halo of the Sgr~A~East shell as a SNR.


title: Thermal stability and nova cycles in permanent superhump systems

abstract:
  Archival data on permanent superhump systems are compiled to test the thermal
stability of their accretion discs. We find that their discs are almost
certainly thermally stable as expected. This result confirms Osaki's suggestion
(1996) that permanent superhump systems form a new subclass of cataclysmic
variables (CVs), with relatively short orbital periods and high mass transfer
rates. We note that if the high accretion rates estimated in permanent
superhump systems represent their mean secular values, then their mass transfer
rates cannot be explained by gravitational radiation, therefore, either
magnetic braking should be extrapolated to systems below the period gap or they
must have mass transfer cycles. Alternatively, a new mechanism that removes
angular momentum from CVs below the gap should be invoked.
  We suggest applying the nova cycle scenarios offered for systems above the
period gap to the short orbital period CVs. Permanent superhumps have been
observed in the two non-magnetic ex-novae with binary periods below the gap.
Their post-nova magnitudes are brighter than their pre-outburst values. In one
case (V1974 Cyg) it has been demonstrated that the pre-nova should have been a
regular SU UMa system. Thus it is the first nova whose accretion disc was
observed to change its thermal stability. If the superhumps in this system
indicate persistent high mass transfer rates rather than a temporary change
induced by irradiation from the hot post-nova white dwarf, it is the first
direct evidence for mass transfer cycles in CVs. The proposed cycles are driven
by the nova eruption.