title: Gravitational Lensing of High Redshift Sources

abstract:
  The combination of deep exposures and high resolution offered by telescopes
in space allows the detection of lensing over a wide range of source redshifts
and lens masses. As an example, we model a lens candidate found in the southern
Hubble Deep Field. The system consists of a source galaxy lensed into an arc,
0.9 arcseconds from an elliptical galaxy with an implied mass-to-light ratio of
order 15 in solar units. This lens system may be a preview of the large number
of lensed high-redshift galaxies that will be detected with the Next Generation
Space Telescope (NGST). Popular models of structure formation imply that
roughly 3% of observed sources at z>5 should be multiply imaged. Thus, NGST
should detect several lensed objects in each field of view. In addition, NGST
will be a valuable tool for weak lensing measurements if it can resolve the
background galaxies. We estimate the angular size distribution of high redshift
sources within hierarchical models of structure formation and find that most
will be resolved. (abridged)


title:
Identifying the Reionization Redshift from the Cosmic Star Formation
  Rate

abstract:
  We show that the cosmic star formation rate per comoving volume should
exhibit a distinct drop around the reionization redshift, when the HII regions
in the intergalactic medium around individual ionizing sources first
overlapped. The drop results from the increase in the temperature of the
intergalactic medium as it was photo-ionized, and the consequent suppression of
the formation of low-mass galaxies. We show quantitatively that the detection
of this drop, which is marked by a corresponding fall in the number counts of
faint galaxies, should become feasible over the coming decade with the Next
Generation Space Telescope.


title: The spectra and energies of classical double radio lobes

abstract:
  We compare two temporal properties of classical double radio sources: i)
radiative lifetimes of synchrotron-emitting particles and ii) dynamical source
ages. We discuss how these can be quite discrepant from one another, rendering
use of the traditional spectral ageing method inappropriate: we contend that
spectral ages give meaningful estimates of dynamical ages only when these ages
are << 10^7 years. In juxtaposing the fleeting radiative lifetimes with source
ages which are significantly longer, a refinement of the paradigm for radio
source evolution is required. The changing spectra along lobes are explained,
not predominantly by synchrotron ageing but, by gentle gradients in a magnetic
field mediated by a low-gamma matrix which illuminates an energy-distribution
of particles, controlled largely by classical synchrotron loss in the high
magnetic field of the hotspot. The energy in the particles is an order of
magnitude higher than that inferred from the minimum-energy estimate, implying
that the jet-power is of the same order as the accretion luminosity produced by
the quasar central engine. This refined paradigm points to a resolution of the
findings of Rudnick et al (1994) and Katz-Stone & Rudnick (1994) that both the
Jaffe-Perola and Kardashev-Pacholczyk model spectra are invariably poor
descriptions of the curved spectral shape of lobe emission, and indeed that for
Cygnus A all regions of the lobes are characterised by a `universal spectrum'.
[abridged]


title: Oxygen Absorption in Cooling Flows

abstract:
  The inhomogeneous cooling flow scenario predicts the existence of large
quantities of gas in massive elliptical galaxies, groups, and clusters that
have cooled and dropped out of the flow. Using spatially resolved, deprojected
X-ray spectra from the ROSAT PSPC we have detected strong absorption over
energies ~0.4-0.8 keV intrinsic to the central ~1 arcmin of the galaxy, NGC
1399, the group, NGC 5044, and the cluster, A1795. These systems have amongst
the largest nearby cooling flows in their respective classes and low Galactic
columns. Since no excess absorption is indicated for energies below ~0.4 keV
the most reasonable model for the absorber is warm, collisionally ionized gas
with T=10^{5-6} K where ionized states of oxygen provide most of the
absorption. Attributing the absorption only to ionized gas reconciles the large
columns of cold H and He inferred from Einstein and ASCA with the lack of such
columns inferred from ROSAT, and also is consistent with the negligible atomic
and molecular H inferred from HI, and CO observations of cooling flows. The
prediction of warm ionized gas as the product of mass drop-out in these and
other cooling flows can be verified by Chandra, XMM, and ASTRO-E.


title: Iron Gradients in Cooling Flow Galaxies and Groups

abstract:
  Previous studies of the Fe abundances in the hot gas of galaxies and groups
have reported conflicting results with most studies finding very sub-solar Fe
abundances that disagree with standard theory. To investigate the possible role
of Fe abundance gradients on these measurements we present deprojection
analysis of the ROSAT PSPC data of 10 of the brightest cooling flow galaxies
and groups. The PSPC allows for spatially resolved spectral analysis on a
half-arcminute scale, and interesting constraints on both the temperatures and
Fe abundances are possible because the ~1 keV temperatures of these systems are
well matched to the bandpass of the PSPC. In 9 out of 10 systems we find clear
evidence that the Fe abundance decreases with increasing radius: Z_Fe ~1 solar
- (several) solar within the central radial bin (r < ~10 kpc) which decreases
to Z_Fe ~0.5 solar at the largest radii examined (r ~50-100 kpc). The Fe
abundances (and temperatures) are consistent with the average values for these
systems that we obtained in our previous analyses of the ASCA data using
multitemperature models which confirms that previous inferences of very
sub-solar Fe abundances from ASCA arise from the incorrect assumption of
isothermal gas and not the presence of Fe abundance gradients. We discuss why
this ``Fe Bias'' affects much more seriously the measurements of Z_Fe from ASCA
data than from ROSAT data. We show that the Fe abundance profiles for these
galaxies and groups are consistent with a gas-dynamical model where the gas is
enriched by stellar ejecta and supernovae in the ``solar supernova
proportion'', the stars formed with a Galactic IMF, and the gas is diluted by
mixing with primordial gas at large radii.


title:
ROSAT Evidence for Intrinsic Oxygen Absorption in Cooling Flow Galaxies
  and Groups

abstract:
  Using spatially resolved, deprojected ROSAT PSPC spectra of 10 of the
brightest cooling flow galaxies and groups with low Galactic column densities
we have detected intrinsic absorption over energies ~0.4-0.8 keV in half of the
sample. Since no intrinsic absorption is indicated for energies below ~0.4 keV,
the most reasonable model for the absorber is collisionally ionized gas at
temperatures T=10^{5-6} K with most of the absorption arising from ionized
states of oxygen but with a significant contribution from carbon and nitrogen.
The soft X-ray emission of this warm gas can explain the sub-Galactic column
densities of cold gas inferred within the central regions of most of the
systems. Attributing the absorption to ionized gas reconciles the large columns
of cold H and He inferred from EINSTEIN and ASCA with the lack of such columns
inferred from ROSAT. Within the central ~10-20 kpc, where the constraints are
most secure, the estimated mass of the ionized absorber is consistent with most
(perhaps all) of the matter deposited by a cooling flow over the lifetime of
the flow. Since the warm absorber produces no significant H or He absorption
the large absorber masses are consistent with the negligible atomic and
molecular H inferred from HI and CO observations of cooling flows. It is also
found that if T > ~2x10^5 K then the optical and UV emission implied by the
warm gas does not violate published constraints. Finally, we discuss how the
prediction of warm ionized gas as the product of mass drop-out in these and
other cooling flows can be verified with new CHANDRA and XMM observations.
(Abridged)


title:
The Diffusive Galactic GeV/TeV Gamma-Ray Background: Sources vs.
  Transport

abstract:
  The diffuse Galactic gamma-ray background, as observed with EGRET on CGRO,
exceeds the model predictions significantly above 1 GeV. This is particularly
true for the inner Galaxy. We shall discuss here the contribution of the
Galactic Cosmic Ray (GCR) sources, considered as unresolved, and in addition
the possibility that the transport of the GCRs out of the Galaxy is not uniform
over the Galactic disk. In both cases the spectrum of the diffuse gamma-rays is
harder than the GCR spectrum in the neighborhood of the Solar system, as
observed in situ. The source contribution is a necessary and, as it turns out,
significant part of the diffuse background, whereas the transport effect is one
of several conceivable additional causes for the hard diffuse gamma-ray
spectrum observed.


title: Where are the type 2 AGNs?

abstract:
  Most of the energy of the X-ray background is contributed by sources with a
hard X-ray spectrum, harder than the spectrum observed in nearby type 1 AGNs.
The hard background contributors are conventionally identified with type 2
AGNs, obscured by circumnuclear matter both in the optical and in the soft X
rays. However, preliminary results of identification programs seem to find
systematically few type 2 AGNs in samples selected with obscuration insensitive
criteria. We discuss this issue, and assess the various scenarios which are
being proposed to solve the discrepancy.


title: Numerical Simulations of Globular Cluster Formation

abstract:
  We examine various physical processes associated with the formation of
globular clusters by using the three-dimensional Smoothed Particle
Hydrodynamics (SPH) code. Our code includes radiative cooling of gases, star
formation, energy feedback from stars including stellar winds and supernovae,
and chemical enrichment by stars. We assume that, in the collapsing galaxy,
isothermal cold clouds form through thermal condensations and become
proto-globular clouds. We calculate the size of proto-globular clouds by
solving the linearized equations for perturbation. We compute the evolution of
the inner region of the proto-cloud with our SPH code for various initial
radius and initial composition of gases. When the initial gases contain no
heavy elements, the evolution of proto-clouds sensitively depends on the
initial radius. For a smaller initial radius, the initial star burst is so
intense that the subsequent star formation occurs in the central regions to
form a dense star cluster as massive as the globular cluster. When the initial
gases contain some heavy elements, the metallicity of gases affects the
evolution and the final stellar mass. If the initial radius of the
proto-globular clouds was relatively large, the formation of a star cluster as
massive as the globular clusters requires the initial metallicity as high as
[Fe/H] $\geq -2$. The self-enrichment of heavy elements in the star cluster
does not occur in all cases.


title: Shock Excitation in Interacting Galaxies: Mkn 266

abstract:
  We present near infrared data on the luminous interacting system Mkn 266 (NGC
5256), comprising 2um continuum, and Br_gamma and 1-0S(1) emission line images,
together with K-band spectra. We have fitted stellar templates to the
continuum, allowing us to account for all the stellar features and hence detect
even faint gas excitation emission lines, including 8 and 11 H_2 lines in the
SW and NE nuclei respectively. Population diagrams for the excited H_2
molecules indicate that most of the 1-0S(1) in each of the nuclei has a thermal
origin. We discuss this with reference to the observed morphologies, especially
that of the 1-0S(1) line. In particular, the core of 1-0S(1) in the NE nucleus
is more compact than the 2um continuum; while in the SW nucleus the 1-0S(1) is
significantly offset by 500pc from the continuum (and other) emission. Lastly
we address the issue of the region midway between the two nuclei, where
previously a strong source of radio continuum has been observed. These results
are set in the context of interacting galaxies where shock excited emission
might be expected to occur as a direct consequence of the interaction.


title: Properties of Gamma-Ray Burst Classes

abstract:
  The three gamma-ray burst (GRB) classes identified by statistical clustering
analysis (Mukherjee et al. 1998) are examined using the pattern recognition
algorithm C4.5 (Quinlan 1986). Although the statistical existence of Class 3
(intermediate duration, intermediate fluence, soft) is supported, the
properties of this class do not need to arise from a distinct source
population. Class 3 properties can easily be produced from Class 1 (long, high
fluence, intermediate hardness) by a combination of measurement error,
hardness/intensity correlation, and a newly-identified BATSE bias (the fluence
duration bias). Class 2 (short, low fluence, hard) does not appear to be
related to Class 1.


title: ISO Spectroscopy of H2 in Star Forming Regions

abstract:
  We have studied molecular hydrogen emission in a sample of 21 YSOs using
spectra obtained with the Infrared Space Observatory (ISO). H2 emission was
detected in 12 sources and can be explained as arising in either a shock,
caused by the interaction of an outflow from an embedded YSO with the
surrounding molecular cloud, or in a PDR surrounding an exposed young
early-type star. The distinction between these two mechanisms can not always be
made from the pure rotational H2 lines alone. Other tracers, such as PAH
emission or [S I] 25.25 micron emission, are needed to identify the H2 heating
mechanism. No deviations from a 3:1 ortho/para ratio of H2 were found. Both
shocks and PDRs show a warm and a hot component in H2, which we explain as
thermal emission from warm molecular gas (warm component), or UV-pumped
infrared fluorescence in the case of PDRs and the re-formation of H2 for shocks
(hot component).


title:
Non-parametric star formation histories for 5 dwarf spheroidal galaxies
  of the local group

abstract:
  We use recent HST colour-magnitude diagrams of the resolved stellar
populations of a sample of local dSph galaxies (Carina, LeoI, LeoII, Ursa Minor
and Draco) to infer the star formation histories of these systems, $SFR(t)$.
Applying a new variational calculus maximum likelihood method which includes a
full Bayesian analysis and allows a non-parametric estimate of the function one
is solving for, we infer the star formation histories of the systems studied.
This method has the advantage of yielding an objective answer, as one need not
assume {\it a priori} the form of the function one is trying to recover. The
results are checked independently using Saha's $W$ statistic. The total
luminosities of the systems are used to normalize the results into physical
units and derive SN type II rates. We derive the luminosity weighted mean star
formation history of this sample of galaxies.


title: The Fluence Duration Bias

abstract:
  The fluence duration bias causes fluences and durations of faint gamma-ray
bursts to be systematically underestimated relative to their peak fluxes. Using
Monte Carlo analysis, we demonstrate how this effect explains characteristics
of structure of the fluence vs. 1024 ms peak flux diagram. Evidence of this
bias exists in the BATSE fluence duration database, and provides a partial
explanation for the existence of burst class properties.


title: GRB Spectral Hardness and Afterglow Properties

abstract:
  A possible relationship between the presence of a radio afterglow and
gamma-ray burst spectral hardness is discussed. The correlation is marginally
significant; the spectral hardness of the bursts with radio afterglows
apparently results from a combination of the break energy Ebreak and the
high-energy spectral index beta. If valid, this relationship would indicate
that the afterglow does carry information pertaining to the GRB central engine.


title: A Simple BATSE Measure of GRB Duty Cycle

abstract:
  We introduce a definition of gamma-ray burst (GRB) duty cycle that describes
the GRB's efficiency as an emitter; it is the GRB's average flux relative to
the peak flux. This GRB duty cycle is easily described in terms of measured
BATSE parameters; it is essentially fluence divided by the quantity peak flux
times duration. Since fluence and duration are two of the three defining
characteristics of the GRB classes identified by statistical clustering
techniques (the other is spectral hardness), duty cycle is a potentially
valuable probe for studying properties of these classes.


title: Radio Pulsar Death Line Revisited: Is PSR J2144-3933 Anomalous?

abstract:
  We reinvestigate the radio pulsar ``death lines'' within the framework of two
different types of polar cap acceleration models, i.e., the vacuum gap model
and the space-charge-limited flow model, with either curvature radiation or
inverse Compton scattering photons as the source of pairs. General relativistic
frame-dragging is taken into account in both models. We find that the inverse
Compton scattering induced space-charge-limited flow model can sustain strong
pair production in some long-period pulsars, which allows the newly detected
8.5s pulsar PSR J2144-3933 to be radio loud, without assuming a special neutron
star equation-of-state or ad hoc magnetic field configurations.


title:
Structure of the globular cluster M15 and constraints on a massive
  central black hole

abstract:
  Globular clusters could harbor massive central black holes (BHs), just as
galaxies do. So far, no unambiguous detection of a massive BH has been reported
for any globular cluster. However, the dense core-collapsed cluster M15 seems
to be a good candidate. I review the available photometric and kinematic data
for this cluster. Both are consistent with a BH of approximately 2000 solar
masses, although such a BH is not unambiguously required by the data. I discuss
some ongoing studies with Keck and HST which should shed more light on this
issue in the coming years.


title: Gravitational Radiation from Strongly Magnetized White Dwarfs

abstract:
  The magnetic fields of white dwarfs distort their shape generating an
anisotropic moment of inertia. A magnetized white dwarf which rotates obliquely
relative to the symmetry axis has a mass quadrupole moment which varies in
time, so it will emit gravitational radiation. LISA may be able to detect the
gravitational waves from two nearby, quickly rotating white dwarfs.


title:
Stochastic Acceleration and Non-Thermal Radiation in Clusters of
  Galaxies

abstract:
  We calculate the distribution of electrons in clusters of galaxies, resulting
from thermalization processes in the presence of stochastic acceleration due to
plasma waves. We show that the electron distribution can deviate from a
Maxwell-Boltzmann distribution, due to the effect of the stochastic energy
gain, provided waves can be sustained against damping. The non-thermal tail of
electrons can generate as bremsstrahlung emission a flux of hard X-rays
compatible with the ones recently detected in some clusters of galaxies.


title: The Pulse Scale Conjecture and the Case of BATSE Trigger 2193

abstract:
  The pulses that compose gamma-ray bursts (GRBs) are hypothesized to have the
same shape at all energies, differing only by scale factors in time and
amplitude. This "Pulse Scale Conjecture" is confirmed here between energy
channels of the dominant pulse in GRB 930214c (BATSE trigger 2193), the single
most fluent single-pulsed GRB that occurred before May 1998. Furthermore,
pulses are hypothesized to start at the same time independent of energy. This
"Pulse Start Conjecture" is also confirmed in GRB 930214c. Analysis of GRB
930214c also shows that, in general, higher energy channels show shorter
temporal scale factors. Over the energy range 100 KeV - 1 MeV, it is found that
the temporal scale factors between a pulse measured at different energies are
related to that energy by a power law, possibly indicating a simple
relativistic mechanism is at work. To test robustness, the Pulse Start and
Pulse Scale Conjectures were also tested on the four next most fluent
single-pulse GRBs. Three of the four clearly passed, with a second smaller
pulse possibly confounding the discrepant test. Models where the pulse rise and
decay are created by different phenomena do not typically predict pulses that
satisfy both the Pulse Start Conjecture and the Pulse Scale Conjecture, unless
both processes are seen to undergo common time dilation.


title:
HST Observations and Photoionization Modeling of the LINER Galaxy NGC
  1052

abstract:
  We present a study of available Hubble Space Telescope (HST) spectroscopic
and imaging observations of the low ionization nuclear emission line region
(LINER) galaxy NGC 1052. The WFPC2 imagery clearly differentiates extended
nebular Halpha emission from that of the compact core. Faint Object
Spectrograph (FOS) observations provide a full set of optical and UV data
(1200-6800 Angstroms).
  These spectral data sample the innermost region (0."86 x 0."86 ~ 82pc x 82pc)
and exclude the extended Halpha emission seen in the WFPC2 image. The derived
emission line fluxes allow a detailed analysis of the physical conditions
within the nucleus. The measured flux ratio for Halpha/Hbeta,
F{Halpha}/F{Hbeta}=4.53, indicates substantial intrinsic reddening,
E(B-V)=0.42, for the nuclear nebular emission. This is the first finding of a
large extinction of the nuclear emission line fluxes in NGC 1052. If the
central ionizing continuum is assumed to be attenuated by a comparable amount,
then the emission line fluxes can be reproduced well by a simple
photoionization model using a central power law continuum source with a
spectral index of alpha = -1.2 as deduced from the observed flux distribution.
A multi-density, dusty gas gives the best fit to the observed emission line
spectrum. Our calculations show that the small contribution from a highly
ionized gas observed in NGC 1052 can also be reproduced solely by
photoionization modeling. The high gas covering factor determined from our
model is consistent with the assumption that our line of sight to the central
engine is obscured.


title:
Halo Profiles and the Nonlinear Two- and Three-Point Correlation
  Functions of Cosmological Mass Density

abstract:
  We investigate the nonlinear two- and three-point correlation functions of
the cosmological density field in Fourier space and test the popular
hierarchical clustering model, that the three-point amplitude Q is independent
of scale. In high-resolution N-body simulations of both n=-2 scale-free and
cold dark matter models, we find that Q at late times is not constant but
increases with wavenumber far into the nonlinear regime. Self-similar scaling
also does not hold as rigorously for the three-point function as for the
two-point function in the n=-2 simulation, perhaps a manifestation of the
finite simulation volume. We suggest that a better understanding of the two-
and three-point correlation functions in the nonlinear regime lies in the link
to the density profiles of dark matter halos. We demonstrate and quantify how
the slopes of the correlation functions are affected by the slope of the halo
profiles using simple halo shapes and analytic clustering models.


title:
Non-Gaussianity and the recovery of the mass power spectrum from the
  Ly$\alpha$ forest

abstract:
  We investigate the effect of non-Gaussianity on the reconstruction of the
initial mass field from the Ly$\alpha$ forest. We show that the transmitted
flux of QSO absorption spectra are highly non-Gaussian in terms of the
statistics, the kurtosis spectrum and scale-scale correlation. These
non-Gaussianities can not be completely removed by the conventional algorithm
of Gaussianization, and the scale-scale correlations are largely retained in
the mass field recovered by the Gaussian mapping. Therefore, the mass power
spectrum recovered by the conventional algorithm is systematically lower than
the initial mass spectrum on scales at which the local scale-scale correlation
is substantial. To reduce the non-Gaussian contamination, we present two
methods. The first is to perform the Gaussianization scale-by-scale using the
discrete wavelet transform (DWT) decomposition. We show that the non-Gaussian
features of the Ly$\alpha$ forest basically will no longer exist in the
scale-by-scale Gaussianized mass field. The second method is to choose a proper
orthonormal basis (representation) to suppress the effect of the non-Gaussian
correlations. In the quasilinear regime of cosmic structure formation, the DWT
power spectrum is efficient for suppressing the non-Gaussian contamination.
These two methods significantly improve the recovery of the mass power spectrum
from the Ly$\alpha$ forest.


title:
Infrared Imaging of z=2.43 Radio Galaxy B3 0731+438 with the Subaru
  Telescope - Detection of H$\alpha$ Ionization Cones of a Powerful Radio
  Galaxy

abstract:
  We report on infrared imaging observations of the z=2.429 radio galaxy B3
0731+438 with the Subaru telescope. The images were taken with the K'-band
filter and the 2.25 um narrow-band filter to examine the structure and
properties of the Ha+[N II] 6548,6583 emission-line components. The Ha+[N II]
emission-line image shows biconical lobes with an extent of 40 kpc, which are
aligned with the radio axis. The rest-frame equivalent widths of the emission
lines at these cones are as large as 1100 AA, and can be well explained by a
gas-cloud model photoionized by power-law continuum radiation. The isotropic
ionizing photon luminosity necessary to ionize the hydrogen gas in these cones
amounts to 1e57(photons/s), which is larger than that in the majority of
radio-loud QSOs. From these results, we propose that the Ha alignment effect in
this object is produced by biconical gas clouds, which are swept up by the
passage of radio jets, and are ionized by strong UV radiation from a hidden
AGN. The continuum image consists of two components, a stellar-like point
source and an extended diffuse galaxy. These are supposed to be a type-2 AGN
and its host galaxy. The SED is fitted by a combination of spectra of a
reddened dust-scattered AGN and an instantaneous starburst population of 500
Myr old. The stellar mass of the galaxy is estimated to be 3e11 M_solar, which
is as large as that of typical 3C radio galaxies at z=1.


title: Gas Streams and Spiral Structure in the Milky Way

abstract:
  The observed gas dynamics in the Milky Way can only be explained by a bar in
the galactic center. Such a bar is directly visible in the near-IR maps of the
bulge, where it causes a distinctive asymmetric light distribution pattern.
Another large-scale structure is the grand-design 4-arm spiral pattern, most
clearly observed in the spatial distribution of molecular gas and HII-regions.
  In order to model the observed gas flow structure, we constructed a model for
the stellar mass distribution. For the inner 5 $\kpc$ we used the 3D
deprojected near-IR light distribution, as observed by the COBE/DIRBE
experiment, and added an analytical disk model outside the box as well as a
halo model. With this frozen mass distribution, we computed the stationary gas
flow for various deprojection parameters and pattern speeds. For all reasonable
parameter choices, we obtain a 4-armed spiral pattern, which can be matched to
the observed spiral arms.
  In the bar region, our model can explain the non-circular motion visible in
the terminal velocity curve as well as part of the forbidden velocities. Inside
the corotation, we also find 4 spiral arms, the nearest arm corresponds to the
3-kpc-arm, although only qualitatively. The missing southern 3-kpc-arm at the
far end of the galaxy is explained by running parallel to another arm. Close to
the center, we find gas on circular orbits forming a disk. Such a disk has been
observed in emission of the CS molecule, however only part of the disk appears
to be occupied by dense enough gas to be traced by CS. Further we compare our
model to the distribution of OH/IR stars in the inner galaxy.


title: Delta Scuti Stars in Stellar Systems: a Review

abstract:
  We present a list of delta Scuti stars in double and multiple systems,
ranging from the very wide binaries to the very close ones such as
spectroscopic and eclipsing systems including the optical visual pairs which
are of no further use here. Our aim is to group the information from the
binarity on the one hand and the pulsational characteristics on the other hand
for as complete a sample as possible of delta Scuti stars in stellar systems. A
selection of 18 well-documented cases, taking care that every type of binary is
being represented, is discussed more extensively.


title: Evolution of the Magnetic Field in Accreting Neutron Stars

abstract:
  There has been sufficient observational indication suggesting a causal
connection between the binary history of neutron stars and the evolution of
their magnetic field. In particular, it is believed that the generation of the
low-field millisecond pulsars is a consequence of the processing of normal
high-field neutron stars in binary systems. We try to understand the mechanism
of field evolution in neutron stars that are members of binary systems with an
aim to understand the problem of millisecond pulsar generation. To this end we
have looked at four related problems : i. the effect of diamagnetic screening
on the final field of a neutron star accreting material from its binary
companion; ii. evolution of magnetic flux located in the crust of an accreting
neutron star; iii. application of the above-mentioned model to real systems and
a comparison with observations; iv. an investigation into the consequences of
magnetic flux being initially located in the core of the star and its
observational implications.


title: On the Viewing Angle Dependence of Blazar Variability

abstract:
  Internal shocks propagating through an ambient radiation field, are subject
to a radiative drag that, under certain conditions, can significantly affect
their dynamics and, consequently, the evolution of the beaming cone of emission
produced behind the shocks. The resultant change of the Doppler factor combined
with opacity effects leads to a strong dependence of the variability pattern
produced by such systems, specifically, the shape of the light curves and the
characteristics of correlated emission, on viewing angle. One implication is
that objects oriented at relatively large viewing angles to the observer should
exhibit a higher level of activity at high synchrotron frequencies (above the
self-absorption frequency) and at gamma-ray energies below the threshold energy
to pair production, than at lower (radio/millimeter) frequencies.


title:
New Constraints from High Redshift Supernovae and Lensing Statistics
  upon Scalar Field Cosmologies

abstract:
  We explore the implications of gravitationally lensed QSOs and high-redshift
SNe Ia observations for spatially flat cosmological models in which a
classically evolving scalar field currently dominates the energy density of the
Universe. We consider two representative scalar field potentials that give rise
to effective decaying $\Lambda $ (``quintessence'') models:
pseudo-Nambu-Goldstone bosons ($V(\phi)=M^4(1+\cos (\phi /f)) $) and an inverse
power-law potential ($V(\phi)=M^{4+\alpha}\phi ^{-\alpha}$). We show that a
large region of parameter space is consistent with current data if $\Omega_{m0}
> 0.15$. On the other hand, a higher lower bound for the matter density
parameter suggested by large-scale galaxy flows, $\Omega_{m0} > 0.3$,
considerably reduces the allowed parameter space, forcing the scalar field
behavior to approach that of a cosmological constant.


title: Pulse shape and spectrum of PSR J0218+4232 observed with BeppoSAX

abstract:
  We present the results of a BeppoSAX observation of PSR J0218+4232 which, for
the first time, provides detailed information on the pulsar's temporal and
spectral emission properties in the broad band 1-10 keV. We detected X-ray
pulses with a pulsed fraction of 73 +/- 12 %. The pulse profile is
characterized by two peaks phase separated by 0.47 +/-0.05. The pulsed spectrum
is best described by a power-law of photon index 0.61 +/- 0.32 with an
unabsorbed (2-10 keV) X-ray flux of 4.1E-13 erg/cm^2/s implying a luminosity of
Lx = 1.3E32 Theta (d/5.7 kpc})^2 erg/s and an X-ray efficiency of 4.8E-4 Theta
(d/5.7 kpc})^2 where Theta is the solid angle spanned by the emission beam.


title:
Weak Lensing by High-Redshift Clusters of Galaxies - I: Cluster Mass
  Reconstruction

abstract:
  We present the results of a weak lensing survey of six high-redshift (z >
0.5), X-ray selected clusters of galaxies. We have obtained ultra-deep R-band
images of each cluster with the Keck Telescope, and have measured a weak
lensing signal from each cluster. From the background galaxy ellipticities we
create two-dimensional maps of the surface mass density of each cluster. We
find that the substructure seen in the mass reconstructions typically agree
well with substructure in both the cluster galaxy distributions and X-ray
images of the clusters. We also measure the one-dimensional radial profiles of
the lensing signals and fit these with both isothermal spheres and "universal"
CDM profiles. We find that the more massive clusters are less compact and not
as well fit by isothermal spheres as the less massive clusters, possibly
indicating that they are still in the process of collapse.


title: BVRcIc photometry of GRB 980703 and GRB 990123 host galaxies

abstract:
  We present a photometry of GRB 980703 and GRB 990123 host galaxies which was
performed about 20 days and a half year after gamma-ray bursts occured,
respectively. The contributions of the optical transiets (OT) were negligible
in both cases. We derived broad band BVR_cI_c spectra of the host galaxies and
compared them to continuum spectra of different Hubble-type galaxies and
averaged spectra of starburst galaxies. For H_0=60 km s^{-1} Mpc^{-1} and three
Friedmann cosmological models with matter density and cosmological constant
parameters (Omega_m,Omega_Lambda) = (1,0),(0,0),(0,1) we estimated M_Brest and
star-forming rates (SFRs) using the fluxes in photometric bands for the host
galaxies. Within the range of cosmological parameters our estimates of the
absolute magnitudes are: M_Brest = -20.60 .. -21.73 for the GRB 980703 host
galaxy and M_Brest = -20.20 .. -21.82 for the GRB 990123 host galaxy. We
obtained estimates of K-correction values and absolute magnitudes of the host
galaxies using SEDs for star-forming galaxies.


title: Modeling the Photoionized Interface in Blister HII Regions

abstract:
  We present a grid of photoionization models for the emission from
photoevaporative interfaces between the ionized gas and molecular cloud in
blister HII regions. For the density profiles of the emitting gas in the
models, we use a general power law form calculated for photoionized,
photoevaporative flows by Bertoldi (1989). We find that the spatial emission
line profiles are dependent on the incident flux, the shape of the ionizing
continuum and the elemental abundances. In particular, we find that the peak
emissivity of the [SII] and [NII] lines are more sensitive to the elemental
abundances than are the total line intensities. The diagnostics obtained from
the grid of models can be used in conjunction with high spatial resolution data
to infer the properties of ionized interfaces in blister HII regions. As an
example, we consider a location at the tip of an ``elephant trunk'' structure
in M16 (the Eagle Nebula) and show how narrow band HST-WFPC2 images constrain
the HII region properties. We present a photoionization model that explains the
ionization structure and emission from the interface seen in these high spatial
resolution data.


title:
Gravitational microlensing source limb-darkening and
  limb-polarization,~I: angle-averaged amplification functions

abstract:
  There is increasing interest in extended source effects in microlensing
events, as probes of the unresolved sources. Previous work has either presumed
a uniform source, or else required an approximate or numerical treatment of the
amplification function averaged over the source disk. In this paper, I present
analytic expressions for the angle-averaged amplification functions for the
rotationally-symmetric intensity and polarization cases.
  These integrals will allow us to use the technology of inverse problems to
study the source limb-darkening and limb-polarization functions.


title: X-ray Surveys of the obscured Universe

abstract:
  Deep X-ray surveys have shown that the cosmic X-ray background (XRB) is
largely due to accretion onto supermassive black holes, integrated over cosmic
time. However, the characteristic hard spectrum of the XRB can only be
explained if most AGN spectra are heavily absorbed. The absorbed AGN will
suffer severe extinction and therefore, unlike classical QSOs, will not be
prominent at optical wavelengths. Most of the accretion power is being absorbed
by gas and dust and will have to be reradiated in the FIR/sub-mm band. AGN
could therefore contribute a substantial fraction to the recently discovered
cosmic FIR/sub-mm background. Here it is shown that a number of high-redshift
absorbed X-ray sources selected in the ROSAT Deep Survey of the Lockman Hole
have broad-band spectral energy distributions very similar to the local ULIRG
NGC6240, lending additional support to the background models for the obscured
universe.


title:
Best Unbiased Estimators for the Three-Point Correlators of the Cosmic
  Microwave Background Radiation

abstract:
  Measuring the three-point correlators of the Cosmic Microwave Background
(CMB) anisotropies could help to get a handle on the level of non-Gaussianity
present in the observational datasets and therefore would strongly constrain
models of the early Universe. However, typically, the expected non-Gaussian
signal is very small. Therefore, one has to face the problem of extracting it
from the noise, in particular from the `cosmic variance' noise. For this
purpose, one has to construct the best unbiased estimators for the three-point
correlators that are needed for concrete detections of non-Gaussian features.
In this article, we study this problem for both the CMB third moment and the
CMB angular bispectrum. We emphasize that the knowledge of the best estimator
for the former does not permit one to infer the best estimator for the latter
and vice versa. We present the corresponding best unbiased estimators in both
cases and compute their corresponding cosmic variances.


title: Mirror Dark Matter and Core Density of Galaxies

abstract:
  We present a particle physics realization of a recent suggestion by Spergel
and Steinhardt that collisional but dissipationless dark matter may resolve the
core density problem in dark matter-dominated galaxies such as the dwarf
galaxies. The realization is the asymmetric mirror universe model introduced to
explain the neutrino puzzles and the microlensing anomaly. The mirror baryons
are the dark matter particles with the desired properties. The time scales are
right for resolution of the core density problem and formation of mirror stars
(MACHOs observed in microlensing experiments). The mass of the region
homogenized by Silk damping is between a dwarf and a large galaxy.


title: Vorticity Budget of Weak Thermal Convection in Keplerian disks

abstract:
  By employing the equations of mean-square vorticity (enstrophy) fluctuations
in strong shear flows, we demonstrate that unlike energy production of
turbulent vorticity in nonrotating shear flows, the turbulent vorticity of weak
convection in Keplerian disks cannot gain energy from vortex stretching/tilting
by background shear unless the asscoiated Reynolds stresses are negative. This
is because the epicyclic motion is an energy sink of the radial component of
mean-square turbulent vorticity in Keplerian disks when Reynolds stresses are
positive. Consequently, weak convection cannot be self-sustained in Keplerian
flows. This agrees with the results implied from the equations of mean-square
velocity fluctuations in strong shear flows. Our analysis also sheds light on
the explanation of the simulation result in which positive kinetic helicity is
produced by the Balbus-Hawley instability in a vertically stratified Keplerian
disk. We also comment on the possibility of outward angular momentum transport
by strong convection based on azimuthal pressure perturbations and directions
of energy cascade.


title:
New Techniques for Relating Dynamically Close Galaxy Pairs to Merger and
  Accretion Rates : Application to the SSRS2 Redshift Survey

abstract:
  We introduce two new pair statistics, which relate close galaxy pairs to the
merger and accretion rates. We demonstrate the importance of correcting these
(and other) pair statistics for selection effects related to sample depth and
completeness. In particular, we highlight the severe bias that can result from
the use of a flux-limited survey. The first statistic, denoted N_c, gives the
number of companions per galaxy, within a specified range in absolute
magnitude. N_c is directly related to the galaxy merger rate. The second
statistic, called L_c, gives the total luminosity in companions, per galaxy.
This quantity can be used to investigate the mass accretion rate. Both N_c and
L_c are related to the galaxy correlation function and luminosity function in a
straightforward manner. We outline techniques which account for various
selection effects, and demonstrate the success of this approach using Monte
Carlo simulations. If one assumes that clustering is independent of luminosity
(which is appropriate for reasonable ranges in luminosity), then these
statistics may be applied to flux-limited surveys.
  These techniques are applied to a sample of 5426 galaxies in the SSRS2
redshift survey. Using close dynamical pairs, we find N_c(-21<M_B<-18) =
0.0226+/-0.0052 and L_c(-21<M_B<-18) = 0.0216+/-0.0055 10^{10} h^2 L_sun at
z=0.015. These are the first secure estimates of low-z close pair statistics.
If N_c remains fixed with redshift, simple assumptions imply that ~ 6.6% of
present day galaxies with -21<M_B<-18 have undergone mergers since z=1. When
applied to redshift surveys of more distant galaxies, these techniques will
yield the first robust estimates of evolution in the galaxy merger and
accretion rates. [Abridged]


title: Starquake-Induced Glitches in Pulsars

abstract:
  The neutron star crust is rigid material floating on a neutron-proton liquid
core. As the star's spin rate slows, the changing stellar shape stresses the
crust and causes fractures. These starquakes may trigger pulsar glitches as
well as the jumps in spin-down rate that are observed to persist after some
glitches. Earlier studies found that starquakes in spinning-down neutron stars
push matter toward the magnetic poles, causing temporary misalignment of the
star's spin and angular momentum. After the star relaxes to a new equilibrium
orientation, the magnetic poles are closer to the equator, and the magnetic
braking torque is increased. The magnitude and sign of the predicted torque
changes are in agreement with the observed persistent spin-down offsets. Here
we examine the relaxation processes by which the new equilibrium orientation is
reached. We find that the neutron superfluid in the inner crust slows as the
star's spin realigns with the angular momentum, causing the crust to spin more
rapidly. For plausible parameters the time scale and the magnitude of the
crust's spin up agree with the giant glitches in the Vela and other pulsars.


title:
The abundance of high-redshift objects as a probe of non-Gaussian
  initial conditions

abstract:
  The observed abundance of high-redshift galaxies and clusters contains
precious information about the properties of the initial perturbations. We
present a method to compute analytically the number density of objects as a
function of mass and redshift for a range of physically motivated non-Gaussian
models. In these models the non-Gaussianity can be dialed from zero and is
assumed to be small. We compute the probability density function for the
smoothed dark matter density field and we extend the Press and Schechter
approach to mildly non-Gaussian density fields. The abundance of high-redshift
objects can be directly related to the non-Gaussianity parameter and thus to
the physical processes that generated deviations from the Gaussian behaviour.
Even a skewness parameter of order 0.1 implies a dramatic change in the
predicted abundance of $z\gap 1$ objects. Observations from NGST and X-ray
satellites (XMM) can be used to accurately measure the amount of
non-Gaussianity in the primordial density field.


title: Thermal X-Ray Pulses Resulting From Pulsar Glitches

abstract:
  The non-spherically symmetric transport equations and exact thermal evolution
model are used to calculate the transient thermal response to pulsars. The
three possible ways of energy release originated from glitches, namely the
`shell', `ring' and `spot' cases are compared. The X-ray light curves resulting
from the thermal response to the glitches are calculated. Only the `spot' case
and the `ring' case are considered because the `shell' case does not produce
significant modulative X-rays. The magnetic field ($\vec B$) effect, the
relativistic light bending effect and the rotational effect on the photons
being emitted in a finite region are considered. Various sets of parameters
result in different evolution patterns of light curves. We find that this
modulated thermal X-ray radiation resulting from glitches may provide some
useful constraints on glitch models.


title: Nanoarcsecond single-dish imaging of the Vela pulsar

abstract:
  We have measured the properties of the diffractive scintillation toward the
Vela pulsar under the extremely strong scattering conditions encountered at 660
MHz. We obtain a decorrelation bandwidth of $\nu_d = 244 \pm 4$ Hz and
diffractive decorrelation timescale of $t_{\rm diff} = 3.3\pm 0.3$ s. Our
measurement of the modulation indices $m=0.87\pm 0.003\pm 0.05$ and $m=0.93\pm
0.03 \pm 0.05$ (one for each polarization stream), are at variance with the
modulation index of the Vela pulsar obtained at 2.3 GHz by Gwinn et al. (1997)
{\it if} the deviation from a modulation index of unity is ascribed to a source
size effect.


title:
On the Determination of Star Formation Rates in Evolving Galaxy
  Populations

abstract:
  The redshift dependence of the luminosity density in certain wavebands (e.g.
UV and H-alpha) can be used to infer the history of star formation in the
populations of galaxies producing this luminosity. This history is a useful
datum in studies of galaxy evolution. It is therefore important to understand
the errors that attend the inference of star formation rate densities from
luminosity densities. This paper explores the self-consistency of star
formation rate diagnostics by reproducing commonly used observational
procedures in a model with known galaxy populations, evolutionary histories and
spectral emission properties. The study reveals a number of potential sources
of error in the diagnostic processes arising from the differential evolution of
different galaxy types. We argue that multi-wavelength observations can help to
reduce these errors.


title: A Third Star in the T Tauri System

abstract:
  New speckle-holographic images of the T Tauri Infrared Companion (T Tauri
IRC; T Tauri S) reveal it to be a double system with a sky-projected separation
of 0".05, corresponding to a linear distance of 7 AU. The presence of this
third star may account for the relative paucity of dust surrounding the IRC.


title: A direct view of the AGN powering IRAS12393+3520

abstract:
  We report the first direct X-ray evidence that an AGN is hidden in the center
of IRAS12393+3520. An ASCA observation of this target unveiled a bright (0.5-10
keV luminosity 3.9 x 10^42 erg/s) and variable source, with minimum observed
doubling/halving time scale comprised in the range 30-75 ks. A model composed
by a simple power-law, with photon index ~1.8 and an absorption edge, whose
threshold energy is consistent with K-shell photoionization of OVII, provides
an adequate fit of the spectrum. This suggests that we are observing the
emission from the nuclear region through a warm absorber of N_H a few
10^{21}/cm/cm. If it has internal dust with Galactic gas-to-dust ratio, it
could explain the lack of broad Hbeta emission, even in the episodic presence
of a broad Halpha emission line. Optical spectra obtained over several years
show indeed variations in the strength of this broad Halpha component. A
distribution of dusty, optically thick matter on spatial scales a few hundreds
parsec, which does not intercept the line of sight towards the nucleus, is
probably required to account simultaneously for the relative [OIII] luminosity
deficit in comparison to the X-rays. The high IR to X-ray luminosity ratio is
most likely due to intense star formation in the circumnuclear region.
IRAS12393+3520 might thus exhibit simultaneously nuclear activity and
remarkable star formation.


title: Asteroseismology and Oblique Pulsator Model of Beta Cephei

abstract:
  We discuss the oscillation features of Beta Cephei, which is a magnetic star
in which the magnetic axis seems to be oblique to the rotation axis. We
interpret the observed equi-distant fine structures of the frequency spectrum
as a manifestation of a magnetic perturbation of an eigenmode, which would be a
radial mode in the absence of the magnetic field. Besides these frequency
components, we interpret another peak in the frequency spectrum as an
independent quadrupole mode. By this mode identification, we deduce the mass,
the evolutionary stage, the rotational frequency, the magnetic field strength,
and the geometrical configuration of Beta Cephei.


title: A near infrared photometric plane for ellipticals and bulges of spirals

abstract:
  We report the existence of a single plane in the space of global photometric
parameters describing elliptical galaxies and the bulges of early type spiral
galaxies. The three parameters which define the plane are obtained by fitting
the Sersic form to the brightness distribution obtained from near-infrared K
band images. We find, from the range covered by their shape parameters, that
the elliptical galaxies form a more homogeneous population than the bulges.
Known correlations like the Kormendy relation are projections of the
photometric plane. The existence of the plane has interesting implications for
bulge formation models.


title: Deductive-reductive determination of the model of our observed Universe

abstract:
  According to the observations, in our expansive and isotropic relativistic
Universe for the gravitational phenomena in a Newtonian approximation the
Newtonian non-modified relations are valid. The Friedmann general equations of
isotropic and homogeneous universe dynamics describe an infinite number of
models of expansive and isotropic relativistic universe in the Newtonian
approximation, but only in one of them the Newtonian non-modified relations are
valid. These facts give - till now not considered - possibility for unambiguous
deductive-reductive determination of the Friedmannian model, which describes
our observed Universe.


title:
The X-ray properties of luminous infrared galaxies and their
  contribution to the X-ray background

abstract:
  We present a study of a sample of luminous infrared galaxies (LIGs, L_IR >
10^11 L_sun) observed in the hard (2-10 keV) X rays. The main results are: 1)
most LIGs are powered both by AGN and starburst activity; 2) the AGNs in our
sample are absorbed in the infrared by a lower N_H than in the X rays or,
alternatively, the dust-to-gas ratio is lower than galactic; 3) the study of a
subsample of sources observed in the 20-200 keV band indicates that most of the
AGNs hosted by the LIGs are heavily obscured up to 100 keV and, therefore,
their contribution to the X-ray background must be small.


title: Evolution of the cluster X-ray luminosity function from the WARPS survey

abstract:
  The evolution of the X-ray luminosity function of clusters of galaxies has
been measured to z=0.85 using over 150 X-ray selected clusters found in the
WARPS survey. We find no evidence for evolution of the luminosity function at
any luminosity or redshift. The observations constrain the evolution of the
space density of moderate luminosity clusters to be very small, and much less
than predicted by most models of the growth of structure with Omega=1.


title:
Diffusive Shock Acceleration of Electrons and Radio Emission from Large
  Diameter Shell-Type Supernova Remnants

abstract:
  In present study I examine the capability of diffusive shock acceleration
mechanism to explain existing data on radio emission from evolved large
diameter shell-type adiabatic supernova remnants (SNRs). Time-dependent
''onion-shell'' model for the radio emission of SNRs is developed, which is
based on the assumptions: a) acceleration takes place from thermal energies and
test-particle approximation is valid; b) the problem of injection is avoided by
introducing, like Bell (1978), two injection parameters; c) to take into
consideration very late stages of SNR evolution the analytic approximation of
Cox and Andersen (1982) for the shell structure is used; c)no radiative
cooling. Constructed Surface Brightness - Diameter $(\Sigma -D)$ tracks are
compared with the empirical $\Sigma -D$ diagram. The main conclusion of the
study is that the DSA mechanism is capable of explaining all the statistics of
radio SNRs including very large diameter remnants and giant galactic loops.


title: Ongoing Large Surveys for Metal-Poor Stars in the Galactic Halo

abstract:
  We report on two major surveys for metal-poor stars in the galactic halo, the
HK survey, and the Hamburg/ESO survey, which have been undertaken in order to
provide targets for high-resolution spectroscopy with the Subaru HDS and other
large telescopes. We compare basic properties of these two surveys and their
current status, and add some historical remarks.
  The candidate selection procedures of both surveys are described in detail.
We evaluate the candidate selection by comparing effective yields (EYs) of the
survey techniques for the identification of metal-poor stars. It is found that
EY for stars below [Fe/H]=-2.0 in the HES can be up to 80% for stars selected
by automatic classification from machine-scanned unwidened plates, whereas in
the HK survey, where stars are selected by visual inspection of widened survey
plates, the EY is between 11% and 32%, depending on whether a pre-selection
based on BV photometry has been applied.
  Finally, we describe techniques used for determining stellar parameters of
the survey stars by means of moderate resolution follow-up spectroscopy, and
additional UBV photometry. While follow-up observations of HES stars have just
been started, the HK survey has already produced a list of ~4700 stars with
estimates of [Fe/H] typically precise to +/-0.2 dex, some 1000 of which have
[Fe/H]<-2.0, and roughly 100 of which have [Fe/H]<-3.0.


title:
One-Point Probability Distribution Functions of Supersonic Turbulent
  Flows in Self-Gravitating Media

abstract:
  Turbulence is essential for understanding the structure and dynamics of
molecular clouds and star-forming regions. There is a need for adequate tools
to describe and characterize the properties of turbulent flows. One-point
probability distribution functions (pdf's) of dynamical variables have been
suggested as appropriate statistical measures and applied to several observed
molecular clouds. However, the interpretation of these data requires comparison
with numerical simulations. To address this issue, SPH simulations of driven
and decaying, supersonic, turbulent flows with and without self-gravity are
presented. In addition, random Gaussian velocity fields are analyzed to
estimate the influence of variance effects. To characterize the flow
properties, the pdf's of the density, of the line-of-sight velocity centroids,
and of the line centroid increments are studied. This is supplemented by a
discussion of the dispersion and the kurtosis of the increment pdf's, as well
as the spatial distribution of velocity increments for small spatial lags. From
the comparison between different models of interstellar turbulence, it follows
that the inclusion of self-gravity leads to better agreement with the observed
pdf's in molecular clouds. The increment pdf's for small spatial lags become
exponential for all considered velocities. However, all the processes
considered here lead to non-Gaussian signatures, differences are only gradual,
and the analyzed pdf's are in addition projection dependent. It appears
therefore very difficult to distinguish between different physical processes on
the basis of pdf's only, which limits their applicability for adequately
characterizing interstellar turbulence.


title: Design and expected performance of the ANTARES neutrino telescope

abstract:
  The ANTARES Collaboration is aiming at the construction and the operation of
a large undersea neutrino telescope for neutrino astronomy, neutrino
oscillation and indirect dark matter searches. Started 3 years ago an intensive
R&D program has shown the feasibility of such a detector in the deep waters of
the Mediterranean sea. We have now started the design and the construction of a
0.1 km(^{2}) detector for which the expected performance will be briefly
described here.


title: Unsupervised Induction and Gamma-Ray Burst Classification

abstract:
  We use ESX, a product of Information Acumen Corporation, to perform
unsupervised learning on a data set containing 797 gamma-ray bursts taken from
the BATSE 3B catalog. Assuming all attributes to be distributed logNormally,
Mukherjee et al. (1998) analyzed these same data using a statistical cluster
analysis. Utilizing the logarithmic values for T90 duration, total fluence, and
hardness ratio HR321 their results showed the instances formed three classes.
Class I contained long/bright/intermediate bursts, class II consisted of
short/faint/hard bursts and class III was represented by
intermediate/intermediate/soft bursts.
  When ESX was presented with these data and restricted to forming a small
number of classes, the two classes found by previous standard techniques were
determined. However, when ESX was allowed to form more than two classes, four
classes were created. One of the four classes contained a majority of short
bursts, a second class consisted of mostly intermediate bursts, and the final
two classes were subsets of the Class I (long) bursts determined by Mukherjee
et al. We hypothesize that systematic biases may be responsible for this
variation.


title:
Chemical enrichment and star formation in the Milky Way disk; I. Sample
  description and chromospheric age-metallicity relation

abstract:
  The age-metallicity relation of the solar neighbourhood is studied using a
sample of 552 late-type dwarfs. This sample was built from the intersection of
photometric catalogues with chromospheric activity surveys of the Mount Wilson
group. For these stars, metallicities were estimated from uvby data, and ages
were calculated from their chromospheric emission levels using a new
metallicity-dependent chromospheric activity-age relation developed by
Rocha-Pinto & Maciel (1998). A careful estimate of the errors in the
chromospheric age is made. The errors in the chromospheric indices are shown to
include partially the effects of the stellar magnetic cycles, although a
detailed treatment of this error is still beyond our knowledge. It is shown
that the results are not affected by the presence of unresolved binaries in the
sample. We derive an age-metallicity relation which confirms the mean trend
found by previous workers. The mean metallicity shows a slow, steady increase
with time, amounting at least 0.56 dex in 15 Gyr. The initial metallicity of
the disk is around -0.70 dex, in agreement with the G dwarf metallicity
distribution. According to our data, the intrinsic cosmic dispersion in metal
abundances is around 0.13 dex, a factor of two smaller than that found by
Edvardsson et al. (1993). We show that chromospheric ages are compatible with
isochrone ages, within the expected errors, so that the difference in the
scatter cannot be caused by the accuracy of our ages and metallicities. This
reinforces some suggestions that the Edvarsson et al.'s sample is not suitable
to the determination of the age-metallicity relation.


title:
Chemical enrichment and star formation in the Milky Way disk; II. Star
  formation history

abstract:
  A chromospheric age distribution of 552 late-type dwarfs is transformed into
a star formation history by the application of scale height corrections,
stellar evolutionary corrections and volume corrections. We show that the disk
of our Galaxy has experienced enhanced episodes of star formation at 0-1 Gyr,
2-5 Gyr and 7-9 Gyr ago, although the reality of the latter burst is still
uncertain. The star sample birthsites are distributed over a very large range
of distances because of orbital diffusion, and so give an estimate of the
global star formation rate. These results are compared with the
metal-enrichment rate, given by the age-metallicity relation, with the expected
epochs of close encounters between our Galaxy and the Magellanic Clouds, and
with previous determinations of the star formation history. Simulations are
used to examine the age-dependent smearing of the star formation history due to
age uncertainties, and the broadening of the recovered features, as well as to
measure the probability level that the history derived to be produced by
statistical fluctuations of a constant star formation history. We show with
more than 98% level of confidence that the the star formation of our Galaxy was
not constant.


title:
Tests of the Accelerating Universe with Near-Infrared Observations of a
  High-Redshift Type Ia Supernova

abstract:
  We have measured the rest-frame B,V, and I-band light curves of a
high-redshift type Ia supernova (SN Ia), SN 1999Q (z=0.46), using HST and
ground-based near-infrared detectors.
  A goal of this study is the measurement of the color excess, E_{B-I}, which
is a sensitive indicator of interstellar or intergalactic dust which could
affect recent cosmological measurements from high-redshift SNe Ia. Our
observations disfavor a 30% opacity of SN Ia visual light by dust as an
alternative to an accelerating Universe. This statement applies to both
Galactic-type dust
  (rejected at the 3.4 sigma confidence level) and greyer dust (grain size >
0.1 microns; rejected at the 2.3 to 2.6 sigma confidence level) as proposed by
Aguirre (1999). The rest-frame $I$-band light cur ve shows the secondary
maximum a month after B maximum typical of nearby SNe Ia of normal luminosi ty,
providing no indication of evolution as a function of redshift out to z~0.5. A
n expanded set of similar observations could improve the constraints on any
contribution of extragalactic dust to the dimming of high-redshift SNe Ia.


title: Do wavelets really detect non-Gaussianity in the 4-year COBE data?

abstract:
  We investigate the detection of non-Gaussianity in the 4-year COBE data
reported by Pando, Valls-Gabaud & Fang (1998), using a technique based on the
discrete wavelet transform. Their analysis was performed on the two DMR faces
centred on the North and South Galactic poles respectively, using the
Daubechies 4 wavelet basis. We show that these results depend critically on the
orientation of the data, and so should be treated with caution. For two
distinct orientations of the data, we calculate unbiased estimates of the
skewness, kurtosis and scale-scale correlation of the corresponding wavelet
coefficients in all of the available scale domains of the transform. We obtain
several detections of non-Gaussianity in the DMR-DSMB map at greater than the
99 per cent confidence level, but most of these occur on pixel-pixel scales and
are therefore not cosmological in origin. Indeed, after removing all multipoles
beyond $\ell = 40$ from the COBE maps, only one robust detection remains.
Moreover, using Monte-Carlo simulations, we find that the probability of
obtaining such a detection by chance is 0.59. We repeat the analysis for the
53+90 GHz coadded COBE map. In this case, after removing $\ell > 40$
multipoles, two non-Gaussian detections at the 99 per cent level remain.
Nevertheless, again using Monte-Carlo simulations, we find that the probability
of obtaining two such detections by chance is 0.28. Thus, we conclude the
wavelet technique does {\em not} yield strong evidence for non-Gaussianity of
cosmological origin in the 4-year COBE data.


title:
Atomic diffusion in metal poor stars. The influence on the Main Sequence
  fitting distance scale, subdwarfs ages and the value of Delta Y/DeltaZ

abstract:
  The effect of atomic diffusion on the Main Sequence (MS) of metal-poor low
mass stars is investigated. Since diffusion alters the stellar surface chemical
abundances with respect to their initial values, one must ensure - by
calibrating the initial chemical composition of the theoretical models - that
the surface abundances of the models match the observed ones of the stellar
population under scrutiny. Since the observed surface abundances of subdwarfs
are different from the initial ones due to the effect of diffusion, while the
globular clusters stellar abundances are measured in Red Giants, which have
practically recovered their initial abundances after the dredge-up, the
isochrones to be employed for studying globular clusters and Halo subdwarfs
with the same observational value of [Fe/H] are different and do not coincide.
We find, however,that the current MS-fitting distances derived from HIPPARCOS
subdwarfs using colour corrections from standard isochrones are basically
unaltered when diffusion is taken properly into account; on the other hand, the
absolute ages, the age dispersion, the age-metallicity relation for Halo
subdwarfs, as well as the value of the helium enrichment ratio Delta Y/Delta Z
obtained from the width of the empirical Halo subdwarfs MS, are all
significantly modified when the properly calibrated isochrones with diffusion
are used.


title:
Interferometric Astrometry of the Detached White Dwarf - M Dwarf Binary
  Feige 24 Using Hubble Space Telescope Fine Guidance Sensor 3: White Dwarf
  Radius and Component Mass Estimates

abstract:
  With HST FGS 3 we have determined a parallax for the white dwarf - M dwarf
interacting binary, Feige 24. The white dwarf (DA) component has an effective
temperature, T_eff of approximately 56,000 K degrees. A weighted average with
past parallax determinations (pi_abs = 14.6 +- 0.4 milliseconds of arc) narrows
the range of possible radius values, compared to past estimates. We obtain R_DA
= 0.0185 +- 0.0008 R_solar with uncertainty in the temperature and bolometric
correction the dominant contributors to the error. FGS photometry provides a
light curve entirely consistent with reflection effects. A recently refined
model Mass-Luminosity Relation (Baraffe et al. 1998) for low mass stars
provides a mass estimate for the M dwarf companion, M_dM = 0.37 +- 0.20
M_solar, where the mass range is due to metallicity and age uncertainties.
Radial velocities from Vennes and Thorstensen (1994) provide a mass ratio from
which we obtain M_DA = 0.49+0.19-0.05 ~ M_solar. Independently, our radius and
recent log_g determinations yield 0.44 < M_DA < 0.47 M_solar. In each case the
minimum DA mass is that derived by Vennes & Thorstensen from their radial
velocities and Keplerian circular orbits with inclination <= 90 degrees.
Locating Feige 24 on an M - R plane suggests a carbon core. Our radius and
these mass estimates yield a gamma_grav inconsistent with that derived by
Vennes & Thorstensen. We speculate on the nature of a third component whose
existence would resolve the discrepancy.


title: ROTSE All Sky Surveys for Variable Stars I: Test Fields

abstract:
  The ROTSE-I experiment has generated CCD photometry for the entire Northern
sky in two epochs nightly since March 1998. These sky patrol data are a
powerful resource for studies of astrophysical transients. As a demonstration
project, we present first results of a search for periodic variable stars
derived from ROTSE-I observations. Variable identification, period
determination, and type classification are conducted via automatic algorithms.
In a set of nine ROTSE-I sky patrol fields covering about 2000 square degrees
we identify 1781 periodic variable stars with mean magnitudes between m_v=10.0
and m_v=15.5. About 90% of these objects are newly identified as variable.
Examples of many familiar types are presented. All classifications for this
study have been manually confirmed. The selection criteria for this analysis
have been conservatively defined, and are known to be biased against some
variable classes. This preliminary study includes only 5.6% of the total
ROTSE-I sky coverage, suggesting that the full ROTSE-I variable catalog will
include more than 32,000 periodic variable stars.


title: External Shock Model for Gamma-Ray Bursts during the Prompt Phase

abstract:
  The hard X-ray and gamma-ray phenomenology of gamma-ray bursts (GRBs) can be
explained by an external shock model where a single relativistic blast wave
interacts with the surrounding medium. Besides reproducing the generic spectral
behavior of GRB profiles, the external shock model provides quantitative fits
to the peak flux distribution, the > 1 s t_{50} duration distribution, and the
distribution of the peaks E_pk of the vF_v spectra of GRBs measured with BATSE.
The apparent paradox between a relativistic beaming scenario and the empirical
finding that E_pk values are preferentially measured within the triggering
range of a GRB detector is resolved by this model when blast wave physics and
detector triggering criteria are taken into account. Some surprising
implications follow, namely that the fireball event rate is ~1 per 10^4 years
per Milky Way galaxy for unbeamed sources, and proportionally more if fireball
outflows are collimated. This is ~ 3 orders of magnitude larger than normally
quoted. Most of the clean and dirty fireball transients are undetected due to
telescope sensitivity and design limitations.
  Strongly variable GRB time histories with good radiative efficiencies are
possible because of the strongly enhanced emissions when a blast wave interacts
with density inhomogeneities located nearly along the line-of-sight to the
observer. Arguments against short timescale variability in an external shock
model are answered, and difficulties in an internal shock/colliding shell model
are mentioned.


title:
Emission-Line Galaxy Surveys as Probes of the Spatial Distribution of
  Dwarf Galaxies. I. The University of Michigan Survey

abstract:
  Objective-prism surveys which select galaxies on the basis of line-emission
are extremely effective at detecting low-luminosity galaxies and constitute
some of the deepest available samples of dwarfs. In this study, we confirm that
emission-line galaxies (ELGs) in the University of Michigan (UM)
objective-prism survey (MacAlpine et al. 1977-1981) are reliable tracers of
large-scale structure, and utilize the depth of the samples to examine the
spatial distribution of low-luminosity (M$_{B} > $ -18.0) dwarfs relative to
higher luminosity giant galaxies (M$_{B} \leq$ -18.0) in the Updated Zwicky
Catalogue (Falco et al. 1999). New spectroscopic data are presented for 26 UM
survey objects. We analyze the relative clustering properties of the overall
starbursting ELG and normal galaxy populations, using nearest neighbor and
correlation function statistics. This allows us to determine whether the
activity in ELGs is primarily caused by gravitational interactions. We conclude
that galaxy-galaxy encounters are not the sole cause of activity in ELGs since
ELGs tend to be more isolated and are more often found in the voids when
compared to their normal galaxy counterparts. Furthermore, statistical analyses
performed on low-luminosity dwarf ELGs show that the dwarfs are less clustered
when compared to their non-active giant neighbors. The UM dwarf samples have
greater percentages of nearest neighbor separations at large values and lower
correlation function amplitudes relative to the UZC giant galaxy samples. These
results are consistent with the expectations of galaxy biasing.


title:
On the Origin of Quasi-Periodic Oscillations and Broad-band Noise in
  Accreting Neutron Stars and Black Holes

abstract:
  Accreting neutron stars and black holes share a number of rapid variability
characteristics, such as quasi-periodic oscillations and broad-band noise. The
frequencies of these features were recently shown to be comparable to
fundamental general relativistic frequencies of perturbed test-particle orbits
near the compact objects. In this paper we propose a dynamical model for
producing modulations in the inner disk properties at these characteristic
frequencies. We postulate that a transition radius exists in the accretion disk
close to the compact object and investigate its response to a broad spectrum of
density fluctuations. We find that the transition radius acts as a low
band-pass filter with a broadband response that is constant at frequencies
lower than the inverse radial drift timescale and decreases above it as the
inverse of the frequency. Moreover, the response shows strong resonances at
frequencies nearly equal to the orbital, periastron-precession, and
nodal-precession frequencies, thereby accounting for the observed power-density
spectra of accreting compact objects and the correlations between their
characteristic frequencies.


title: HST Observations of M Subdwarfs

abstract:
  We present the results of an HST snapshot program to search for very-low-mass
stellar companions to nearby M subdwarfs. None of our nine targetted metal-poor
primaries have companions more massive than the hydrogen burning limit,
implying that the halo binary fraction is equal to or less than the Galactic
disk binary fraction below 0.3 M_sun. In addition, the more distant tertiary
VB12, an sdM3.0 companion to an F subdwarf double, is also unresolved. We show
that the relation between WFPC2 F555W and F850LP photometry and ground V,I
photometry is consistent with theoretical expectations. We also report that two
recently observed Hyades M dwarfs appear single.


title: Cross-correlating the Sloan Digital Sky Survey with the Microwave Sky

abstract:
  While most of the microwave background fluctuations on angular scales greater
than a few arcminutes were generated at z > 800, the low redshift universe does
distort the microwave background. Since the Sloan Digital Sky Survey (SDSS)
traces the structures in the low-redshift universe, we can gain additional
insights into the physics of the low-redshift universe by cross-correlating
microwave background maps with template maps produced from the SDSS. We present
a formalism for cross-correlating data from the Microwave Anisotropy Probe
(MAP) with the Sloan Survey for the thermal Sunyaev-Zel'dovich (SZ) effect, the
Integrated Sachs-Wolfe (ISW) effect, and weak lensing. This formalism is used
to compute the signal-to-noise for cross-correlating these effects with various
classes of tracer objects from the SDSS. It is found that the anticipated
samples of SDSS quasars and galaxies with photometrically-determined redshifts
would be good tracers for cross-correlating with the CMB. We find that the
SZ-galaxy cross-correlation would give good constraints on pressure
fluctuations in supercluster-scale gas. Cross-correlating weakly-lensed quasars
with maps of the convergence of the CMB is found to give strong constraints on
Omega_0 as well as the equation of state w. We find that the ISW
cross-correlation gives poor signal-to-noise using these techniques.


title: Radio Properties of z>4 Optically-Selected Quasars

abstract:
  We report on two programs to address differential evolution between the
radio-loud and radio-quiet quasar populations at high (z>4) redshift. Both
programs entail studying the radio properties of optically-selected quasars.
First, we have observed 32 optically-selected, high-redshift (z>4) quasars with
the VLA at 6 cm (5 GHz). These sources comprise a statistically complete and
well-understood sample. We detect four quasars above our 3-sigma limit of ~0.15
mJy, which is sufficiently sensitive to detect all radio-loud quasars at the
probed redshift range. Second, we have correlated 134 z>4 quasars, comprising
all such sources that we are aware of as of mid-1999, with FIRST and NVSS.
These two recent 1.4 GHz VLA sky surveys reach 3-sigma limits of approximately
0.6 mJy and 1.4 mJy respectively. We identify a total of 15 z>4 quasars, of
which six were not previously known to be radio-loud. The depth of these
surveys does not reach the radio-loud/radio-quiet demarcation luminosity
density (L(1.4 GHz) = 10^32.5 h(50)^(-2) ergs/s/Hz) at the redshift range
considered; this correlation therefore only provides a lower limit to the
radio-loud fraction of quasars at high-redshift. The two programs together
identify eight new radio-loud quasars at z>4, a significant increase over the
seven currently in the published literature. We find no evidence for radio-loud
fraction depending on optical luminosity for -25 > M_B > -28 at z~2, or for
-26>M_B>-28 at z>4. Our results also show no evolution in the radio-loud
fraction between z~2 and z>4 (-26>M_B>-28).


title:
Starburst-driven galactic winds: I. Energetics and intrinsic X-ray
  emission

abstract:
  We have performed an extensive hydrodynamical parameter study of
starburst-driven galactic winds, motivated by the latest observation data on
the best-studied starburst galaxy M82. We study how the wind dynamics,
morphology and X-ray emission depend on the host galaxy's ISM distribution,
starburst star formation history and strength, and presence and distribution of
mass-loading by dense clouds. We find that the soft X-ray emission from
galactic winds comes from low filling factor (ff < 2 per cent) gas, which
contains only a small fraction (f < 10 per cent) of the mass and energy of the
wind, irrespective of whether the wind models are strongly mass-loaded or not.
X-ray observations of galactic winds therefore do not directly probe the gas
that contains the majority of the energy, mass or metal-enriched gas in the
outflow. The soft X-ray emission comes from gas at a wide range different
temperatures and densities. Estimates of the physical properties of the hot gas
in starburst galaxies, based on fitting the standard simple spectral models to
existing X-ray spectra, should therefore be treated with extreme suspicion. The
majority of the thermal and kinetic energy of these winds is in a volume
filling hot, T approx 10^7 K, component which is extremely difficult to probe
observationally due to its low density and hence low emissivity. Most of the
total energy is in the kinetic energy of this hot gas, a factor which must be
taken into account when attempting to constrain wind energetics
observationally. We also find that galactic winds are efficient at transporting
large amounts of energy out of the host galaxy, in contrast to their
inefficiency at transporting mass out of star-forming galaxies. (Abridged)


title: Gamma-Ray Bursts via Pair Plasma Fireballs from Heated Neutron Stars

abstract:
  In this paper we model the emission from a relativistically expanding
electron-positron pair plasma fireball originating near the surface of a heated
neutron star. This pair fireball is deposited via the annihilation of neutrino
pairs emanating from the surface of the hot neutron star. The heating of
neutron stars may occur in close neutron star binary systems near their last
stable orbit. We model the relativistic expansion and subsequent emission of
the plasma and find 10^51 to 10^52 ergs in gamma-rays are produced with
spectral and temporal properties consistent with observed gamma-ray bursts.


title: Flattening modulus of a neutron star by rotation and magnetic field

abstract:
  We calculated the ellipticity of the deformed star due to the rotation or
magnetic field. These two effects are compared to each other within general
relativity. It turned out that the magnetic distortion is important for
recently observed candidates of magnetars, while the magnetic effect can be
neglected for well-known typical pulsars.


title:
The star formation history as a function of type: constraints from
  galaxy counts

abstract:
  Deep galaxy counts are among the best constraints on the cosmic star
formation history (SFH) of galaxies. The evolution of the star formation
activity is followed on a wide range of redshifts (0< z < 4) covering most of
the history of the Universe. Two incompatible interpretations of the
observations are currently competing. After applying star formation rate (SFR)
conversion factors to the CFRS, Halpha or ISO samples, many authors conclude to
a strong increase (~ a factor 10) of the SFR from z=0 to z=1. They also find
some evidence for a peak at z~1 and for a rapid decrease at higher redshifts.
On the other side, the Hawaii deep surveys favor only a mild increase between
z=0 and 1 (Cowie et al., 1996, 1999). To understand the reason for these
discrepant interpretations, we consider three classes of galaxies: E/S0,
Sa-Sb-Sbc, Sc-Sd-Im and bursting dwarfs. We use the new version of our
evolutionary synthesis code, PEGASE which takes into account metallicity and
dust effects. The main results are: i) Late-type galaxies contribute
significantly to the local SFR, especially bursting dwarfs (Fioc and
Rocca-Volmerange, 1999). Because of that, the cosmic SFR can not decrease by a
factor 10 from z=0 to 1. This is in agreement with Cowie's result. ii) The SFR
of intermediate-type galaxies has strongly decreased since z=1. Though the
decrease is less than what find Lilly et al., 1996, this suggests that the CFRS
and Halpha surveys are dominated by such bright early spirals. iii) The
contribution of early-type galaxies increases rapidly from z=1 to their
redshift of formation (> 2-3 for cosmological reasons). Their intense star
formation rates at high-z give strong constraints on early ionization phases,
primeval populations or metal enrichments.(abridged)


title:
Locally Optimally-Emitting Clouds and the Variable Broad Emission Line
  Spectrum of NGC 5548

abstract:
  [abridged] We test the ``locally optimally-emitting clouds'' (LOC) model
against the extensive spectroscopic data base of the Seyfert~1, NGC 5548. The
time-averaged, integrated-light UV broad emission line spectrum from the 1993
{\em HST} monitoring campaign is reproduced via the optimization of three
global geometric parameters: the outer radius, the index controlling the radial
cloud covering fraction of the continuum source, and the integrated cloud
covering fraction. We make an {\em ad~hoc} selection from the range of
successful models, and for a simple spherical BLR geometry we simulate the
emission line light curves for the 1989 {\em IUE} and 1993 {\em HST} campaigns,
using the respective observed UV continuum light curves as drivers. We find
good agreement between the predicted and observed light curves and lags --- a
demonstration of the LOC picture's viability as a means to understanding the
BLR environment. Finally, we discuss the next step in developing the LOC
picture which involves the marriage of echo-mapping techniques with spectral
simulation grids such as those presented here, using the constraints provided
by a high quality, temporally well-sampled spectroscopic data set.


title: Collapsars

abstract:
  A variety of stellar explosions powered by black hole accretion are
discussed. All involve the failure of neutrino energy deposition to launch a
strong supernova explosion. A key quantity which determines the type of high
energy transient produced is the ratio of the engine operation time, $\rm
t_{engine}$, to the time for the explosion to break out of the stellar surface,
$\rm t_{bo}$. Stars with sufficient angular momentum produce collapsars --
black holes accreting rapidly through a disk -- in their centers. Collapsars
can occur in stars with a wide range of radii depending on the amount of
pre-collapse mass loss. The stellar radius and jet properties determine the
degree of beaming of the explosion. In some cases the stellar envelope serves
to focus the explosion to narrow beaming angles. The baryon loading of various
models for classical GRBs formed in massive stars is examined and the
consequences are explored. For $\rm t_{engine} > t_{bo}$, highly relativistic
outflow is possible and classical GRBs accompanied by supernovae can be
produced. In other cases hyper-energetic, asymmetric supernovae are produced.
Longer GRBs ($t \gtaprx 100 \s$) can be produced by fallback following a weak
neutrino-driven supernova explosion.


title: Power Density Spectra of GRBs

abstract:
  Power density spectra (PDSs) of long gamma-ray bursts (GRBs) provide useful
information on GRBs, indicating their self-similar temporal structure. The best
power-law PDSs are displayed by the longest bursts (T_90>100 s) in which the
range of self-similar time scales covers more than 2 decades. Shorter bursts
have apparent PDS slopes more strongly affected by statistical fluctuations.
The underlying power law can then be reproduced with high accuracy by averaging
the PDSs for a large sample of bursts. This power law has a slope approximately
equal to -5/3 and a sharp break at about 1 Hz.
  The power-law PDS provides a new sensitive tool for studies of GRBs. In
particular, we calculate the PDSs of bright bursts in separate energy channels.
The PDS flattens in the hard channel (h\nu>300 keV) and steepens in the soft
channel (h\nu<50 keV), while the PDS of bolometric light curves approximately
follows the -5/3 law.
  We then study dim bursts and compare them to the bright ones. We find a
strong correlation between the burst brightness and the PDS slope. This
correlation shows that the bursts are far from being standard candles and dim
bursts should be intrinsically weak. The time dilation of dim bursts is
probably related to physical processes occurring in the burst rather than to a
cosmological redshift.


title:
Chandra X-ray Observations of the Hydra A Cluster: An Interaction
  Between the Radio Source and the X-Ray-Emitting Gas

abstract:
  We present Chandra X-ray Observations of the Hydra A cluster of galaxies, and
we report the discovery of structure in the central 80 kpc of the cluster's
X-ray-emitting gas. The most remarkable structures are depressions in the X-ray
surface brightness, $\sim 25-35$ kpc diameter, that are coincident with Hydra
A's radio lobes. The depressions are nearly devoid of X-ray-emitting gas, and
there is no evidence for shock-heated gas surrounding the radio lobes. We
suggest the gas within the surface brightness depressions was displaced as the
radio lobes expanded subsonically, leaving cavities in the hot atmosphere. The
gas temperature declines from 4 keV at 70 kpc to 3 keV in the inner 20 kpc of
the brightest cluster galaxy (BCG), and the cooling time of the gas is $\sim
600$ Myr in the inner 10 kpc. These properties are consistent with the presence
of a $\sim 34 \msunyr$ cooling flow within a 70 kpc radius. Bright X-ray
emission is present in the BCG surrounding a recently-accreted disk of nebular
emission and young stars. The star formation rate is commensurate with the
cooling rate of the hot gas within the volume of the disk, although the sink
for the material cooling at larger radii remains elusive.


title: Properties of disk galaxies in a hierarchical formation scenario

abstract:
  We used galaxy evolutionary models in a hierarchical inside-out formation
scenario to study the origin of the main properties and correlations of disk
galaxies. We found that most of these properties and correlations are the
result of three (cosmological) initial factors and their dispersions: the
virial mass, the halo mass aggregation history (MAH), and the angular momentum
given through the spin parameter \lambda. The MAH determines mainly the halo
structure and the color indexes while \lambda determines mainly the surface
brightness and the bulge-to-disk ratio. We calculated star formation (SF) using
a gravitational instability criterion and a self-regulation mechanism in the
turbulent ISM. The efficiency of SF in this model is almost independent from
the mass. We show that the luminosity-dependent dust absorption empirically
determined by Wang & Heckman explains the observed color-magnitude and color
Tully-Fisher (TF) relations without the necessity of introducing a
mass-dependent SF efficiency. The disks in centrifugal equilibrium form within
growing CDM halos with a gas accretion rate proportional to the MAH. The disks
present exponential surface density and brightness profiles, negative radial
color index gradients, and nearly flat rotation curves. We also calculated the
secular formation of a bulge due to gravitational instabilities in the stellar
disk. The intensive properties of our models agree with the observational data
and the trends of the Hubble sequence are reproduced. The predicted infrared TF
and luminosity-radius relations also agree with observations. The main
shortcomings of our inside-out hierarchical models are the excessive radial
color gradients and the dark halo dominion in the rotation curve
decompositions.


title: ROSAT observations of X-ray emission from planetary nebulae

abstract:
  We have searched the entire ROSAT archive for useful observations to study
X-ray emission from Galactic planetary nebulae (PNs). The search yields a
sample of 63 PNs, which we call the ROSAT PN sample. About 20-25% of this
sample show X-ray emission; these include 13 definite detections and three
possible detections (at a 2-sigma level). All X-ray sources in these PNs are
concentrated near the central stars. Only A 30, BD+30 3639, and NGC 6543 are
marginally resolved by the ROSAT instruments. Three types of X-ray spectra are
seen in PNs. Type 1 consists of only soft X-ray emission (<0.5 keV), peaks at
0.1-0.2 keV, and can be fitted by blackbody models at temperatures 1-2 10^5 K.
Type 2 consists of harder X-ray emission, peaks at >0.5 keV, and can be fitted
by thin plasma emission models at temperatures of a few 10^6 K. Type 3 is a
composite of a bright Type 1 component and a fainter Type 2 component.
Unresolved soft sources with Type 1 spectra or the soft component of Type 3
spectra are most likely photospheric emission from the hot central stars.
Absorption cross sections are large for these soft-energy photons; therefore,
only large, tenuous, evolved PNs with hot central stars and small absorption
column densities have been detected. The origin of hard X-ray emission from PNs
is uncertain. PNs with Type 2 spectra are small, dense, young nebulae with
relatively cool (<<10^5 K) central stars, while PNs with Type 3 X-ray spectra
are large, tenuous, evolved nebulae with hot central stars. The hard X-ray
luminosities are also different between these two types of PNs, indicating
perhaps different origins of their hard X-ray emission. Future Chandra and XMM
observations with high spatial and spectral resolution will help to understand
the origin of hard X-ray emission from PNs.


title: Hydrodymanic collimation of GRB fireballs

abstract:
  Analytic solutions are presented for the hydrodynamic collimation of a
relativistic fireball by a surrounding baryonic wind emanating from a torus.
The opening angle is shown to be the ratio of the power output of the inner
fireball to that of the exterior baryonic wind. The gamma ray burst 990123
might thus be interpreted as a baryon-pure jet with an energy output of order
10$^{50}$ erg or less, collimated by a baryonic wind from a torus with an
energy output of order $10^{52}$ erg.


title:
The deuterium abundance in QSO absorption systems: a mesoturbulent
  approach

abstract:
  A new method, based on simulated annealing technique and aimed at the inverse
problem in the analysis of intergalactic or interstellar complex spectra of
hydrogen and metal lines, is outlined. We consider the process of line
formation in clumpy stochastic media accounting for fluctuating velocity and
density fields self-consistently. Two examples of the analysis of `H+D'-like
absorptions seen at z = 3.514 and 3.378 towards APM 08279+5255 are presented.


title: Broad-band BeppoSAX observation of the low-mass X-ray binary X1822-371

abstract:
  Results of a 1997 September 9-10 BeppoSAX observation of the 5.57 hr low-mass
X-ray binary (LMXRB) X1822-371 are presented. The 0.3-40 keV spectrum is
unusually complex and cannot be fit by any of the standard models applied to
other LMXRB. At least two components are required. One component has a shape
consistent with that expected from the Comptonization of an input soft (Wein)
spectrum while the other, contributing ~40% of the 1-10 keV flux, is consistent
with being a blackbody. In addition, there is a ``dip'' in the spectrum which
can be modeled by a 1.33 +0.05 -0.11 keV absorption edge with an optical depth,
tau, of 0.28 +/- 0.06. If the same model is fit to ASCA Solid-State Imaging
Spectrometer spectra obtained in 1993 and 1996, then reasonable fits are also
obtained, with a similar absorption feature required. The nature of this
feature is highly uncertain; its energy corresponds to the K-edges of highly
ionized Ne x and neutral Mg, or to an L-edge of moderately ionized Fe.
Surprisingly, no strong (tau > 0.05) Fe-K or (tau > 0.18) O-K edges are
visible. The folded lightcurve of X1822-371 is similar to previous
observations, except that no strong softening is seen near the eclipse. An
updated orbital ephemeris is provided.


title:
Transformations between the theoretical and observational planes in the
  HST-NICMOS and WFPC2 photometric systems

abstract:
  Color-temperature relations and bolometric corrections in the HST-NICMOS
F1110W, F160W and F222M and in the WFPC2 F439W, F555W and F814W photometric
systems, using two different sets of model atmospheres, have been derived. This
database of homogeneous, self-consistent transformations between the
theoretical and observational planes also allows combinations of visual and
infrared quantities, without any further transformation between the two
different photometric systems. The behavior of the inferred quantities with
varying the stellar parameters, the adopted model atmospheres and the
instrumental configurations are investigated. Suitable relations to transform
colors and bolometric corrections from HST to ground-based photometric systems
are also provided.


title: Evolution of D and 3He in the Galaxy

abstract:
  The predictions of Galactic chemical evolution models for D and $^3$He are
described in connection with those on the other Galactic quantities for which
observational constraints are available.
  Models in agreement with the largest set of data predict deuterium depletions
from the Big Bang to the present epoch smaller than a factor of 3 and do not
allow for D/H primordial abundances larger than $\sim4\times10^{-5}$. Models
predicting higher D consumption do not reproduce other observed features of our
Galaxy.
  If both the primordial D and $^3$He are low, models assuming that 90% of
low-mass stars experience an extra-mixing during the red giant phase reproduce
all the $^3$He observed abundances. The same percentage allows to fit also the
observed carbon isotopic ratios, thus supporting the self-consistency of the
extra-mixing mechanism.


title: New Extreme Synchrotron BL Lac Objects

abstract:
  We report on the BeppoSAX observations of four "extreme" BL Lacs, selected to
have high synchrotron peak frequencies. All have been detected also in the PDS
band. For 1ES 0120+340, PKS 0548-322 and H 2356-309 the spectrum is well fitted
by a convex broken power-law, thus locating the synchrotron peak around 1 - 4
keV. 1ES 1426+428 presents a flat energy spectral index (alpha_x= 0.92) up to
100 keV, thus constraining the synchrotron peak to lie near or above that
value. For their extreme properties, all sources could be strong TeV emitters.


title:
Search for Astronomical Sites in Developing Countries and their
  Preventive Protection

abstract:
  The archives of meteorological satellites permit to find around the world dry
sites well adapted to astronomical observations (in the visible, IR, and
millimetric ranges), as a pre-selection of sites. The GSM (Grating Scale
Monitor) technique permits to qualify some of them as astronomical sites for a
future setting-up of astronomical observatories. Such sites are found in new
astronomical countries or in Developing Countries. At the same time, their
preventive protection from light pollution and/or radio interference has to be
viewed. In practice, once the pre-selection is made, the government of these
countries ought to be alerted for example by IAU and/or UN Office for Outer
Space Affairs. The local site testing through GSM should be carried out in
cooperation with astronomers or scientists of these countries under the
umbrella of IAU. This should be an approach to help to introduce astronomy and
astrophysics in Developing Countries.


title: The enigmatic Soft X-ray AGN RX J0134.2--4258

abstract:
  We report the discovery and analysis of the follow-up ROSAT pointed
observation, an ASCA observation and optical and radio observations of the
enigmatic soft X-ray AGN RX J0134.2-4258. In the optical, RX J0134.2-4258
appears as an extreme 'Narrow-Line Seyfert 1 galaxy' (NLS1), with very strong
FeII emission, very blue optical continuum spectrum and almost no Narrow Line
Region emission. While its spectrum was one of the softest observed from an AGN
during the ROSAT All-Sky Survey, its spectrum was found to be dramatically
harder during a pointed observation although the count rate remained constant.
We found in the pointed observation that the spectrum is softer when it is
fainter and spectral fitting demonstrates that it is the hard component that is
variable. The ASCA observation confirms the presence of a hard X-ray power law,
the slope of which is rather flat compared with other NLS1s. Survey and
followup radio observations reveal that RX J0134.2-4258 is also unusual in that
it is a member of the rare class of radio-loud NLS1s, and, with R=71, it holds
the current record for largest radio-to-optical ratio in NLS1s. We discuss
possible scenarios to explain its strange behaviour.