title: Competitive accretion in embedded stellar cluster

abstract:
  We investigate the physics of gas accretion in young stellar clusters.
Accretion in clusters is a dynamic phenomenon as both the stars and the gas
respond to the same gravitational potential. Accretion rates are highly
non-uniform with stars nearer the centre of the cluster, where gas densities
are higher, accreting more than others. This competitive accretion naturally
results in both initial mass segregation and a spectrum of stellar masses.
Accretion in gas-dominated clusters is well modelled using a tidal-lobe radius
instead of the commonly used Bondi-Hoyle accretion radius. This works as both
the stellar and gas velocities are under the influence of the same
gravitational potential and are thus comparable. The low relative velocity that
results means that the tidal radius is smaller than the Bondi-Hoyle radius in
these systems. In contrast, when the stars dominate the potential and are
virialised, the Bondi-Hoyle radius is smaller than the tidal radius and thus
Bondi-Hoyle accretion is a better fit to the accretion rates.


title: The optical counterpart to SAX J1808.4-3658: observations in quiescence

abstract:
  We report the first extensive set of optical photometric observations of the
counterpart to SAX J1808.4-3658 (V4580 Sagittarius) in quiescence. The source
was detected at V~21, 5 magnitudes fainter than at the peak of its 1998
outburst. However, a comparable ~6% semi-amplitude 2hr modulation of its flux
is revealed. This has the same phasing and approximately sinusoidal modulation
as seen during outburst and with photometric minimum when the pulsar is behind
the companion. The lack of a double-humped morphology rules out an ellipsoidal
origin, implying that the bulk of the optical flux does not arise from the
companion. Moreover, applying crude modelling to the disc and X-ray irradiated
face of the donor shows that the internal energy release of a remnant disc
(with mass transfer driven by gravitational radiation) is sufficient to explain
most of the optical emission, and with the modulation due to the varying
contribution of the heated star's face. We note that this model is also
consistent with the much lower X-ray to optical flux ratio in quiescence versus
outburst, and with the phasing of the optical modulation.


title: Intergalactic Magnetic Fields from Quasar Outflows

abstract:
  Outflows from quasars inevitably pollute the intergalactic medium (IGM) with
magnetic fields. The short-lived activity of a quasar leaves behind an
expanding magnetized bubble in the IGM. We model the expansion of the remnant
quasar bubbles and calculate their distribution as a function of size and
magnetic field strength at different redshifts. We generically find that by a
redshift z=3, about 5-20% of the IGM volume is filled by magnetic fields with
an energy density >10% of the mean thermal energy density of a photo-ionized
IGM (at T=10^4 K). As massive galaxies and X-ray clusters condense out of the
magnetized IGM, the adiabatic compression of the magnetic field could result in
the field strength observed in these systems without a need for further dynamo
amplification. The intergalactic magnetic field could also provide a nonthermal
contribution to the pressure of the photo-ionized gas that may account for the
claimed discrepancy between the simulated and observed Doppler width
distributions of the Ly-alpha forest.


title: The Origin of the Highest Energy Cosmic Rays

abstract:
  Contrary to expectations, several cosmic ray events with energies above
$10^{20}$ eV have been observed. The flux of such events is well above the
predicted Greisen-Zatsepin-Kuzmin cutoff due to the pion production (via the
$\Delta$ resonace) of extragalactic cosmic ray protons off the cosmic microwave
background. In addition to the relatively high flux of events, the isotropic
distribution of arrival directions and an indication of hadronic primaries
strongly challenge all models proposed to resolve this puzzle. Models based on
astrophysical accelerators need to invoke strong Galactic and extragalactic
magnetic fields with specific properties which are yet to be observed, while
models based on physics beyond the standard model of particle physics generally
predict photon primaries contrary to experimental indications. The resolution
of this puzzle awaits a significant increase in the data at these energies
which will be provided by future experiments.


title:
A Next Generation High-speed Data Acquisition System for Multi-channel
  Infrared and Optical Photometry

abstract:
  We report the design, operation, and performance of a next generation
high-speed data acquisition system for multi-channel infrared and optical
photometry based on the modern technologies of Field Programmable Gate Arrays,
the Peripheral Component Interconnect bus, and the Global Positioning System.
This system allows either direct recording of photon arrival times or binned
photon counting with time resolution up to 1-$\mu$s precision in Universal
Time, as well as real-time data monitoring and analysis. The system also allows
simultaneous recording of multi-channel observations with very flexible,
reconfigurable observational modes. We present successful 20-$\mu$s resolution
simultaneous observations of the Crab Nebula Pulsar in the infrared (H-band)
and optical (V-band) wavebands obtained with this system and 100-$\mu$s
resolution V-band observations of the dwarf nova IY Uma with the 5-m Hale
telescope at the Palomar Observatory.


title: Dynamical Friction in dE Globular Cluster Systems

abstract:
  The dynamical friction timescale for globular clusters to sink to the center
of a dwarf elliptical galaxy (dE) is significantly less than a Hubble time if
the halos have King-model or isothermal profiles and the globular clusters
formed with the same radial density profile as the underlying stellar
population. We examine the summed radial distribution of the entire globular
cluster systems and the bright globular cluster candidates in 51 Virgo and
Fornax Cluster dEs for evidence of dynamical friction processes. We find that
the summed distribution of the entire globular cluster population closely
follows the exponential profile of the underlying stellar population. However,
there is a deficit of bright clusters within the central regions of dEs
(excluding the nuclei), perhaps due to the orbital decay of these massive
clusters into the dE cores. We also predict the magnitude of each dE's nucleus
assuming the nuclei form via dynamical friction. The observed trend of
decreasing nuclear luminosity with decreasing dE luminosity is much stronger
than predicted if the nuclei formed via simple dynamical friction processes. We
find that the bright dE nuclei could have been formed from the merger of
orbitally decayed massive clusters, but the faint nuclei are several magnitudes
fainter than expected. These faint nuclei are found primarily in M_V > -14 dEs
which have high globular cluster specific frequencies and extended globular
cluster systems. In these galaxies, supernovae-driven winds, high central dark
matter densities, extended dark matter halos, the formation of new star
clusters, or tidal interactions may act to prevent dynamical friction from
collapsing the entire globular cluster population into a single bright nucleus.


title:
A Short Timescale Candidate Microlensing Event in the POINT-AGAPE Pixel
  Lensing Survey of M31

abstract:
  We report the discovery of a short-duration microlensing candidate in the
northern field of the POINT-AGAPE pixel lensing survey towards M31. The
full-width half-maximum timescale is very short, just 1.8 days. Almost
certainly, the source star has been identified on Hubble Space Telescope
archival images, allowing us to infer an Einstein crossing time of 10.4 days, a
maximum magnification of about 18, and a lens-source proper motion greater than
0.3 microarcsec/day. The event lies projected at 8' from the center of M31,
which is beyond the bulk of the stellar lens population. The lens is likely to
reside in one of three locations. It may be a star in the M31 disk, or a
massive compact halo object (Macho) in either M31 or the Milky Way. The most
probable mass is 0.06 solar masses for an M31 Macho, 0.02 solar masses for a
Milky Way Macho and 0.2 solar masses for an M31 stellar lens. Whilst the
stellar interpretation is plausible, the Macho interpretation is the most
probable for halo fractions above 20%.


title: Lensing Properties of Scale-Free Galaxies

abstract:
  The multiple images of lensed quasars provide evidence on the mass
distribution of the lensing galaxy. The lensing invariants are constructed from
the positions of the images, their parities and their fluxes. They depend only
on the structure of the lensing potential. The simplest is the magnification
invariant, which is the sum of the signed magnifications of the images. Higher
order invariants are the sums of products of the signed magnifications with
positive or negative powers of the position coordinates of the images. We
consider the case of the four and five image systems produced by elliptical
power-law galaxies. This paper provides simple contour integrals for evaluating
all their lensing invariants. For practical evaluation, this offers
considerable advantages over the algebraic methods used previously. To
illustrate the power of the contour integral method, we calculate full
expansions in the source offset for all lensing invariants in the presence of
arbitrary external shear. As an example, we use the elliptical power-law
galaxies to fit to the data on the four images of the Einstein Cross
(G2237+030). The lensing invariants play a role by reducing the dimensionality
of the parameter space in which the chi-squared minimisation proceeds with
consequent gains in accuracy and speed.


title: No Need for Dark Matter in Galaxies?

abstract:
  Unhappily, there has been a maelstrom of problems for dark matter theories
over the last few years and many serious difficulties still have no resolution
in sight. This article reviews the evidence for dark matter in galaxies. The
haloes built up by hierarchical merging in dark matter cosmogonies are cusped
and dominated by dark matter at the center. Evidence from the microlensing
optical depth towards Baade's Window and from dynamical modelling of the
Galactic bar already suggests that the Galactic halo is not cusped. Similarly,
evidence from the stability of unbarred disk galaxies, as well as the survival
of fast bars in barred galaxies, suggests that the this result holds good more
generally. Judged on the data from galactic scales alone, the case for dark
matter is weak and non-standard theories of gravity provide a better
description. Of course, non-standard theories of gravity have their own
problems, but not on galactic scales.


title: Superluminal motion in a compact steep spectrum radio source 3C 138

abstract:
  We present the results of 5 GHz VLBI observations of a compact steep spectrum
source 3C 138. The data are consistent with the western end being the location
of the central activity. The observed offset between different frequencies in
the central region of 3C 138 can be accounted for by a frequency dependent
shift of the synchrotron self-absorbed core. Our new measurements confirm the
existence of a superluminal motion, but its apparent velocity of 3.3c is three
times slower than the reported one. This value is consistent with the absence
of parsec-scale counter-jet emission in the inner region, but seems still too
high to allow the overall counter-jet to be seen in terms of Doppler boosting
of an intrinsically identical jet. Either an interaction of jet with central
dense medium, or an intrinsically asymmetrical jet must be invoked to reconcile
the detected superluminal speed with the observed large scale asymmetry in 3C
138.


title:
X-ray Observations of the New Pulsar--Supernova Remnant System PSR
  J1119-6127 and SNR G292.2-0.5

abstract:
  PSR J1119-6127 is a recently discovered 1700-year-old radio pulsar that has a
very high inferred surface dipolar magnetic field. We present a detailed
analysis of a pointed ASCA observation and archival ROSAT data of J1119-6127
and its surroundings. Both data sets reveal extended emission coincident with
the newly-discovered radio supernova remnant G292.2-0.5, reported in a
companion paper by Crawford et al. (astro-ph/0012287).. A hard point source,
offset ~1.5' from the position of the radio pulsar, is seen with the ASCA GIS.
No pulsations are detected at the radio period with a pulsed fraction upper
limit of 61% (95% confidence). The limited statistics prevent a detailed
spectral analysis, although a power-law model with photon index (Gamma=~1-2)
describes the data well. Both the spectral model and derived X-ray luminosity
are consistent with those measured for other young radio pulsars, although the
spatial offset renders an identification of the source as the X-ray counterpart
of the pulsar uncertain.


title: Arecibo imaging of compact high-velocity clouds

abstract:
  Ten isolated compact high-velocity clouds (CHVCs) of the type cataloged by
Braun & Burton (1999) have been imaged with the Arecibo telescope and were
found to have a nested core/halo morphology. We argue that a combination of
high-resolution filled-aperture and synthesis data is crucial to determining
the intrinsic properties of the CHVCs. We identify the halos as Warm Neutral
Medium surrounding one or more cores in the Cool Neutral Medium phase. These
halos are clearly detected and resolved by the Arecibo filled-aperture imaging,
which reaches a limiting sensitivity (1 sigma) of N_H about 2x10^17 cm^-2 over
the typical 70 km/s linewidth at zero intensity. The FWHM linewidth of the halo
gas is found to be 25 km/s, consistent with a WNM thermal broadening within
10^4 K gas. Substantial asymmetries are found at high N_H (>10^18.5 cm^-2)
levels in 60% of our sample. A high degree of reflection-symmetry is found at
low N_H (<10^18.5 cm^-2) in all sources studied at these levels. The
column-density profiles of the envelopes are described well by the sky-plane
projection of a spherical exponential in atomic volume density, which allows
estimating the characteristic central halo column density, N_H(0) =
4.1+/-3.2x10^19 cm^-2, and characteristic exponential scale-length,
h_B=420+/-90 arcsec. For plausible values of the thermal pressure at the
CNM/WNM interface, these edge profiles allow distance estimates to be made for
the individual CHVCs studied here which range between 150 and 850 kpc.
(abridged)


title: Simulating galaxy surveys with FIRST (PACS & SPIRE)

abstract:
  The next generation of submillimetre/millimetre instruments will provide us
with a deeper insight into the mechanisms that rule galaxy formation. As the
brightest starbursts are thought to be heavily obscured at optical wavelengths,
the opening of this new window will complement the present observations, and
enable a detailed investigation of the hierarchical merging of galaxies at
remote epochs. In this context, we will present preliminary results of
simulations currently developed for deep galaxy surveys with FIRST.
  N-body simulations of galaxy mergers are being developed to produce realistic
morphologies and star formation histories. A Schmidt law is used to account for
the evolution of the stellar and gas content of each particle. These
simulations will be gathered in a library, including the temporal information
with a timestep of 10 Myr. We then build consistent spectra accounting for this
star formation history at each resolution element of the simulations. These
morphologies will be included in the framework of the GalICS hybrid model of
hierarchical galaxy formation, which reproduces the main observational
constraints. We intend to present synthetic maps with the characteristics of
the FIRST PACS/SPIRE instruments and discuss the optimal strategy for deep
surveys complemented with the ground-based ALMA project.


title:
The X-ray nebula of the filled center supernova remnant 3C58 and its
  interaction with the environment

abstract:
  An \xmm observation of the plerionic supernova remnant 3C58 has allowed us to
study the X-ray nebula with unprecedented detail. A spatially resolved spectral
analysis with a resolution of 8\arcsec has yielded a precise determination of
the relation between the spectral index and the distance from the center. We do
not see any evidence for bright thermal emission from the central core. In
contrast with previous ASCA and {\em Einstein} results, we derive an upper
limit to the black-body 0.5-10 keV luminosity and emitting area of $1.8\times
10^{32}$ \ergsec and $1.3\times 10^{10}$ cm$^2$, respectively, ruling out
emission from the hot surface of the putative neutron star and also excluding
the "outer-gap" model for hot polar caps. We have performed for the first time
a spectral analysis of the outer regions of the X-ray nebula, where most of the
emission is still non-thermal, but where the addition of a soft (kT=0.2-0.3
keV) optically thin plasma component is required to fit the spectrum at $E<1$
keV. This component provides 6% of the whole remnant observed flux in the
0.5-10.0 keV band. We show that a Sedov interpretation is incompatible with the
SN1181-3C58 association, unless there is a strong deviation from electron-ion
energy equipartition, and that an origin of this thermal emission in terms of
the expansion of the nebula into the ejecta core nicely fits all the radio and
X-ray observations.


title: Spectra and evolution of two X-ray sources in M81

abstract:
  We analyzed the spectral and temporal features of X-ray sources in M81 using
data from the relevant ROSAT-PSPC/HRI observations. We discuss the main
features of the point-like nucleus and of the second brightest source in the
field (X-9), whose identification is still unclear.


title:
AGAPEROS: Searching for variable stars in the LMC Bar with the Pixel
  Method. I. Detection, astrometry and cross-identification

abstract:
  We extend the work developed in previous papers on microlensing with a
selection of variable stars. We use the Pixel Method to select variable stars
on a set of 2.5 x 10**6 pixel light curves in the LMC Bar presented elsewhere.
The previous treatment was done in order to optimise the detection of long
timescale variations (larger than a few days) and we further optimise our
analysis for the selection of Long Timescale and Long Period Variables
(LT&LPV). We choose to perform a selection of variable objects as comprehensive
as possible, independent of periodicity and of their position on the colour
magnitude diagram. We detail the different thresholds successively applied to
the light curves, which allow to produce a catalogue of 632 variable objects.
We present a table with the coordinate of each variable, its EROS magnitudes at
one epoch and an indicator of blending in both colours, together with a finding
chart.
  A cross-correlation with various catalogues shows that 90% of those variable
objects were undetected before, thus enlarging the sample of LT&LPV previously
known in this area by a factor of 10. Due to the limitations of both the Pixel
Method and the data set, additional data -- namely a longer baseline and near
infrared photometry -- are required to further characterise these variable
stars, as will be addressed in subsequent papers.


title:
Internal color gradients and the color-magnitude relation of early-type
  galaxies

abstract:
  The traditional use of fixed apertures in determining the well known
color-magnitude (CM) relation of early type galaxies, coupled with the presence
of radial color gradients within these systems, introduces a bias in the CM
relation itself. The effect of this bias is studied here deriving a CM relation
which is based on color measurements carried out homogeneously within an
aperture of radius equal to that of the galaxy effective radius. A sample of 48
giant early-type galaxies in the Coma cluster, with CCD observations in the U-
and V-band, is used for this derivation. It is found that internal radial color
gradients in early-type galaxies cannot be neglected when discussing the colors
of these systems, and that the CM relation derived using color measurements
within the effective radius is significantly flatter than those based on
fixed-aperture color measurements. With the presently available data it is
impossible to determine whether the relation is completely flat, or whether a
small correlation is still present between galaxy color and luminosity.


title: Modeling the production of flares in $\gamma$-ray quasars

abstract:
  Theories of high energy radiation production in quasar jets can be verified
by studies of both time-averaged spectra and variability patterns. While the
former has been explored extensively, the latter is in its infancy. In this
paper, we study the production of short-term flares in the shock-in-jet model.
We examine how the flares' profiles depend on such parameters as
shock/dissipation lifetime, electron-injection time profile, adiabaticity, and
half-opening angle of the jet. In particular, we demonstrate the large
difference between flare profiles produced in the radiative and adiabatic
regimes. We apply our model to the $\sim$day timescale flares observed in
optically violently variable (OVV) quasars, checking whether the
external-radiation-Compton (ERC) model for $\gamma$-ray flares at energies $>
30$ MeV (EGRET range) can be reconciled with the flares observed at lower
energies. Specifically, we show that the strict correlation between X-ray and
$\gamma$-ray flares strongly supports the dominance of the synchrotron
self-Compton mechanism in the X-ray band. We also derive conditions that must
be satisfied by the ERC model in order to explain a lag of the $\gamma$-ray
peak behind the optical one, as claimed to be observed in PKS 1406-076.
Finally, we predict that in ERC models where the MeV peak is related to the
break in electron distribution due to inefficient cooling of electrons below a
certain energy, the flares should decay significantly more slowly in the soft
$\gamma$-ray band than at energies greater than 30 MeV.


title:
Bispectrum speckle interferometry observations and radiative transfer
  modelling of the red supergiant NML Cyg: Multiple dust-shell structures
  evidencing previous superwind phases

abstract:
  (abridged) NML Cyg is a highly evolved OH/IR supergiant and supposed to be
among the most luminous supergiants in the galaxy. We present the first
diffraction limited 2.13micron observations of NML Cyg with 73mas resolution.
The speckle interferograms were obtained with the SAO 6m telescope, image
reconstruction is based on the bispectrum speckle interferometry method.
Radiative transfer calculations have been carried out to model the spectral
energy distribution, our 2.13micron visibility function, and mid-infrared
visibility functions. The observed dust shell properties do not appear to be in
accordance with single-shell models but seem to require multiple components.
Considering previous periods of enhanced mass-loss, various density
enhancements in the dust shell were taken into account. An extensive grid of
models was calculated for different locations and strenghts of such superwind
regions in the dust shell. To match the observations from the optical to the
sub-mm domain requires at least two superwind regions embedded in the shell.
The best model includes a dust shell with a temperature of 1000K at its inner
radius of 6.2Rstar, a close embedded superwind shell extending from 15.5Rstar
to 21.7Rstar with amplitude 10 (factor of density enhancement), and a far-out
density enhancement at 186Rstar with amplitude 5. The angular diameter of the
inner dust-shell rim amounts to 105mas. Within the various parts of the dust
shell, 1/r^2 density distributions could be maintained differing only in their
amplitude A. The present-day mass-loss rate was determined to be 1.2 10^-4
Msol/yr. The inner embedded superwind shell corresponds to a phase of enhanced
mass-loss which began ~59yr ago and lasted for ~18yr, and the outer superwind
region to a high mass-loss period which terminated 529yr ago.


title: FUSE Limits on FUV Emission from Warm Gas in Clusters of Galaxies

abstract:
  We have obtained FUV spectra of two clusters of galaxies with FUSE, the Far
Ultraviolet Spectroscopic Explorer. The Coma cluster was observed for a total
of 28.6 ksec, the Virgo cluster for 10.9 ksec. Neither spectrum shows
significant O VI (1032, 1038 A) emission at the cluster redshift. Such emission
would be expected from the warm [(5-10) x 10^5 K] component of the intracluster
medium that has been proposed to explain the excess EUV and SXR flux present in
EUVE and ROSAT observations of these clusters. Our 2-sigma upper limits on the
O VI 1032 A flux from Coma and Virgo exclude all published warm-gas models of
the EUV excess in these clusters.


title: Decay of cosmological constant as Bose condensate evaporation

abstract:
  We consider the process of decay of symmetric vacuum state as evaporation of
a Bose condensate of physical Higgs particles, defined over asymmetric vacuum
state. Energy density of their selfinteraction is identified with cosmological
constant $\Lambda$ in the Einstein equation. $\Lambda$ decay then provides
dynamical realization of spontaneous symmetry breaking. The effective mechanism
is found for damping of coherent oscillations of a scalar field, leading to
slow evaporation regime as the effective mechanism for $\Lambda$ decay
responsible for inflation without special fine-tuning of the microphysical
parameters. This mechanism is able to incorporate reheating, generation of
proper primordial fluctuations, and nonzero cosmological constant today.


title:
How to distinguish a nearly flat Universe from a flat Universe using the
  orientation independence of a comoving standard ruler

abstract:
  Several recent observations using standard rulers and standard candles now
suggest, either individually or in combination, that the Universe is close to
flat, i.e. that the curvature radius is about as large as the horizon radius
(\sim 10h^{-1}Gpc) or larger. Here, a method of distinguishing an almost flat
universe from a precisely flat universe using a single observational data set,
without using any microwave background information, is presented. The method
(i) assumes that a standard ruler should have no preferred orientation (radial
versus tangential) to the observer, and (ii) requires that the (comoving)
length of the standard ruler be known independently (e.g. from low redshift
estimates). The claimed feature at fixed {\em comoving} length in the power
spectrum of density perturbations, detected among quasars, Lyman break galaxies
or other high redshift objects, would provide an adequate standard candle to
prove that the Universe is curved, if indeed it is curved. For example, a
combined intrinsic and measurement uncertainty of 1% in the length of the
standard ruler $\llss$ applied at a redshift of $z=3$ would distinguish an
hyperbolic $(\Omega_m=0.2,\Omega_\Lambda=0.7)$ or a spherical
$(\Omega_m=0.4,\Omega_\Lambda=0.7)$ universe from a flat one to 1-P > 95%
confidence.


title: Are there any Young Galaxies in our local universe?

abstract:
  We find no evidence for the presence of genuinely young galaxies in our Local
universe. We argue that the metal deficiencies and the behavior of the N/O and
C/O ratios meet several plausible explanations involving the primary and
secondary nature of these elements and their dispersion and mixing by massive
stars and intermediate mass stars. Colors are to be analysed using surface
photometry and comparing several model scenario since an instant burst model
unavoidably provides a lower limit to the age. Ultimately colour-magnitude
diagram techniques will be the only direct test to be performed at least on a
small number local starburst galaxies. Our present analysis provides important
warning for assessing the true age of distant galaxies.


title: Inner magnetospheric accelerators in high magnetic field pulsars

abstract:
  Photon splitting is a QED process that can potentially suppress pair
production in the inner magnetosphere of a pulsar with a super-strong magnetic
field, and hence, may quench radio emission from these objects. While it is
unknown how many splitting modes really operate in super-critical fields, we
derive gap parameters of a high magnetic field pulsar for both the vacuum-type
gap and the space-charge-limited flow accelerator under the assumption that all
three splitting modes permitted by QED operate. The competition between photon
splitting and pair production depends on the gap parameters, and the
``photon-splitting-dominant'' line for both cases are derived. We discuss the
implications of these results and the possible connection of the high magnetic
field pulsars with the anomalous X-ray pulsars and soft gamma-ray repeaters
which are conjectured to be magnetars.


title: On the nature of soft Gamma-ray repeaters

abstract:
  Soft gamma-ray repeaters (SGRs) and Anomalous X-ray pulsars (AXPs) are
generally accepted to be magnetars. Recently, Zhang, Xu & Qiao (2000, ApJ, 545,
L127) proposed an alternative viewpoint about the nature of the SGRs (and
AXPs). In this picture, SGR bursts are attributed to the impacts of some
comet-like objects with the central bare strange star. Here I briefly review
this model, and confront it with the detailed observations of SGRs/AXPs. A
comparison to the magnetar model is also presented. Some theoretical issues
concerning the nature of the SGRs (and AXPs) are discussed.


title: On the comoving distance as an arc-length in four dimensions

abstract:
  The inner product provides a conceptually and algorithmically simple method
for calculating the comoving distance between two cosmological objects given
their redshifts, right ascension and declination, and arbitrary constant
curvature. The key to this is that just as a distance between two points `on'
the surface of the ordinary 2-sphere ${\cal S}^2$ is simply an arc-length
(angle multiplied by radius) in ordinary Euclidean 3-space ${\cal E}^3$, the
distance between two points `on' a 3-sphere ${\cal S}^3$ (a 3-hyperboloid
${\cal H}^3$) is simply an `arc-length' in Euclidean 4-space ${\cal E}^4$
(Minkowski 4-space ${\cal M}^4$), i.e. an `hyper-angle' multiplied by the
curvature radius of the 3-sphere (3-hyperboloid).


title: RXTE Observations of Hercules X-1 During the July 1998 Short-high State

abstract:
  We present RXTE monitoring of the eclipsing X-ray binary Hercules X-1
conducted over the short-high state of July 1998. This was one of the last
major short-high states before the source entered an anomalous low-state of
activity. A comparison with previous epochs finds no evidence for special
behavior during these observations. We determine orbital and pulsar spin
periods to facilitate measurements period derivatives during the subsequent
anomalous low state and during the next epoch of high-state activity.
Spectrally, the decay of the short-high state and concurrent pre-eclipse dips
are consistent with obscuration of a central X-ray source by a cloud of
non-uniform column density. The standard model of a warped accretion disk of
finite vertical scale height fits the characteristics of this absorber well.
Pre-eclipse dips have durations a factor of a few longer than the
characteristic durations of dips during main-high states. Pulse profile
structure increases in complexity towards the tail of the short-high state
suggesting changes in accretion curtain geometry.


title:
Correlation function of ultra-high energy cosmic rays favors point
  sources

abstract:
  We calculate the angular two-point correlation function of ultra-high energy
cosmic rays (UHECR) observed by AGASA and Yakutsk experiments. In both data
sets, there is a strong signal at highest energies, which is concentrated in
the first bin of the size of the angular resolution of the experiment. For the
uniform distribution of sources, the probability of a chance clustering is 4 x
10^{-6}. Correlations are absent or not significant at larger angles. This
favors the models with compact sources of UHECR.


title: How to make CMB maps from huge timelines with small computers

abstract:
  We present in this article two different ways to make CMB maps in practice,
from large timelines. One is to make a simple destriping, fitting the data and
using the scan intercepts to remove the low frequency noise (stripes). The
second, optimal, is to resolve linearly the map-making problem, which in case
of big timelines must be simplified and changed from matrices to vectors for
the calculations. Assuming few conditions on the noise, it is possible to make
fast map-making tools.


title:
Multiwavelength observations revealing the outbursts of the two soft
  X-ray transients XTE J1859+226 and XTE J1118+480

abstract:
  We report here on multiwavelength observations of the two new soft X-ray
transients (SXTs) XTE J1859+226 and XTE J1118+480, which we observed with
HST/RXTE/UKIRT. For XTE J1118+480 we also used EUVE since it is located at a
very high galactic latitude and suffers from very low extinction. The two
sources exhibited very different behaviour. XTE J1859+226 seems quite normal
and therefore a good object for testing the accretion mechanisms in place
during the outbursts, XTE J1118+480 is much more unusual because it exhibits i)
a low X-ray to optical ratio and ii) a strong non-thermal contribution in the
radio to optical domain, which is likely to be due to synchrotron emission. We
concentrate here on the near-infrared (NIR) and optical observations of these
two systems.


title:
Analysis of ProNaOS submillimeter maps in the M42 Orion Nebula -
  Temperature - spectral index inverse correlation in several regions

abstract:
  We have mapped with the French submillimeter balloon-borne telescope ProNaOS
the thermal emission of the dust in the M42 Orion Nebula. The map obtained is
50' by 40' with an angular resolution of about 3' in the four efficient
wavelengths 200 $\mic$, 260 $\mic$, 360 $\mic$ and 580 $\mic$. The temperature
and index we obtain are highly variable within the studied region: the
temperature varies from 12 K to 70 K, and the spectral index from 1.1 to 2.2.
The statistical analysis of the temperature and spectral index spatial
distribution in this region, as well as in the other regions mapped by ProNaOS,
shows an evidence of an inverse correlation between these two parameters. We
deduce the column densities of gas and the masses. Being invisible in the IRAS
100 $\mic$ survey, some cold clouds are likely to be the seeds for future star
formation activity going on in the complex.


title: On the nature of the microquasar V4641 Sagittari

abstract:
  We present photometric and spectroscopic optical and near-infrared (NIR)
observations (Based on observations collected at the European Southern
Observatory, Chile (ESO ID 63.H-0493 and 64.H-0382)) taken during the outburst
of the recently flaring source V4641 Sgr = SAX J1819.3-2525 (in't Zand et al.,
2000), on September 1999. This source was independently detected as the RXTE
source XTE J1819-254 (Markwardt et al. 1999), and afterwards identified with
the variable object V4641 Sgr (Kato et al. 1999). It underwent a bright optical
outburst on 1999 Sept. 15.7 UT, going from magnitude 14 to 8.8 in the V-band
(Stubbings 1999), and reaching 12.2 Crab in the X-rays (Smith et al. 1999) and
Ks nearly 13. This outburst was therefore bright, but very brief, with an
e-fold decay time of 0.6 days. A radio source was resolved, making of V4641 Sgr
a new microquasar (Hjellming et al. 2000). We discuss the nature of this
system, showing that our observations suggest a distance farther than
previously derived from the radio observations (Hjellming et al. 2000). The
distance of the system would be between 3 and 8 kpc, the companion star being a
B3-A2 main sequence star. Another possibility is that the companion star is
crossing the Hertzsprung gap (type B3-A2 IV), and in this case the distance
cited above would be the minimum distance of the system. The system is
therefore an Intermediate or High Mass X-ray Binary System (IMXB or HMXB). The
inconsistency regarding the distance between the radio and optical/NIR
observations could be explained by the detection of an interaction between
matter ejected before the X-ray outburst and the surrounding medium of the
source. If this is confirmed, this source could be added to the short list of
microquasars where such an interaction has been detected.


title: 10^20 eV Cosmic Ray and Particle Physics with IceCube

abstract:
  We show that a kilometer-scale neutrino observatory, though optimized for
detecting neutrinos of TeV to PeV energy, can reveal the science associated
with the enigmatic super-EeV radiation in the Universe. Speculations regarding
its origin include heavy relics from the early Universe, particle interactions
associated with the Greisen cutoff, and topological defects which are remnant
cosmic structures associated with phase transitions in grand unified gauge
theories. We show that it is a misconception that new instruments optimized to
EeV energy can exclusively do this important science. Because kilometer-scale
neutrino telescopes such as IceCube can reject the atmospheric neutrino
background by identifying the very high energy of the signal events, they have
sensitivity over the full solid angle, including the horizon where most of the
signal is concentrated. This is critical because upgoing neutrino-induced
muons, considered in previous calculations, are absorbed by the Earth. Previous
calculations have underestimated the event rates of IceCube for EeV signals by
over one order of magnitude.


title:
K-shell dielectronic resonances in photoabsorption: differential
  oscillator strengths for Li-like C IV, O VI, and Fe XXIV

abstract:
  Recently X-ray photoabsorption in KLL resonances of O VI was predicted
[Pradhan, Astrophys.J. Lett. 545, L165 (2000)], and detected by the Chandra
X-ray Observatory [Lee et al, Astrophys. J. {\it Lett.}, submitted].
  The required resonance oscillator strengths f_r, are evaluated in terms of
the differential oscillator strength df/de that relates bound and continuum
absorption. We present the f_r values from radiatively damped and undamped
photoionization cross sections for Li-like C,O, and Fe calculated using
relativistic close coupling Breit-Pauli R-matrix method. The KLL resonances of
interest here are: 1s2p (^3P^o) 2s [^4P^o_{1/2,3/2}, ^2P^o_{1/2,3/2}] and 1s2p
(^1P^o) 2s [^2P^o_{1/2,3/2}]. The KLL photoabsorption resonances in Fe XXIV are
fully resolved up to natural autoionization profiles for the first time. It is
demonstrated that the undamped f_r independently yield the resonance radiative
decay rates, and thereby provide a precise check on the resolution of
photoionization calculations in general. The predicted photoabsorption features
should be detectable by the X-ray space observatories and enable column
densities in highly ionized astrophysical plasmas to be determined from the
calculated f_r. The dielectronic satellites may appear as redward broadening of
resonances lines in emission and absorption.


title: X-ray Emitting Groups in the Infall Region of Abell 2199

abstract:
  Using a large redshift survey covering 95 square degrees, we demonstrate that
the infall region of Abell 2199 contains Abell 2197, one or two X-ray emitting
groups, and up to five additional groups identified in redshift surveys. Our
survey shows that the X-ray emitting systems, located at projected radii of
$1.^\circ4, 1.^\circ9$, and $5.^\circ1$ (2.2, 3.1, and $8.0 h^{-1}$Mpc), are
connected kinematically to A2199. A2197 is itself an optically rich cluster;
its weak X-ray emission suggests that it is much less massive than A2199. The
absence of a sharp peak in the infall pattern at the position of A2197 supports
this hypothesis. The outermost group is well outside the virial region of A2199
and it distorts the infall pattern in redshift space. The two X-ray emitting
groups are roughly colinear, suggesting the existence of an extended ($8.0
h^{-1}$Mpc) filament. The identification of these infalling groups provides
direct support of hierarchical structure formation; studies of these systems
will provide insights into structure evolution. Groups in the infall regions of
nearby clusters may offer a unique probe of the physics of the warm/hot ionized
medium (WHIM) which is difficult to observe directly with current instruments.


title:
Induced Gravity Model Based on External Impinging Neutrinos: Calculation
  of G in Terms of Collision Phenomena and Inferences to Inertial Mass and
  Atomic Quantization

abstract:
  Herein, we present a particle-based mechanism and mathematical formulation of
gravity, focusing on the neutrino as the gravity-inducing particle. The
mechanism is based on the primacy of momentum conservation and postulates an
omni-directional distribution throughout the universe of fast small particles
of finite mass that have a low probability of colliding with nucleons. The
measured acceleration between two neighboring mass bodies results from an
alteration of this distribution caused by nucleons of each body interacting
with some of those particles. Based on findings establishing that the neutrino
has mass, we evaluate the various neutrinos as external particle candidates. We
show that for mass quantities up to several times that of the sun the form of
the time rate of momentum transfer to each body is proportional to the product
of the two body masses because of the probability nature of any collision
process, and inversely proportional to the square of the distance between them
because of the mathematical properties of an altered particle flux. A derived
expression involving the neutrino momentum flux, the neutrino-nucleon collision
cross section, and the nucleon mass replaces the constant G from the classical
gravitational model. The neutrino momentum flux that is required to account for
gravity is so large as to cause us herein to re-evaluate conventional notions
in kinematics and the cause of inertial properties and to examine
neutrino-nucleon collisions as a possible source of electromagnetic standing
waves essential to establish electron shell states. This reasoning indicates
that in a much more massive body that is accreting mass, a coulombic collapse
to a black hole will ensue when external neutrinos lose the ability to
penetrate in sufficient numbers to the central region.


title: Collimated Fast Winds in Wide Binary Progenitors of Planetary Nebulae

abstract:
  We discuss the formation of planetary nebulae (PNe) having a pair of lobes,
or multi-lobes, in their inner region surrounded by an elliptical or spherical
shell or halo. We suggest that most of these PNe are formed by wide binary
systems, with final orbital periods in the range of 40-10,000 years, such that
there is no strong tidal interaction. The outer more spherical structure is
formed from the early asymptotic giant branch (AGB) wind. Toward the end of the
AGB the mass loss rate increases, and wind velocity possibly decreases, making
the conditions for the formation of an accretion disk around the wide
companion, more favorable. We assume that the accreting companion blows jets or
a collimated fast wind (CFW), which lead to the formation of a pair of lobes in
the inner region. In cases of a precessing accretion disk, a multi-lobes
structure can be formed. We conduct a population synthesis study of such
systems, and find that overall 5-20 percents of all PNe are formed by such
binary systems. We also estimate that 20-40 percents of these systems possess
observable departure from axisymmetry, e.g., the central star is not in the
center of the nebula.


title:
Spectral and temporal properties of X-ray emission from the
  ultra-luminous source X-9 in M81

abstract:
  We have analysed the spectra and the variability of individual X-ray sources
in the M-81 field using data from the available ROSAT-PSPC and ROSAT-HRI
observations of this nearby spiral galaxy. Here we present the results on the
second brightest source in the field (X-9 - Fabbiano, 1988 ApJ 325 544), whose
identification and interpretation is still unclear. Our work includes the study
of the shape of X-9 from HRI data, the light curve and hardness ratio
evolution, and the spectral analysis.


title: Time delay in QSO 0957+561 from 1984-99 optical data

abstract:
  Photometric optical data of QSO 0957+561 covering the period 1984-99 are
analyzed to discern between the two values of the time delay (417 and 424 days)
mostly accepted in the recent literature. The observations, performed by groups
from three different institutions-Princeton University, Harvard-Smithsonian
Center for Astrophysics, and Instituto de Astrofisica de Canarias-and including
new unpublished 1998-9 data from the IAC80 Telescope, were obtained in five
filters (V, R, I, g, and r). The different light curves have been divided into
observational seasons and two restriction have been applied to better calculate
the time delay: (i) points with a strange photometric behavior have been
removed; and (ii) data sets without large gaps have been selected. Simulated
data were generated to test several numerical methods intended to compute the
time delay. The methods giving the best results-the discrete correlation
function, delta-square, z-transformed discrete correlation function, and linear
interpolation-were then applied to real data. A first analysis of the 23
different time delays derived from each technique shows that the time delay
must be into the interval 420-424 days. From our statistical study, a most
probable value of 422.6 +/- 0.6 days is inferred.


title:
Deep VLT search for globular clusters in NGC 5128: color-magnitude
  diagrams and globular cluster luminosity function

abstract:
  At the distance of NGC 5128 (3.6 \pm 0.2 Mpc) it is possible to resolve
globular clusters with high resolution imaging from the ground, thus allowing
the globular cluster candidate selection primarily through their morphological
properties. I report the discovery of 71 globular clusters in NGC 5128 on VLT
UT1+FORS1 images, including the faintest members (Mv ~ -5) known to date in
this galaxy as well as 5 previously known clusters. U- and V-band photometry
has been measured for all the candidates and the luminosity function, spanning
-10.1<Mv<-4.9 and -9.3<Mu<-3.3, constructed. These are the deepest globular
cluster luminosity functions in an elliptical galaxy determined so far. The
Kolmogorov-Smirnov statistics show that the difference between the globular
cluster luminosity functions of NGC 5128 and the MW is not larger than the
difference between the ones of M31 and the MW. The (U-V)o color histogram shows
a bimodal distribution. For 23 globular clusters I obtained K-band images with
SOFI at the NTT in La Silla. Their positions in the (U-V) vs.(V-K) color-color
diagram indicate that they are indeed old globular clusters. Assuming that the
globular clusters in NGC 5128 span a similar age range as the ones in the Milky
Way and adopting a linear fit between the metallicity and (U-V)o color, the
metal-rich clusters peak at [Fe/H]=-0.6 dex and the metal-poor ones peak at
[Fe/H]=-1.7 dex.


title:
X-ray states and radio emission in the black hole candidate XTE
  J1550-564

abstract:
  We report on radio and X-ray observations of the black hole candidate (BHC)
XTE J1550-564 performed during its 2000 X-ray outburst. Observations have been
conducted with the Australia Telescope Compact Array (ATCA) and have allowed us
to sample the radio behavior of XTE J1550-564 in the X-ray Low Hard and
Intermediate/Very High states. We observed optically thin radio emission from
XTE J1550-564 five days after a transition to an Intermediate/Very High state,
but we observed no radio emission six days later, while XTE J1550-564 was still
in the Intermediate/Very High state. In the Low Hard state, XTE J1550-564 is
detected with an inverted radio spectrum. The radio emission in the Low Hard
state most likely originates from a compact jet; optical observations suggest
that the synchrotron emission from this jet may extend up to the optical range.
The total power of the compact jet might therefore be a significant fraction of
the total luminosity of the system. We suggest that the optically thin
synchrotron radio emission detected five days after the transition to the
Intermediate/Very High state is due to a discrete ejection of relativistic
plasma during the state transition. Subsequent to the decay of the optically
thin radio emission associated with the state transition, it seems that in the
Intermediate/Very High state the radio emission is quenched by a factor greater
than 50, implying a suppression of the outflow. We discuss the properties of
radio emission in the X-ray states of BHCs.


title: The Cannonball Model of Gamma Ray Bursts: Lines in the X-Ray Afterglow

abstract:
  Recent observations suggest that gamma-ray bursts (GRBs) and their afterglows
are produced by jets of highly relativistic cannonballs (CBs), emitted in
supernova explosions. The fully ionized CBs cool to a temperature below 4500 K
within a day or two, at which point electron-proton recombination produces an
intense Lyman-$\alpha$ emission. The line energy is Doppler-shifted by the CBs'
motion to X-ray energies in the observer's frame. The measured line energies,
corrected for their cosmological redshift, imply Doppler factors in the range
600 to 1000, consistent with those estimated -in the CB model- from the
characteristics of the $\gamma$-ray bursts. All other observed properties of
the lines are also well described by the CB model. Scattering and
self-absorption of the recombination lines within the CB also produce a
wide-band flare-up in the GRB afterglow, as the observations indicate. A very
specific prediction of the CB model is that the X-ray lines ought to be narrow
and move towards lower line energies as they are observed: their current
apparently large widths would be the effect of time integration, and/or of the
blending of lines from CBs with different Doppler factors.


title:
The far-infrared--submm spectral energy distribution of high-redshift
  quasars

abstract:
  We combine submm photometric data of z>4 quasars, to obtain a mean
far-infrared (FIR) (rest-frame) spectral energy distribution (SED) of thermal
emission from dust, parameterised by a single temperature (T) and power-law
emissivity index (beta). Best-fit values are T=41+/-5K, beta=1.95+/-0.3. The
redshift spread of this set of quasars is exploited to allow us to sample the
SED at a greater number of rest wavelengths than is possible for a single
object. This parameterisation is of use to any study that extrapolates from a
flux at a single submm wavelength, for example to infer dust masses and FIR
luminosities. We then interpret the submm component as arising from dust heated
by star-formation in the quasar's host galaxy, and investigate a simple scheme
of AGN--host coevolution, in which the timescale for formation of the host
galaxy is c.0.5-1.0Gyr, with star formation proceeding at a constant rate
c.1000Msol/yr. The luminous quasar phase occurs towards the end of the
star-forming period, just before the galaxy's reservoir of cold gas is
depleted. Given the youth of the Universe at z=4 (1.6Gyr), the coexistence of a
massive black hole and a luminous starburst can be a powerful constraint on
models of quasar host-galaxy formation.


title: How big were the first cosmological objects?

abstract:
  We calculate the cooling times at constant density for halos with virial
temperatures from 100 K to 10^5 K that originate from a 3-sigma fluctuation of
a CDM power spectrum in three different cosmologies. Our intention is to
determine the first objects that can cool to low temperatures, but not to
follow their dynamical evolution. We identify two generations of halos: those
with low virial temperatures, Tvir < 9000 K that remain largely neutral, and
those with larger virial temperatures that become ionized. The
lower-temperature, lower-mass halos are the first to cool to 75 percent of
their virial temperature. The precise temperature and mass of the first objects
are dependent upon the molecular hydrogen (H2) cooling function and the
cosmological model. The higher-mass halos collapse later but, in this paradigm,
cool much more efficiently once they have done so, first via electronic
transitions and then via molecular cooling: in fact, a greater residual
ionization once the halos cool below 9000 K results in an enhanced H2
production and hence a higher cooling rate at low temperatures than for the
lower-mass halos, so that within our constant-density model it is the former
that are the first to cool to really low temperatures. We discuss the possible
significance of this result in the context of CDM models in which the shallow
slope of the initial fluctuation spectrum on small scales leads to a wide range
of halo masses (of differing overdensities) collapsing over a small redshift
interval. This ``crosstalk'' is sufficiently important that both high- and
low-mass halos collapse during the lifetimes of the massive stars which may be
formed at these epochs. Further investigation is thus required to determine
which generation of halos plays the dominant role in early structure formation.


title: The origin of the runaway high-mass X-ray binary HD153919/4U1700-37

abstract:
  Based on its Hipparcos proper motion, we propose that the high-mass X-ray
binary HD153919/4U1700-37 originates in the OB association Sco OB1. At a
distance of 1.9 kpc the space velocity of 4U1700-37 with respect to Sco OB1 is
75 km/s. This runaway velocity indicates that the progenitor of the compact
X-ray source lost about 7 Msun during the (assumed symmetric) supernova
explosion. The system's kinematical age is about 2 +/- 0.5 million years which
marks the date of the supernova explosion forming the compact object. The
present age of Sco OB1 is <8 Myr; its suggested core, NGC 6231, seems to be
somewhat younger (~5 Myr). If HD153919/4U1700-37 was born as a member of Sco
OB1, this implies that the initially most massive star in the system terminated
its evolution within <6 million years, corresponding to an initial mass >30
Msun. With these parameters the evolution of the binary system can be
constrained.


title: The Influence of Hydrodynamical Winds on Hot Accretion Disk Solutions

abstract:
  The effect of a possible hydrodynamical wind on the nature of hot accretion
disk solutions is studied. It is found that the advection dominated branch, in
the presence of a wind, maintains the self-similar solution for the disk
structure with the temperature, $\theta \propto 1/r$, optical depth $\tau
\propto r^{P-1/2}$ and accretion rate $\dot m \propto r^P$. Based on global
solutions, cooling due to wind energy loss and advection are found to be
equally important. For a wide range of viscosity and wind parameters, the
temperature is about one-tenth of the virial value and $P \approx 0.9$,
independent of the mass accretion rate and radius. In the context of cooling by
unsaturated Comptonization of soft photons, solutions also exist where
radiative cooling, advection, and wind cooling are important. In this case,
wind regulated solutions are possible. Here, the radial dependence of the
critical mass accretion rate above which solutions do not exist is unchanged
from those solutions without winds. The wind/advection dominated solutions are
locally unstable to a new type of instability called ``wind driven insbility''
where the presence of a wind causes the disk to be unstable to long wavelength
perturbations of the surface density. The growth rate of this instability is
inversely proportional to the ratio of the radiative cooling to the
gravitational energy dissipation rates and it can grow on a timescale much
longer than the viscous timescale in the disk for sufficiently small radiative
cooling efficiencies.


title: A Determination of the Local Density of White Dwarf Stars

abstract:
  The most recent version of the Catalog of Spectroscopically Identified White
Dwarfs lists 2249 white dwarf stars. Among these stars are 118 white dwarfs
that have either reliable trigonometric parallaxes or color-based distance
moduli which place them at a distance within 20 pc of the Sun. Most of these
nearby white dwarfs are isolated stars, but 35 (30 % of the sample) are in
binary systems, including such well known systems as Sirius A/B, and Procyon
A/B. There are also three double degenerate systems in this sample of the local
white dwarf population. The sample of local white dwarfs is largely complete
out to 13 pc and the local density of white dwarf stars is found to be (5.5 +/-
0.8) x 10^-3 pc^-3 with a corresponding mass density of (3.7 +/- 0.5) x10^-3 M
pc^-3.


title: Accretion in stellar clusters and the IMF

abstract:
  We present a simple physical mechanism that can account for the observed
stellar mass spectrum for masses $\ms \simgreat 0.5 \solm$. The model depends
solely on the competitive accretion that occurs in stellar clusters where each
star's accretion rate depends on the local gas density and the square of the
accretion radius. In a stellar cluster, there are two different regimes
depending on whether the gas or the stars dominate the gravitational potential.
When the cluster is dominated by cold gas, the accretion radius is given by a
tidal-lobe radius. This occurs as the cluster collapses towards a $\rho\propto
R^{-2}$ distribution. Accretion in this regime results in a mass spectrum with
an asymptotic limit of $\gamma=-3/2$ (where Salpeter is $\gamma=-2.35$). Once
the stars dominate the potential and are virialised, which occurs first in the
cluster core, the accretion radius is the Bondi-Hoyle radius. The resultant
mass spectrum has an asymptotic limit of $\gamma=-2$ with slightly steeper
slopes ($\gamma\approx-2.5$) if the stars are already mass-segr egated.
Simulations of accretion onto clusters containing 1000 stars show that as
expected, the low-mass stars accumulate the majority of their masses during the
gas dominated phase whereas the high-mass stars accumulate the majority of
their massed during the stellar dominated phase. This results in a mass
spectrum with a relatively shallow $\gamma\approx 3/2$ power-law for low-mass
stars and a steeper, power-law for high-mass stars $ -2.5\simless\gamma\le -2$.
This competitive accretion model also results in a mass segregated cluster.


title: Observations and theoretical models of gamma-ray bursts

abstract:
  GRBs have remained a puzzle for many high-energy astrophysicists since their
discovery in 1967. With the advent of the X-ray satellites BeppoSAX and
RossiXTE, it has been possible to carry out deep multi-wavelength observations
of the counterparts associated with the long GRBs class just within a few hours
of occurence, thanks to the observation of the fading X-ray emission that
follows the more energetic gamma-ray photons once the GRB event has ended. The
fact that this emission (the afterglow) extends at longer wavelengths, has led
to the discovery optical/IR/radio counterparts in 1997-2000, greatly improving
our understanding of these sources. Now it is widely accepted that GRBs
originate at cosmological distances with energie releases of 10^51-10^53 ergs.
About 25 host galaxies have been detected so far for long-duration GRBs. The
observed afterglow satisfies the predictions of the "standard" relativistic
fireball model, and the central engines that power these extraordinary events
are thought to be the collapse of massive stars rather than the merging of
compact objects as previously also suggested. Short GRBs still remain a mystery
as no counterparts have been detected.


title:
DIRECT Distances to Nearby Galaxies Using Detached Eclipsing Binaries
  and Cepheids. VI. Variables in the Central Part of M33

abstract:
  The DIRECT project aims to determine direct distances to two important
galaxies in the cosmological distance ladder - M31 and M33 - using detached
eclipsing binaries (DEBs) and Cepheids.
  We present the results of the first large-scale CCD-based search for
variables in M33. We have observed two fields located in the central region of
M33 for a total of 95 nights on the F. L. Whipple Observatory 1.2-m telescope
and 36 nights on the Michigan-Dartmouth-MIT 1.3-m telescope. We have found a
total of 544 variables, including 251 Cepheids and 47 eclipsing binaries.
  The catalog of variables is available online, along with finding charts and
BVI light curve data (consisting of 8.2x10^4 individual measurements). The
complete set of CCD frames is available upon request.


title: Exploring the Small Scale Structure of N103B

abstract:
  We present the preliminary results of a 40.8 ks Chandra ACIS observation of
the young supernova remnant (SNR) N103B located in the Large Magellanic Cloud.
The image reveals structure at the sub-arcsecond level, including several
bright knots and filamentary structures. The remnant has the characteristic
spectrum of a Type Ia SNR, containing strong lines of Fe, He-and H-like Si and
S, Ar, and Ca. Narrow band images reveal non-uniformities in the remnant.


title:
NICMOS Observations of Extragalactic Cepheids. I. Photometry Database
  and a Test of the Standard Extinction Law

abstract:
  We present the results of near-infrared observations of extragalactic
Cepheids made with the Near Infrared Camera and Multi-Object Spectrometer on
board the Hubble Space Telescope. The variables are located in the galaxies IC
1613, IC 4182, M 31, M 81, M 101, NGC 925, NGC 1365, NGC 2090, NGC 3198, NGC
3621, NGC 4496A and NGC 4536. All fields were observed in the F160W bandpass;
additional images were obtained in the F110W and F205W filters. Photometry was
performed using the DAOPHOT II/ALLSTAR package.
  Self-consistent distance moduli and color excesses were obtained by fitting
Period-Luminosity relations in the H, I and V bands. Our results support the
assumption of a standard reddening law adopted by the HST Key Project on the
Extragalactic Distance Scale. A companion paper will determine true distance
moduli and explore the effects of metallicity on the Cepheid distance scale.


title: Astronomical engineering: a strategy for modifying planetary orbits

abstract:
  The Sun's gradual brightening will seriously compromise the Earth's biosphere
within ~ 1E9 years. If Earth's orbit migrates outward, however, the biosphere
could remain intact over the entire main-sequence lifetime of the Sun. In this
paper, we explore the feasibility of engineering such a migration over a long
time period. The basic mechanism uses gravitational assists to (in effect)
transfer orbital energy from Jupiter to the Earth, and thereby enlarges the
orbital radius of Earth. This transfer is accomplished by a suitable
intermediate body, either a Kuiper Belt object or a main belt asteroid. The
object first encounters Earth during an inward pass on its initial highly
elliptical orbit of large (~ 300 AU) semimajor axis. The encounter transfers
energy from the object to the Earth in standard gravity-assist fashion by
passing close to the leading limb of the planet. The resulting outbound
trajectory of the object must cross the orbit of Jupiter; with proper timing,
the outbound object encounters Jupiter and picks up the energy it lost to
Earth. With small corrections to the trajectory, or additional planetary
encounters (e.g., with Saturn), the object can repeat this process over many
encounters. To maintain its present flux of solar energy, the Earth must
experience roughly one encounter every 6000 years (for an object mass of 1E22
g). We develop the details of this scheme and discuss its ramifications.


title: Multi-Epoch Multiwavelength Spectra and Models for Blazar 3C~279

abstract:
  Of the blazars detected by EGRET in GeV gamma rays, 3C 279 is not only the
best-observed by EGRET, but also one of the best-monitored at lower
frequencies. We have assembled eleven spectra, from GHz radio through GeV gamma
rays, from the time intervals of EGRET observations. Although some of the data
have appeared in previous publications, most are new, including data taken
during the high states in early 1999 and early 2000. All of the spectra show
substantial gamma-ray contribution to the total luminosity of the object; in a
high state, the gamma-ray luminosity dominates over that at all other
frequencies by a factor of more than 10. There is no clear pattern of time
correlation; different bands do not always rise and fall together, even in the
optical, X-ray, and gamma-ray bands.
  The spectra are modeled using a leptonic jet, with combined synchrotron
self-Compton + external Compton gamma-ray production. Spectral variability of
3C 279 is consistent with variations of the bulk Lorentz factor of the jet,
accompanied by changes in the spectral shape of the electron distribution. Our
modeling results are consistent with the UV spectrum of 3C 279 being dominated
by accretion disk radiation during times of low gamma-ray intensity.


title: The Structure and Star-Formation History of NGC 5461

abstract:
  We compute photoionization models for the giant extragalactic H II region NGC
5461, and compare their predictions to several observational constraints. Since
we aim at reproducing not only the global properties of the region, but its
local structure also, the models are constrained to reproduce the observed
density profile, and our analysis takes into consideration the bias introduced
by the shapes and sizes of the slits used by different observers. We find that
an asymmetric nebula with a gaussian density distribution, powered by a young
burst of 3.1 Myr, satisfactorily reproduces most of the constraints, and that
the star-formation efficiency inferred from the model agrees with current
estimates. Our results strongly depend on the assumed density law, since
constant density models overestimate the hardness of the ionizing field,
affecting the deduced properties of the central stellar cluster. We illustrate
the features of our best model, and discuss the possible sources of errors and
uncertainties affecting the outcome of this type of studies.


title: X-ray Observations of Young Supernova Remnants

abstract:
  This brief review of recent X-ray observations of young supernova remnants
highlights results obtained by the Chandra and XMM Newton Observatories since
their launch last year. Their impressive capabilities are illustrated by
results for spectral imaging, and for spatially resolved spectroscopy that
isolates emission from individual ejecta knots and from the forward shock. I
also review X-ray dynamical studies of supernova remnants, which should undergo
significant advances during this new era.


title:
A VLA H92alpha Study of the Arched Filament Complex Near the Galactic
  Center

abstract:
  The VLA has been used at 8.3 GHz in the DnC and CnB array configurations to
carry out an H92alpha recombination line study (at 8.3 GHz) of the ionized gas
in the Arched Filaments H II complex, which defines the western edge of the
Galactic center Radio Arc. The H92alpha line properties of the ionized gas are
consistent with photoionization from hot stars,and consistent with the physical
properties of other Galactic center H II regions. The LTE electron temperatures
vary only slightly across the entire extent of the source, and have an average
value of 6200 K. The velocity field is very complex, with velocities ranging
from +15 to - 70 km/s and the majority of velocities having negative values.
Large velocity gradients (2-7 km/s/pc, with gradients in some regions >10
km/s/pc) occur along each of the filaments, with the velocities becoming
increasingly negative with decreasing distance from the Galactic center. The
magnitudes of the velocity gradient are consistent with the cloud residing on
an inner, elongated orbit which is due to the Galaxy's stellar bar, or with a
radially infalling cloud. The ionization of the Arched Filaments can be
accounted for completely by the massive Arches stellar cluster, which consists
of > 150 O-stars. This cluster is likely to belocated 10-20 pc from the Arched
Filaments, which can explain the uniformity of ionization conditions in the
ionized gas.


title: Binary Masses as a Test for Pre-Main-Sequence Models

abstract:
  Observations of binaries have traditionally provided the means for
ascertaining stellar masses. Here, we use the published data on 8
pre-main-sequence pairs to gauge the accuracy of our own, recently calculated,
evolutionary tracks (Palla & Stahler 1999). We consider both eclipsing,
double-lined spectroscopic binaries, which provide the mass of each star
separately, and non-eclipsing, double-lined systems, which yield only the
ratio. We also analyze the visual, quadruple system GG Tau, for which the sum
of the two component masses follows from observations of the circumbinary disk.
  In almost all cases, our theoretically derived masses or mass ratios are in
good agreement with the empirical values. For two binaries (NTTS 162814-2427
and P1540), the observational results are still too uncertain for a proper
comparison. We also find that the derived contraction ages within each
pre-main-sequence pair are nearly equal. This result extends earlier findings
regarding visual pairs, and indicates that the components of all binaries form
in proximity, perhaps within the same dense cloud core. Finally, our study
reveals that the Trapezium star BM Ori is very young, since both the star
itself and its companion have contraction ages less than 10^5 years.


title: Public Imaging Surveys: Scientific Opportunities

abstract:
  The start of operation of several large-aperture telescopes has motivated
several groups around the world to conduct deep large imaging surveys,
complementing other wide-area but shallower surveys. A special class of imaging
surveys are the public ones such as those being conducted by the ESO Imaging
Survey (EIS) project established with the primary objective of providing to the
ESO community large datasets from which samples can be drawn for VLT programs.
To date several surveys have been carried out providing multi-passband optical
data over relatively large areas to moderate depth, deep optical/infrared
surveys of smaller fields as well as observations of a large number of selected
stellar fields. These surveys have produced a large amount of well-defined
datasets with known limits from which a variety of data products have been
extracted and distributed. The goal of the present contribution is to give an
overview of the EIS project, to present the end-to-end survey system being
developed by this program and to highlight the effort being made to standardize
procedures, to build software tools to assess the quality of the derived
products and to explore the available datasets in a systematic and consistent
way to search for astronomical objects of potential scientific interest.


title:
Central Matter Distributions in Rich Clusters of Galaxies from z~0 to
  z~0.5

abstract:
  We have analyzed the galaxy number density and luminosity density profiles of
rich clusters of galaxies from redshifts z=0 to z=0.5. We show that the
luminosity profile computed with bright galaxies is significantly cusped in the
center of the clusters, whatever the redshift. This is in agreement with the
dark matter profiles predicted by numerical simulations. The galaxy number
density profile for the bright galaxies is fitted equally well with a core
model or a cusped model. In contrast, the luminosity and the galaxy number
density profiles of the fainter galaxies are significantly better fitted by a
core rather than a cusp model. We did not detect any statistically significant
different fits when applied to data in range from z=0 to z=0.5. The difference
in profile between faint and bright galaxies may be due to the rapid (relative
to the age of the universe at z=0 versus z=0.5) destruction of the faint
galaxies by tidal forces and merging events in the denser central regions of
the clusters. This process could erase the cusp by turning faint galaxies into
diffuse light. In this case, the galaxies (with a cusp visible in the bright
galaxy number density and mainly in luminosity profiles) would trace the total
mass distribution.


title: Abundance Gradients and the Formation of the Milky Way

abstract:
  In this paper we adopt a chemical evolution model, which is an improved
version of the Chiappini, Matteucci and Gratton (1997) model, assuming two main
accretion episodes for the formation of the Galaxy. The present model takes
into account in more detail than previously the halo density distribution and
explores the effects of a threshold density in the star formation process,
during both the halo and disk phases. In the comparison between model
predictions and available data, we have focused our attention on abundance
gradients as well as gas, stellar and star formation rate distributions along
the disk. We suggest that the mechanism for the formation of the halo leaves
detectable imprints on the chemical properties of the outer regions of the
disk, whereas the evolution of the halo and the inner disk are almost
completely disentangled. This is due to the fact that the halo and disk
densities are comparable at large Galactocentric distances and therefore the
gas lost from the halo can substantially contribute to building up the outer
disk. We also show that the existence of a threshold density for the star
formation rate, both in the halo and disk phase, is necessary to reproduce the
majority of observational data in the solar vicinity and in the whole disk.
Moreover, we predict that the abundance gradients along the Galactic disk must
have increased with time and that the average [alpha/Fe] ratio in stars (halo
plus disk) slightly decrease going from 4 to 10 Kpcs from the Galactic center.
We also show that the same ratios increase substantially towards the outermost
disk regions and the expected scatter in the stellar ages decreases, because
the outermost regions are dominated by halo stars.


title:
Evolution on the AGB and beyond: On the formation of H-deficient
  post-AGB stars

abstract:
  The evolution on the AGB and beyond is reviewed with respect to the origin of
Wolf-Rayet central stars. We focus on thermal pulses due to their particular
importance for the evolution of hydrogen deficient stars. It is shown that
overshoot applied to all convection regions is a key ingredient to model these
objects leading to intershell abundances already close to the surface
abundances of Wolf-Rayet central stars. In contrast to standard evolutionary
calculations, overshoot models do show dredge up for very low envelope masses
and efficient dredge up was found even during the post-AGB stage. Three thermal
pulse scenarios for Wolf-Rayet central stars can now be distinguished: an AGB
Final Thermal Pulse (AFTP) occurring at the very end of the AGB evolution, a
Late Thermal Pulse (LTP) occurring during the post-AGB evolution when hydrogen
burning is still on, and a Very Late Thermal Pulse (VLTP) occurring on the
cooling branch when hydrogen burning has already ceased. All scenarios lead to
hydrogen-deficient post-AGB stars with carbon and oxygen abundances as observed
for Wolf-Rayet stars. Hydrogen is either diluted by dredge up (AFTP, LTP) or
completely burnt (VLTP).


title: Galaxy Associations within the Coma Cluster

abstract:
  The mean redshift of the core of the Coma cluster - cz = 6953 km s^{-1} and
its dispersion \sigma=949 km s^{-1} are obtained by means of the analysis of
the substructures of this cluster by using the S-tree method. The existence of
three subgroups of galaxies is revealed, one of them is associated with the cD
galaxy NGC 4874, the other -- with NGC 4889. It is argued that these subgroups
are {\it galaxy associations}, i.e. galactic dynamical entities moving within
the main cluster. Thus we conclude that the non-stationarity of the dynamical
processes ongoing in the Coma core is due to the merging of small-scale galaxy
associations, rather than of two equal sized clusters. We provide the lists of
the galaxies of the associations, the observational study of which can be of
particular interest.


title: Non-axisymmetric accretion on the classical TTS RW Aur A

abstract:
  (Abridged) High-resolution spectroscopic monitoring of RW Aur A was carried
out in 1996, 1998 and 1999 with simultaneous B, V photometry. A multicomponent
spectrum is revealed with a veiled photospheric spectrum, broad emissions,
narrow emission lines of helium, and accretion, wind and shell features.
  Periodic modulations in many spectral features were found. The photospheric
absorption lines show sinusoidal variations in radial velocity with an
amplitude of +-6 km/s and a period of about 2.77 days. The radial velocities of
the narrow emission lines of He vary with the same period but in anti-phase to
the photospheric lines. The equivalent widths of the narrow emissions vary with
a phase-shift with respect to the velocity curve. The strength of the
red-shifted accretion components of Na D and other lines is also modulated with
the same period. The broad emission lines of metals vary mostly with the double
period of about 5.5 days.
  One unexpected result is that no correlation was found between the veiling
and the brightness, although both varied in wide ranges. This is partly due to
a contribution of the shell absorption to the photospheric line profiles, which
make them vary in width and depth thus simulating lower veiling.
  Most of the observed features can be interpreted in the framework of
non-axisymmetric magnetospheric accretion. We consider two possible models. In
the first the asymmetry is induced by orbital motion of an invisible, low mass
secondary, which also influences the gasflows around the star, the second
considers rotational modulation of a single star with an inclined or asymmetric
magnetosphere.


title:
Cosmological parameter determination from Planck and SDSS data in
  LambdaCHDM cosmologies

abstract:
  We study the complementarity between the cosmological information obtainable
with the Planck surveyour and the large scale structure (LSS) redshift surveys
in LambdaCHDM cosmologies. We compute the initial full phase-space neutrino
distribution function for LambdaCHDM models by using numerical simulations. As
initial condition we adopt the HDM density fluctuation power spectrum
normalized on the basis of the analysis of the local cluster X-ray temperature
function and derive the initial neutrino phase-space distribution at each
spatial wave number k by using the Zel'dovich approximation. These initial
neutrino phase-space distributions are implemented in the CMBFAST code for the
integration of the coupled linearized Einstein, Boltzmann and fluid equations
in k-space. We find that the relative bias between the CMB temperature
fluctuations and the underlying matter density fluctuation power spectrum in
COBE/DMR normalization is given by the CDM component normalized accordingly to
the abundance of rich clusters at the present time. We use the Fisher
information matrix approximation to constrain a multi-dimensional
parametrization of the LambdaCHDM model, by jointly considering CMB and large
scale structure data according to the Planck and the SDSS experimental
specifications and by taking into account redshift distortions and nonlinear
effects on the matter power spectrum. We found that, although the CMB
anisotropy and polarization measurements tend to dominate the constraints on
most of the cosmological parameters, the additional small scale LSS data help
to break the parameter degeneracies. This work has been done in the framework
of the Planck LFI activities.


title: A new Monte Carlo code for star cluster simulations: I. Relaxation

abstract:
  We have developed a new simulation code aimed at studying the stellar
dynamics of a galactic central star cluster surrounding a massive black hole.
In order to include all the relevant physical ingredients (2-body relaxation,
stellar mass spectrum, collisions, tidal disruption,...), we chose to revive a
numerical scheme pioneered by Henon in the 70's. It is basically a Monte Carlo
resolution of the Fokker-Planck equation. It can cope with any stellar mass
spectrum or velocity distribution. Being a particle-based method, it also
allows one to take stellar collisions into account in a very realistic way.
This first paper covers the basic version of our code which treats the
relaxation-driven evolution of stellar cluster without a central BH. A
technical description of the code is presented, as well as the results of test
computations. Thanks to the use of a binary tree to store potential and rank
information and of variable time steps, cluster models with up to 2 million
particles can be simulated on a standard personal computer and the CPU time
required scales as N*ln(N) with the particle number N. Furthermore, the number
of simulated stars needs not be equal to N but can be arbitrarily larger. A
companion paper will treat further physical ingredients, mostly relevant to
galactic nuclei.


title: Large Area Lyman Alpha survey: finding young galaxies at z=4.5

abstract:
  Strong Lyman-alpha emission is a signpost of young stars and the absence of
dust and thus indicates young galaxies. To find such a population of young
galaxies at z=4.5 we started the Large Area Lyman Alpha survey (LALA). This
survey achieves an unprecedented combination of volume and sensitivity by using
narrow-band filters on a large format (36x36 square-arcminutes) camera on the 4
meter telescope at KPNO. The volume density and star-formation contribution of
the Lyman-alpha emitters at z=4.5 is comparable to that of Lyman break
galaxies. With many candidates and a few spectroscopic confirmations in hand we
discuss what the properties of Ly-alpha emitters imply for galaxy and star
formation in the early universe.


title:
Common-Envelope Evolution: the Nucleosynthesis in Mergers of Massive
  Stars

abstract:
  We study the merging of massive stars inside a common envelope for binary
systems consisting of a red supergiant with a mass of 15-20 Msun and a
main-sequence companion of 1-5 Msun. We are particularly interested in the
stage when the secondary, having overfilled its Roche lobe inside the common
envelope, starts to transfer mass to the core of the primary at a very high
mass-transfer rate and the subsequent nucleo-synthesis in the core-impact
region. Using a parametrized model for the structure of the envelope at this
stage, we perform 2-dimensional hydrodynamical calculations with the Munich
Prometheus code to calculate the dynamics of the stream emanating from the
secondary and its impact on the core of the primary. We find that, for the
lower end of the estimated mass-transfer rate, low-entropy, hydrogen-rich
material can penetrate deep into the primary core where nucleosynthesis through
the hot CNO cycle can take place and that the associated neutron exposure may
be sufficiently high for significant s-processing. For mass-transfer rates at
the high end of our estimated range and higher densities in the stream, the
stream impact can lead to the dredge-up of helium, but the neutron production
is too low for significant s-processing.


title:
CCD-based observations of PG 0856+121 and a theoretical analysis of its
  oscillation modes

abstract:
  BVRI CCD-based and near-IR (J) imaging, together with unfiltered photometry
of the hot subdwarf B star PG 0856+121 are reported. Two close, faint, red,
point-like sources are resolved. They account for the previously reported IR
excess observed in this hot subdwarf. In addition, the new unfiltered
differential photometry of PG 0856+121 confirms its previously reported
pulsational nature. A comparison with the oscillation modes of stellar models
suggests the possible presence of g modes.


title:
Strong Chaos in the N-body problem and Microcanonical Thermodynamics of
  collisionless self gravitating systems

abstract:
  The dynamical justifications which lie at the basis of an effective
Statistical Mechanics for self gravitating systems are formulated, analyzing
some among the well known obstacles thought to prevent a rigorous Statistical
treatment. It is shown that N-body gravitational systems satisfy a strong chaos
criterion, so supporting the assumption of an increasingly uniform spreading of
orbits over the constant energy surface, i.e., the asymptotic evolution towards
a microcanonical distribution. We then focus on the necessary conditions for
the equivalence of statistical ensembles and remark that this equivalence is
broken for any N-body system whose interaction has a range comparable with its
spatial extension. Once realized that the obstacles originate from the long
range nature of the Newtonian interaction, leading to the non-extensivity of
canonical and grand-canonical thermodynamic potentials, we show that instead a
suitably generalized microcanonical ensemble constitutes an orthode, i.e., a
reliable framework for a correct Thermodynamics. Within this setting we use the
thermodynamic relations and find consistent definitions for Entropy (which
turns out to be extensive, despite the non stable nature of the interaction),
Temperature and Heat Capacity. Then, a Second Law-like criterion is used to
select the hierarchy of secular equilibria describing, for any finite time, the
macroscopic behaviour of self gravitating systems.


title: Early-universe constraints on a time-varying fine structure constant

abstract:
  Higher-dimensional theories have the remarkable feature of predicting a time
(and hence redshift) dependence of the `fundamental' four dimensional constants
on cosmological timescales. In this paper we update the bounds on a possible
variation of the fine structure constant alpha at the time of BBN (z =10^10)
and CMB (z=10^3). Using the recently-released high-resolution CMB anisotropy
data and the latest estimates of primordial abundances of 4He and D, we do not
find evidence for a varying alpha at more than one-sigma level at either epoch.


title:
A Module for Radiation Hydrodynamic Calculations With ZEUS-2D Using
  Flux-Limited Diffusion

abstract:
  A module for the ZEUS-2D code is described which may be used to solve the
equations of radiation hydrodynamics to order unity in v/c, in the flux-limited
diffusion (FLD) approximation. In this approximation, the tensor Eddington
factor f which closes the radiation moment equations is chosen to be an
empirical function of radiation energy density. This is easier to implement and
faster than full-transport techniques, in which f is computed by solving the
transfer equation. However, FLD is less accurate when the flux has a component
perpendicular to the gradient in radiation energy density, and in optically
thin regions when the radiation field depends strongly on angle. The material
component of the fluid is here assumed to be in local thermodynamic
equilibrium. The energy equations are operator-split, with transport terms,
radiation diffusion term, and other source terms evolved separately. Transport
terms are applied using the same consistent transport algorithm as in ZEUS-2D.
The radiation diffusion term is updated using an alternating-direction implicit
method with convergence checking. Remaining source terms are advanced together
implicitly using numerical root-finding. However when absorption opacity is
zero, accuracy is improved by treating compression and expansion source terms
using time-centered differencing. Results are discussed for test problems
including radiation-damped linear waves, radiation fronts propagating in
optically-thin media, subcritical and supercritical radiating shocks, and an
optically-thick shock in which radiation dominates downstream pressure.


title:
In the Shadow of the Accretion Disk: Higher Resolution Imaging of the
  Central Parsec in NGC 4261

abstract:
  The physical conditions in the inner parsec of accretion disks believed to
orbit the central black holes in active galactic nuclei can be probed by
imaging the absorption (by ionized gas in the disk) of background emission from
a radio counterjet. We report high angular resolution VLBI observations of the
nearby (about 40 Mpc) radio galaxy NGC 4261 that confirm free-free absorption
of radio emission from a counterjet by a geometrically thin, nearly edge-on
disk at 1.6, 4.8, and 8.4 GHz. The angular width and depth of the absorption
appears to increase with decreasing frequency, as expected. We derive an
average electron density of ~10E4 per cc at a disk radius of about 0.2 pc,
assuming that the inner disk inclination and opening angles are the same as at
larger radii. Pressure balance between the thermal gas and the magnetic field
in the disk implies an average field strength of 0.1 milligauss at a radius of
0.2 pc. These are the closest-in free-free absorption measurements to date of
the conditions in an extragalactic accretion disk orbiting a black hole with a
well-determined mass. If a standard advection-dominated accretion flow exists
in the disk center, then the transition between thin and thick disk regions
must occur at a radius less than 0.2 pc (4000 Schwarzschild radii).


title: Distances and Metallicities of High- and Intermediate-Velocity Clouds

abstract:
  A table is presented that summarizes published absorption line measurements
for the high- and intermediate velocity clouds (HVCs and IVCs). New values are
derived for N(HI) in the direction of observed probes, in order to arrive at
reliable abundances and abundance limits (the HI data are described in Paper
II). Distances to stellar probes are revisited and calculated consistently, in
order to derive distance brackets or limits for many of the clouds, taking care
to properly interpret non-detections. The main conclusions are the following.
1) Absolute abundances have been measured using lines of SII, NI and OI, with
the following resulting values: ~0.1 solar for one HVC (complex C), ~0.3 solar
for the Magellanic Stream, ~0.5 solar for a southern IVC, and ~ solar for two
northern IVCs (the IV Arch and LLIV Arch). Finally, approximate values in the
range 0.5-2 solar are found for three more IVCs. 2) Depletion patterns in IVCs
are like those in warm disk or halo gas. 3) Most distance limits are based on
strong UV lines of CII, SiII and MgII, a few on CaII. Distance limits for major
HVCs are >5 kpc, while distance brackets for several IVCs are in the range
0.5-2 kpc. 4) Mass limits for major IVCs are 0.5-8x10^5 M_sun, but for major
HVCs they are >10^6 M_sun. 5) The CaII/HI ratio varies by up to a factor 2-5
within a single cloud, somewhat more between clouds. 6) The NaIHI ratio varies
by a factor >10 within a cloud, and even more between clouds. Thus, CaII can be
useful for determining both lower and upper distance limits, but NaI only
yields upper limits.


title: HI spectra and column densities toward HVC and IVC probes

abstract:
  We show 21-cm line profiles in the direction of stars and extragalactic
objects, lying projected on high- and intermediate-velocity clouds (HVCs and
IVCs). About half of these are from new data obtained with the Effelsberg 100-m
telescope, about a quarter are extracted from the Leiden-Dwingeloo Survey (LDS)
and the remaining quarter were observed with other single-dish telescopes. HI
column densities were determined for each HVC/IVC. Wakker (2001) (Paper I) uses
these in combination with optical and ultraviolet high-resolution measurements
to derive abundances. Here, an analysis is given of the difference and ratio of
N(HI) as observed with a 9 arcmin versus a 35 arcmin beam. For HVCs and IVCs
the ratio N(HI-9 arcmin)/N(HI-35 arcmin) lies in the range 0.2-2.5. For
low-velocity gas this ratio ranges from 0.75 to 1.3 (the observed ratio is
0.85-1.4, but it appears that the correction for stray radiation is slightly
off). The smaller range for the low-velocity gas may be caused by confusion in
the line of sight, so that a low ratio in one component can be compensated by a
high ratio in another -- for 11 low-velocity clouds fit by one component the
distribution of ratios has a larger dispersion. Comparison with higher angular
resolution data is possible for sixteen sightlines. Eight sightlines with HI
data at 1 arcmin-2 arcmin resolution show a range of 0.75-1.25 for N(HI-2
arcmin)/N(HI-9 arcmin), while in eight other sightlines N(HI-Ly-alpha)/N(HI-9
arcmin) ranges from 0.74 to 0.98.


title: X-ray Halos and Large Grains in the Diffuse Interstellar Medium

abstract:
  Recent observations with dust detectors on board the interplanetary
spacecraft Ulysses and Galileo have recorded a substantial flux of large
interstellar grains with radii between 0.25 and 2.0 mu entering the solar
system from the local interstellar cloud. The most commonly used interstellar
grain size distribution is characterized by a a^-3.5 power law in grain radii
a, and extends to a maximum grain radius of 0.25 mu. The extension of the
interstellar grain size distribution to such large radii will have a major
effect on the median grain size, and on the amount of mass needed to be tied up
in dust for a given visual optical depth. It is therefore important to
investigate whether this population of larger dust particles prevails in the
general interstellar medium, or if it is merely a local phenomenon. The
presence of large interstellar grains can be mainly inferred from their effect
on the intensity and radial profiles of scattering halos around X-ray sources.
In this paper we examine the grain size distribution that gives rise to the
X-ray halo around Nova Cygni 1992. The results of our study confirm the need to
extend the interstellar grain size distribution in the direction of this source
to and possibly beyond 2.0 mu. The model that gives the best fit to the halo
data is characterized by: (1) a grain size distribution that follows an a^-3.5
power law up to 0.50 mu, followed by an a^-4.0 extension from 0.50 mu to 2.0
mu; and (2) silicate and graphite (carbon) dust-to-gas mass ratios of 0.0044
and 0.0022, respectively, consistent with solar abundances constraints.
Additional observations of X-ray halos probing other spatial directions are
badly needed to test the general validity of this result.


title: Multifrequency studies of the enigmatic gamma-ray source 3EG J1835+5918

abstract:
  The EGRET telescope has repeatedly observed 3EG J1835+5918 as a bright and
steady source of high-energy gamma-ray radiation which has not yet been
indentified. EGRET data from CGRO observation cycle 1 to 7 have been reanalysed
above 100 MeV and above 1 GeV. The gamma-ray source location represents the
latest and probably final positional assessment based on EGRET data. We
especially adress the question of flux variability and spectral variability.
The results of a X-ray/optical identification campaign towards 3EG J1835+5918
are given. The one object which might be associated with the gamma-ray source
3EG J1835+5918 has the characteristics of an isolated neutron star and possible
of a radio-quiet pulsar.


title:
Chandra X-ray Spectroscopic Imaging of Sgr A* and the Central Parsec of
  the Galaxy

abstract:
  We present results of our Chandra observation with the ACIS-I instrument
centered on the position of Sagittarius A* (Sgr A*), the compact nonthermal
radio source associated with the massive black hole (MBH) at the dynamical
center of the Milky Way Galaxy. We have obtained the first
high-spatial-resolution (~1 arcsec), hard X-ray (0.5-7 keV) image of the
central 40 pc (17 arcmin) of the Galaxy and have discovered an X-ray source,
CXOGC J174540.0-290027, coincident with the radio position of Sgr A* to within
0.35 arcsec, corresponding to a maximum projected distance of 16 light-days for
an assumed distance to the center of the Galaxy of 8.0 kpc. We received 222
+/-17 (1 sigma) net counts from the source in 40.3 ks. Due to the low number of
counts, the spectrum is well fit either by an absorbed power-law model with
photon index Gamma = 2.7 (1.8-4.0) and column density NH = (9.8 [6.8-14.2]) x
10^22 cm^-2 (90% confidence interval) or by an absorbed optically thin thermal
plasma model with kT = 1.9 (1.4-2.8) keV and NH = (11.5 [8.4-15.9]) x 10^22
cm^-2. Using the power-law model, the measured (absorbed) flux in the 2-10 keV
band is (1.3 [1.1-1.7]) x 10^-13 ergs cm^-2 s^-1, and the absorption-corrected
luminosity is (2.4 [1.8-5.4]) x 10^33 ergs s^-1. We also briefly discuss the
complex structure of the X-ray emission from the Sgr A radio complex and along
the Galactic plane and present morphological evidence that Sgr A* and Sgr A
West lie within the hot plasma in the central cavity of Sgr A East.


title: CMB (And Other) Challenges To BBN

abstract:
  Primordial nucleosynthesis provides a probe of the universal abundance of
baryons when the universe was only a few minutes old. Recent observations of
anisotropy in the cosmic microwave background (CMB) probe the baryon abundance
when the universe was several hundred thousand years old. Observations of type
Ia supernovae and clusters of galaxies in the very recent past, when the
universe is several billion years old and older, provide a complementary
measure of the baryon density in excellent agreement with the early universe
values. The general agreement among the three measurements represents an
impressive confirmation of the standard model of cosmology. However, there is a
hint that the CMB observations may not be in perfect agreement with those from
big bang nucleosynthesis (BBN). If this "tension" between BBN and the CMB
persists, the standard model of cosmology may need to be modified. Here, in a
contribution dedicated to Silvia Torres-Peimbert and Manuel Peimbert, we
describe how an asymmetry between neutrinos and antineutrinos ("neutrino
degeneracy") has the potential for resolving this possible conflict between BBN
and the CMB.


title:
Constructing, characterizing, and simulating Gaussian and higher-order
  point distributions

abstract:
  The definition and the properties of a Gaussian point distribution, in
contrast to the well-known properties of a Gaussian random field are discussed.
Constraints for the number density and the two-point correlation function
arise. A simple method for the simulation of this so-called Gauss-Poisson point
process is given and illustrated with an example. The comparison of the
distribution of galaxies in the PSCz catalogue with the Gauss-Poisson process
underlines the importance of higher-order correlation functions for the
description for the galaxy distribution. The construction of the Gauss-Poisson
point process is extended to the n-point Poisson cluster process, now
incorporating correlation functions up to the nth-order. The simulation methods
and constraints on the correlation functions are discussed for the n-point case
and detailed for the three-point case. As another approach, well suited for
strongly clustered systems, the generalized halo-model is discussed. The
influence of substructure inside the halos on the two- and three-point
correlation functions is calculated in this model.

categories: astro-ph cond-mat.stat-mech math.PR physics.data-an