title:
Evidence for Circumstellar Disks around Young Brown Dwarfs in the
  Trapezium Cluster

abstract:
  We report the results of deep infrared observations of brown dwarf candidates
in the Trapezium cluster in Orion. Analysis of the JHK color-color diagram
indicates that a large fraction (approx 65% +/- 15%) of the observed sources
exhibit infrared excess emission. This suggests the extreme youth of these
objects and in turn, provides strong independent confirmation of the existence
of a large population of substellar objects in the cluster. Moreover, this
suggests that the majority of these substellar objects are presently surrounded
by circumstellar disks similar to the situation for the stellar population of
the cluster. This evidence for a high initial disk frequency (> 50%) around
cluster members of all masses combined with the smooth continuity of the
cluster's initial mass function across the hydrogen burning limit suggests that
a single physical mechanism is likely responsible for producing the entire
cluster mass spectrum down to near the deuterium burning limit. The results may
also indicate that even substellar objects are capable of forming with systems
of planetary companions.


title:
HST NICMOS Photometry of the reddened bulge globular clusters NGC 6528,
  Terzan 5, Liller 1, UKS 1 and Terzan 4

abstract:
  We present results from NICMOS Hubble Space Telescope observations of the
reddened bulge globular clusters NGC 6528, Terzan 5, Liller 1, UKS 1 and Terzan
4, obtained through the filters F110W and F160W (nearly equivalent to J and H).
For the first time the turnoff region of Liller 1 and the main sequence of
Terzan 5 and Terzan 4 are reached, as well as the horizontal branch of UKS 1.
The magnitude difference between the turnoff and the red horizontal branch
$\Delta m_{110}=m_{110}^{HB}- m_{110}^{TO}$ is used as an age indicator. From
comparisons with new isochrones in the NICMOS photometric system, we conclude
that the two metal-rich clusters NGC 6528 and Terzan 5 are coeval within
uncertainties ($\sim 20%$) with 47 Tucanae. Liller 1 and UKS 1 are confirmed as
metal-rich globular clusters. Terzan 4 is confirmed as an interesting case of a
metal-poor cluster in the bulge with a blue horizontal branch.


title:
The 2dF QSO Redshift Survey - VIII. Absorption systems in the 10k
  catalogue

abstract:
  We examine the highest S/N spectra from the 2QZ 10k release and identify over
100 new low-ionisation heavy element absorbers; DLA candidates suitable for
higher resolution follow-up observations. These absorption systems map the
spatial distribution of high-z metals in exactly the same volumes that the
foreground 2QZ QSOs themselves sample and hence the 2QZ gives us the unique
opportunity to directly compare the two tracers of large scale structure. We
examine the cross-correlation of the two populations to see how they are
relatively clustered, and, by considering the colour of the QSOs, detect a
small amount of dust in these metal systems.


title:
ROSAT PSPC observations of nearby spiral galaxies - II. Statistical
  properties

abstract:
  We present a statistical analysis of the largest X-ray survey of nearby
spiral galaxies in which diffuse emission has been separated from discrete
source contributions. Regression and rank-order correlation analyses are used
to compare X-ray properties such as total, source and diffuse luminosities, and
diffuse emission temperature, with a variety of physical and multi-wavelength
properties, such as galaxy mass, type and activity, and optical and infrared
luminosity. The results are discussed in terms of the way in which hot gas and
discrete X-ray sources scale with the mass and activity of galaxies, and with
the star formation rate. We find that the X-ray properties of starburst
galaxies are dependent primarily on their star-forming activity, whilst for
more quiescent galaxies, galaxy mass is the more important parameter. One of
the most intriguing results is the tight linear scaling between far-infrared
and diffuse X-ray luminosity across the sample, even though the hot gas changes
from a hydrostatic corona to a free wind across the activity range sampled
here.


title: Ambipolar Drift in a Turbulent Medium

abstract:
  The interstellar magnetic field strength and gas density are observed to be
correlated, but there is a large dispersion in this relation. In particular,
the magnetic field is often observed to be weaker than expected. It is usually
assumed that the field is frozen into the gas, except at very low ionization
fraction, in which case ion-neutral drift, or ambipolar diffusion, leads to
appreciable slip of the field and tends to make it more uniform. We show that
turbulence enhances the rate of ambipolar drift, and that this may explain the
surprisingly low fieldstrengths sometimes seen in dense interstellar gas.


title: FUSE Observations of U Geminorum During Outburst and Decline

abstract:
  We have obtained FUV (904 -- 1187 \AA) spectra of U Gem in outburst with
FUSE. Three of the observations were acquired during the plateau phase of the
outburst, while the fourth was obtained during late outburst decline. The
plateau spectra have continuum shapes and fluxes that are approximated by
steady-state accretion disk model spectra with $\dot{m} \simeq$ 7e-09 M_sun/yr.
The spectra also show numerous absorption lines of H I, He II, and two- to
five-times ionized transitions of C, N, O, P, S and Si. There are no emission
features in the spectra, with the possible exception of a weak feature on the
red wing of the O VI doublet. The absorption lines are narrow (FWHM ~ 500
km/s), too narrow to arise from the disk photosphere, and at low velocities (<=
700 km/s). The S VI and O VI doublets are optically thick. The absorption lines
in the plateau spectra show orbital variability: in spectra obtained at orbital
phases 0.53 -- 0.79, low-ionization absorption lines appear and the central
depths of the pre-existing lines increase. The increase in line absorption
occurs at the same orbital phases as previously observed EUV and X-ray light
curve dips. If the absorbing material is in (near-) Keplerian rotation around
the disk, it must be located at large disk radii. The final observation
occurred when U Gem was about two magnitudes from optical quiescence. The
spectra are dominated by emission from an $\simeq$43,000 K, metal-enriched
white dwarf (WD). The inferred radius of the WD is 4.95e+08 cm, close to that
observed in quiescence. Allowing for a hot heated region on the surface of the
WD improves the fit to the spectrum at short (< 960 \AA) wavelengths.


title:
Spectral Consequences of Deviation from Spherical Composition Symmetry
  in Type Ia Supernovae

abstract:
  We investigate the prospects for constraining the maximum scale of clumping
in composition that is consistent with observed Type Ia supernova flux spectra.
Synthetic spectra generated without purely spherical composition symmetry
indicate that gross asymmetries make prominent changes to absorption features.
Motivated by this, we consider the case of a single unblended line forming in
an atmosphere with perturbations of different scales and spatial distributions.
Perturbations of about 1% of the area of the photodisk simply weaken the
absorption feature by the same amount independent of the line of sight.
Conversely, perturbations of about 10% of the area of the photodisk introduce
variation in the absorption depth which does depend on the line of sight. Thus,
1% photodisk area perturbations may be consistent with observed profile
homogeneity but 10% photodisk area perturbations can not. Based on this, we
suggest that the absence of significant variation in the depths of Si II 6355
absorption features in normal Type Ia spectra near maximum light indicates that
any composition perturbations in these events are quite small. This also
constrains future three-dimensional explosion models to produce ejecta profiles
with only small scale inhomogeneities.


title: The DLAs contribution to the metal content of the high z Universe

abstract:
  We investigate the evolutionary history of the Universe's metal content
focusing on the chemical abundance of several elements (N, O, S, Si, Fe, Cr,
Zn) taken from observational data and predictions from chemical evolution
models. The estimated abundances were observed in Damped Lyman alpha Systems
(DLAs) over a wide range of redshift (z ~ 0.5-4.5). These data are compared to
predictions of chemical evolution models. Since the nature of the DLAs is
uncertain, they are represented by two class of models: models with galactic
winds describing dwarf galaxies and with infall representing disk galaxies. In
order to settle constraints for star formation timescales in DLAs, we use the
ratios [alpha/Fe] and [N/alpha] in the comparison to the predictions of the
models. These ratios in DLAs are only partially reproduced by the disk and
dwarf galaxy models suggesting that the DLAs come from a variety of
morphological types of galaxies and not only one. They also imply a typically
long timescale for the star formation in these systems.


title:
Towards A Mean-Field Formulation Of The Babcock-Leighton Type Solar
  Dynamo. I. Alpha Coefficient Versus Durney's Double Ring Approach

abstract:
  We develop a model of the solar dynamo in which, on the one hand, we follow
the Babcock-Leighton approach to include surface processes like the production
of poloidal field from the decay of active regions, and, on the other hand, we
attempt to develop a mean field theory that can be studied in quantitative
detail. One of the main challenges in developing such models is to treat the
buoyant rise of toroidal field and the production of poloidal field from it
near the surface. We build up a dynamo model with two contrasting methods of
treating buoyancy. In one method, we incorporate the generation of the poloidal
field near the solar surface by Durney's procedure of double ring eruption. In
the second method, the poloidal field generation is treated by a positive
alpha-effect concentrated near the solar surface, coupled with an algorithm for
handling buoyancy. The two methods are found to give qualitatively similar
results.


title: The Spectacular BHR 71 Outflow

abstract:
  BHR 71 is a well isolated Bok globule located at ~200 pc, which harbours a
highly collimated bipolar outflow. The outflow is driven by a very young Class
0 protostar with a luminosity of ~9 L_sun. It is one of a very small number
that show enhanced abundances of a number of molecular species, notably SiO and
CH3OH, due to shock processing of the ambient medium. In this paper the
properties of the globule and outflow are discussed.


title:
Detection of the 67.9 keV and 78.4 keV lines associated with the
  radioactive decay of Ti-44 in Cassiopeia A

abstract:
  We report the detection of the Sc-44 nuclear decay lines at 67.9 keV and 78.4
keV associated with the nuclear decay of Ti-44 in Cassiopeia A. The line
emission was observed by the PDS instrument on board BeppoSAX, which recently
observed the supernova remnant for over 500 ks. The detection of the line
emission with a flux of (2.1 +/-0.7) 10^{-5} ph/cm^2/s in each line (90%
confidence) is at the 5 sigma significance level, if we can assume that the
12-300 keV continuum is adequately represented by a single power law. However,
as the nature of the continuum is not clear we investigate various other
possibilities. A more conservative estimate of the line flux is made by
assuming that a power law continuum is at least a good approximation to the
continuum emission for a narrower 30-100 keV energy range. With this limitation
the measured line flux is (1.9 +/- 0.9) 10^{-5} ph/cm^2/s, with the detection
still at the 3.4 sigma significance level. We suggest that together with the
CGRO-COMPTEL measurement of the Ca-44 line at 1157 keV of (3.5 +/- 0.9) 10^{-5}
ph/cm^2/s a flux for all three lines of (2.5 +/- 1.0) 10^{-5} ph/cm^2/s for Cas
A can be adopted. This implies an initial Ti-44 mass of (0.8 - 2.5) 10^{-4}
Msun.


title: The Spectrum and Accurate Location of GRS 1758-258

abstract:
  We observed the "micro-quasar" GRS 1758-258 four times with Chandra. Two
HRC-I observations were made in 2000 September-October spanning an
intermediate-to-hard spectral transition (identified with RXTE). Another HRC-I
and an ACIS/HETGS observation were made in 2001 March following a hard-to-soft
transition to a very low flux state. The accurate position (J2000) of the X-ray
source is RA= 18 01 12.40, Dec= -25 44 36.0 (90% confidence radius = 0".6),
consistent with the purported variable radio counterpart. All images are
consistent with GRS 1758-258 being a point source, indicating that any bright
jet is less than ~1 light-month in projected length, assuming a distance of 8.5
kpc. The March spectrum is well-fit with a multi-color disk-blackbody with an
inner temperature of 0.50+/-0.01 keV, interstellar absorption of nH = (1.59 +/-
0.05)x10^22 cm^-2, and (un-absorbed) 1-10 keV luminosity of
(4.5x10^36)(D/8.5kpc)^2 ergs/sec. No narrow emission lines are apparent in the
spectrum and upper limits to line equivalent widths are given.


title:
Using Pulsars to Detect Massive Black Hole Binaries via Gravitational
  Radiation: Sagittarius A* and Nearby Galaxies

abstract:
  Pulsar timing measurements can be used to detect gravitational radiation from
massive black hole binaries. The ~106d quasi-periodic flux variations in
Sagittarius A* at radio wavelengths reported by Zhao, Bower, & Goss (2001) may
be due to binarity of the massive black hole that is presumed to be responsible
for the radio emission. A 106d equal-mass binary black hole is unlikely based
on its short inspiral lifetime and other arguments. Nevertheless the reported
quasi-periodicity has led us to consider whether the long-wavelength
gravitational waves from a conjectured binary might be detected in present or
future precision timing of millisecond pulsars. While present timing cannot
reach the level expected for an equal-mass binary, we estimate that future
efforts could. This inquiry has led us to further consider the detection of
binarity in the massive black holes now being found in nearby galaxies. For
orbital periods of ~2000d where the pulsar timing measurements are most
precise, we place upper limits on the mass ratio of binaries as small as 0.06.


title:
Infrared Surface Brightness Fluctuations of the Coma Elliptical NGC 4874
  and the Value of the Hubble Constant

abstract:
  We have used the Keck I Telescope to measure K-band surface brightness
fluctuations (SBFs) of NGC 4874, the dominant elliptical galaxy in the Coma
cluster. We use deep HST WFPC2 optical imaging to account for the contamination
due to faint globular clusters and improved analysis techniques to derive
measurements of the SBF apparent magnitude. Using a new SBF calibration which
accounts for the dependence of K-band SBFs on the integrated color of the
stellar population, we measure a distance modulus of 34.99+/-0.21 mag (100+/-10
Mpc) for the Coma cluster. The resulting value of the Hubble constant is 71+/-8
km/s/Mpc, not including any systematic error in the HST Cepheid distance scale.


title: On the Column Density of AGN Outflows: the Case of NGC 5548

abstract:
  We re-analyze the HST high resolution spectroscopic data of the intrinsic
absorber in NGC 5548 and find that the C IV absorption column density is at
least four times larger than previously determined. This increase arises from
accounting for the kinematical nature of the absorber and from our conclusion
that the outflow does not cover the narrow emission line region in this object.
The improved column density determination begins to bridge the gap between the
high column densities measured in the X-ray and the low ones previously
inferred from the UV lines. Combined with our findings for outflows in high
luminosity quasars these results suggest that traditional techniques for
measuring column densities: equivalent width, curve-of-growth and Gaussian
modeling, are of limited value when applied to absorption associated with AGN
outflows.


title: Hubble and FUSE studies of Ly-alpha absorbers at low redshift

abstract:
  Ultraviolet spectrographs aboard the Hubble Space Telescope (HST) and the Far
Ultraviolet Spectroscopic Explorer (FUSE) have proved their value as sensitive
probes of the low-density intergalactic medium (IGM) at low redshifts (z <
0.1). Recent observations in Ly-alpha, Ly-beta, and occasional higher Lyman
lines show that warm photoionized gas in the low-z IGM may contain 20-25% of
the baryons, with a N(HI)^-1.8 distribution in column density. Measurements of
resonance lines of Si III, C III, C IV, and O VI suggest that the metallicity
of these absorbers ranges from 1-10% of solar abundance down to values below
0.003 Z(solar). A comparison of Ly-beta/Ly-alpha ratios (FUSE and HST) yields a
distribution of Doppler parameters with mean b = 31.4 +/- 7.4 km/s and median
28 km/s, comparable to values at z = 2-3. The curve-of-growth (CoG) b-values
are considerably less than widths derived from Ly-alpha profile fitting, with
mean b(CoG)/b(width) = 0.52, which suggests that low-z absorbers contain
sizable non-thermal motions or velocity components arising from cosmological
expansion and infall. A challenge for future UV spectroscopic missions (HST/COS
and SUVO) is to obtain precision measurements of Omega(IGM) and metallicities
for the strong Ly-alpha absorbers that dominate the IGM baryon content. This
program will require accurate determinations of: (1) curves of growth using
higher Lyman series lines; (2) the ionizing radiation field at 1-5 Ryd; and (3)
characteristic sizes and shapes of the absorbers.


title:
On the Neutral Gas Content and Environment of NGC 3109 and the Antlia
  Dwarf Galaxy

abstract:
  As part of a continuing survey of nearby galaxies, we have mapped the neutral
gas content of the low surface brightness, Magellanic-type galaxy NGC 3109 ---
and its environment, including the Antlia dwarf galaxy --- at unprecedented
velocity resolution and brightness sensitivity. The HI mass of NGC 3109 is
measured to be (3.8 +/- 0.5) x 10^8 Msun. A substantial warp in the disk of NGC
3109 is detected in the HI emission image in the form of an extended low
surface brightness feature. We report a positive detection in HI of the nearby
Antlia dwarf galaxy, and measure its total neutral gas mass to be (6.8 +/- 1.4)
x 10^5 Msun. We show the warp in NGC 3109 to lie at exactly the same radial
velocity as the gas in the Antlia dwarf galaxy and speculate that Antlia
disturbed the disk of NGC 3109 during a mild encounter ~1 Gyr in the past. HI
data for a further eight galaxies detected in the background are presented.


title: The optical long `period' of LMC X-3

abstract:
  We have studied the long term variability of LMC X-3 in optical lightcurves
spanning six years, in order to search for optical signatures which could
confirm/refute the suggestion that the `modulation' is the result of accretion
rate variability rather than accretion disc precession. We find that there is
no stable period in the optical lightcurves, that the optical and X-ray
lightcurves are correlated, with an X-ray lag and that there is no modulation
in the optical colours. We argue that these effects agree with the variable
mass accretion rate model proposed by Wilms et al. (2001).


title:
A nuclear many-body theory at finite temperature applied to protoneutron
  star

abstract:
  Thermodynamical properties of nuclear matter are studied in the framework of
an effective many-body field theory at finite temperature, considering the
Sommerfeld approximation. We perform the calculations by using the nonlinear
Boguta and Bodmer model, extended by the inclusion of the fundamental baryon
octet and leptonic degrees of freedom. Trapped neutrinos are included in order
to describe protoneutron star properties through the integration of the
Tolman-Oppenheimer-Volkoff equations, from which we obtain, beyond the standard
plots for the mass and radius of the protoneutron stars as functions of central
density, new plots of these quantities as functions of temperature. Our
predictions include the determination of an absolute value for the limiting
protoneutron star mass; new aspects on nuclear matter phase transition via the
behaviour of the specific heat and, through the inclusion of quark degrees of
freedom, the properties of a hadron-quark phase transition and of the hybrid
protoneutron stars.


title: The mass of the white dwarf in the recurrent nova U Scorpii

abstract:
  We present spectroscopy of the eclipsing recurrent nova U Sco. The radial
velocity semi-amplitude of the primary star was found to be K_W = 93 \pm 10
kms^{-1} from the motion of the wings of the HeII\lambda4686\AA emission line.
By detecting weak absorption features from the secondary star, we find its
radial velocity semi-amplitude to be K_R = 170 \pm 10 kms^{-1}. From these
parameters, we obtain a mass of M_1 = 1.55 \pm 0.24M_\odot for the white dwarf
primary star and a mass of M_2 = 0.88 \pm 0.17M_\odot for the secondary star.
The radius of the secondary is calculated to be R_2 = 2.1\pm0.2R_\odot,
confirming that it is evolved. The inclination of the system is calculated to
be i = 82.7^\circ\pm2.9^\circ, consistent with the deep eclipse seen in the
lightcurves. The helium emission lines are double-peaked, with the blue-shifted
regions of the disc being eclipsed prior to the red-shifted regions, clearly
indicating the presence of an accretion disc. The high mass of the white dwarf
is consistent with the thermonuclear runaway model of recurrent nova outbursts,
and confirms that U Sco is the best Type Ia supernova progenitor currently
known. We predict that U Sco is likely to explode within \sim 700,000 years.


title: 4mu spectra of AGB stars I: Observations

abstract:
  We present times series of high resolution spectra of AGB variables at 4mu.
Line profiles from the major contributors to the spectra of oxygen rich stars
at 4mu, OH, H$_2$O, HCl and SiO, are examined. The velocity as well as shape
variations of these profiles with time are discussed. The line profiles
investigated frequently have emission and multiple absorption components. The
changes with time of the 4mu region lines do not always follow the cyclic
variability seen in NIR spectra and in the photometric light curve. We
interpret and discuss the results qualitatively considering comparing the
spectral variability with that of the well behaved 1.6mu region and of
dynamical model atmospheres. Miras and semiregular variables are compared. The
origins of non-periodic behavior are discussed, including the role of spatial
inhomogeneities in the stellar atmosphere.


title:
Velocity dispersions of dwarf spheroidal galaxies: dark matter versus
  MOND

abstract:
  We present predictions for the line-of-sight velocity dispersion profiles of
dwarf spheroidal galaxies and compare them to observations in the case of the
Fornax dwarf. The predictions are made in the framework of standard dynamical
theory of spherical systems with different velocity distributions. The stars
are assumed to be distributed according to Sersic laws with parameters fitted
to observations. We compare predictions obtained assuming the presence of dark
matter haloes (with density profiles adopted from N-body simulations) versus
those resulting from Modified Newtonian Dynamics (MOND). If the anisotropy of
velocity distribution is treated as a free parameter observational data for
Fornax are reproduced equally well by models with dark matter and with MOND. If
stellar mass-to-light ratio of $1 M_{\sun}/L_{\sun}$ is assumed, the required
mass of the dark halo is $1.5 \times 10^9 M_{\sun}$, two orders of magnitude
bigger than the mass in stars. The derived MOND acceleration scale is $a_0 =
2.1 \times 10^{-8}$ cm/s$^2$. In both cases a certain amount of tangential
anisotropy in the velocity distribution is needed to reproduce the shape of the
velocity dispersion profile in Fornax.


title: The X-ray emission of the Intermediate Polar V709 Cas

abstract:
  We present RXTE and BeppoSAX observations of the Intermediate Polar V709 Cas
acquired in 1997 and 1998 respectively. The X-ray emission from 0.1 to 30keV is
dominated by the strong pulsation at the rotational period of the white dwarf
(312.8s) with no sign of orbital or sideband periodicity, thus confirming
previous ROSAT results. However, we detect changes in the power spectra between
the two epochs. While the second harmonic of the spin period is present during
both observations, the first harmonic is absent in 1997. An increase in the
amplitude of the spin pulsation is found between 1997 and 1998 together with a
decrease in the X-ray flux. The average X-ray spectrum from 0.1 to 100keV is
well described by an isothermal plasma at ~27keV plus complex absorption and an
iron K(alpha) fluorescent line, due to reflection from the white dwarf surface.
The rotational pulsation is compatible with complex absorption dominating the
low energy range, while the high energy spin modulation can be attributed to
tall shocks above the accreting poles. The RXTE spectrum in 1997 also shows the
presence of an absorption edge from ionized iron likely located in the
pre-shock accretion flow. The variations along the spin period of the partial
covering absorber and of reflection are compatible with the classical accretion
curtain scenario. The variations in the spin pulse characteristics and X-ray
flux indicate that V709 Cas experiences changes in the mass accretion rate on
timescales from months to years.


title:
The Local Ly-alpha forest: Association of absorbers with galaxies,
  voids, and superclusters

abstract:
  We describe recent discoveries of low column density [N(HI) < 10^14.5 cm^-2]
H I Ly-alpha absorbers made with the Hubble Space Telescope (HST), which have
allowed us a first look at gas in local intergalactic space between us and the
"Great Wall". Despite the mere 2.4m aperture of HST, these new observations
allow us to detect absorbers with column densities, N(HI) \approx 10^12.5
cm^-2, as low as those found using Keck/HIRES at high-z. Owing to the proximity
of these absorbers to the Earth, the 197 absorbers in our combined GHRS + STIS
sample (Penton, Stocke, & Shull 2001) provide our best view of the relationship
between these absorbers and galaxies, voids, and supercluster filaments. Unlike
previous results based on galaxy surveys near higher-N(HI) absorbers, we find
no evidence that these lower-N(HI) absorbers are extended galaxy halos. Rather,
the majority (78%) are associated with large-scale filamentary structures of
galaxies, while 22% are found in galaxy "voids". Since these Ly-alpha absorbers
are currently the only baryons detected in the voids, we use their properties
to estimate that the voids contain 4.5 (+/-1.5)% of the Universal baryon
density.


title:
Maximum likelihood method for estimating the mass and period
  distributions of extra-solar planets

abstract:
  We investigate the distribution of mass M and orbital period P of extra-solar
planets, taking account of selection effects due to the limited velocity
precision and duration of existing surveys. We fit the data on 63 planets to a
power-law distribution of the form dn=CM^{-alpha}P^{-beta}(dM/M)(dP/P), and
find alpha=0.12+-0.10, beta=-0.26+-0.06 for M<10M_J, where M_J is the Jupiter
mass. The correlation coefficient between these two exponents is -0.32,
indicating that uncertainties in the two distributions are coupled. We estimate
that 3% of solar-type stars have companions in the range 1M_J<M<10M_J,
2d<P<10yr.


title: The Influence of the Starburst on the ISM in NGC 1569

abstract:
  A tremendous starburst, that ended about 4 million years ago, has had a very
strong impact on the dwarf galaxy NGC 1569. Evidence for this event is still
prominent in the different components of the galaxy: For example, photometric
studies of the resolved stellar population close to the very luminous
super-star clusters hint at past star formation rates of up to 3 M_solar/yr. In
ROSAT observations, extended X-ray emission has been found at the same
location. And the spectrum of the strong synchrotron emission shows a kink
characteristic for a recently stopped starburst. We study the impact of the
starburst on the neutral atomic and molecular gas. Our CO(3--2) map obtained
with the Heinrich-Hertz Telescope reveals 3 giant molecular associations as
well as a large amount of extended CO gas. The comparison of this map with
interferometric and single-dish data of the CO(2--1) and CO(1--0) transitions
indicates an unusually warm molecular gas phase. The HI distribution is
remarkably smooth for a star-forming dwarf galaxy. Only one huge bubble has
been found. It is located near the position of the super-star clusters. With
our naturally weighted VLA data cube, we can confirm the existence of a
low-mass HI cloud and a bridge connecting it to NGC 1569. This might be an HI
cloud in the process of merging with the galaxy and being tidally disrupted. We
also report the detection of the remnants of a shell, that can be traced best
in the HI velocity field.


title: The Fine Structure and Outskirts of DDO 154

abstract:
  Mapping of the HI disk of the isolated irregular galaxy DDO 154 with the C
array of the Very Large Array and with the 3.2' upgraded Arecibo beam is
presented. Our results show a truncation (or temporary drop) of the HI disk at
a column density around 10^19 atoms cm^-2, consistent with theoretical
expectations for the truncation produced by the extragalactic UV field. We also
detect a marginally significant levelling off of the HI distribution along the
continuation of the major axis at a column density near 2X10^18 atoms cm^-2.
The VLA results show that the gas beyond ~6' in radius must be relatively
smooth, with no structure larger in size than ~300 pc exhibiting a density
contrast of a factor of 10 or more. However, there is considerable
few-hundred-parsec scale structure in the gas disk at smaller radii, even well
outside the regions where there are visible stars. Two prominent cavities well
removed from any significant stellar populations are studied. While the
energies required for evacuation are consistent with those produced by multiple
supernovae, there is no visible trace of stars within a kpc of the center of
the larger cavity, and the smaller of the two cavities is centered just outside
the 26.5 mag arcsec^-2 B isophote. The velocity dispersion of the gas, measured
within our 270 pc beam, is 7 to 8 km s^-1 throughout the disk (to 6' radius).
This translates to a scaleheight of ~700 pc at the point where the rotation
curve flattens, at a radius of ~4.5 kpc. Velocity profiles are well fit by
single gaussians at all points.


title: Associated absorption and radio source growth

abstract:
  Results are presented from a survey for CIV associated absorption in a
complete low-frequency-selected sample of quasars. In agreement with previous
work, associated absorbers are most common in steep-spectrum and lobe-dominated
quasars, indicative of an anisotropic cloud distribution. Furthermore, we find
the strongest CIV absorption occurs in sources of small radio size, suggesting
that the absorbing clouds are destroyed or displaced as radio sources expand.
Evidence for dust in the clouds is also found, such that quasars with strong
absorption are systematically redder. Finally, we find no evidence for
evolution in the frequency or properties of the absorbers from z~0.7 to z~3.


title:
An efficient photoelectric X-ray Polarimeter for the study of Black
  Holes and Neutron Stars

abstract:
  In astronomy there are basically four kinds of observations to extract the
information carried by electromagnetic radiation: photometry, imaging,
spectroscopy and polarimetry. By optimal exploitation of the first three
techniques, X-ray astronomy has been able to unveil the violent world of
compact high energy sources. Here we report on a new instrument that brings
high efficiency also to X-ray polarimetry, the last unexplored field of X-ray
astronomy. It will then be possible to resolve the internal structures of
compact sources which otherwise would remain inaccessible, even to X-ray
interferometry1. Polarimetry could provide a direct, visual picture of the
state of matter under extreme magnetic and gravitational fields by measuring
the radiation polarized through interaction with the highly asymmetric matter
distribution (accretion disk) and with the magnetic field.
  The new instrument derives the polarization information from the track of the
photoelectrons imaged by a finely subdivided gas detector. Its great
improvement of sensitivity (at least two orders of magnitude) will allow direct
exploration of the most dramatic objects of the X-ray sky.


title: Thermal Desorption of Water-Ice in the Interstellar Medium

abstract:
  Water (H2O) ice is an important solid constituent of many astrophysical
environments. To comprehend the role of such ices in the chemistry and
evolution of dense molecular clouds and comets, it is necessary to understand
the freeze-out, potential surface reactivity, and desorption mechanisms of such
molecular systems. Consequently, there is a real need from within the
astronomical modelling community for accurate empirical molecular data
pertaining to these processes. Here we give the first results of a laboratory
programme to provide such data. Measurements of the thermal desorption of H2O
ice, under interstellar conditions, are presented. For ice deposited under
conditions that realistically mimic those in a dense molecular cloud, the
thermal desorption of thin films (~50 molecular layers) is found to occur with
zero order kinetics characterised by a surface binding energy, E_{des}, of 5773
+/- 60 K, and a pre-exponential factor, A, of 10^(30 +/- 2) molecules cm^-2
s^-1. These results imply that, in the dense interstellar medium, thermal
desorption of H2O ice will occur at significantly higher temperatures than has
previously been assumed.


title:
X-Ray Beaming due to Resonance Scattering in the Accretion Column of
  Magnetic Cataclysmic Variables

abstract:
  Extremely strong ionized Fe emission lines, with the equivalent width
reaching about 4000 eV, were discovered with ASCA from a few Galactic compact
objects, including AX J2315-0592, RX J1802.1+1804 and AX J1842.8-0423. These
objects are thought to be binary systems containing magnetized white dwarfs
(WDs). A possible interpretation of the strong Fe-K line is the line-photon
collimation in the WD accretion column, due to resonance scattering of line
photons. The collimation occurs when the accretion column has a flat shape, and
the effect is augmented by the vertical velocity gradient there, which reduces
the resonant trapping of resonant photons along the magnetic field lines. This
effect was quantitatively confirmed with Monte-Carlo simulations. Furthermore,
with ASCA observations of the polar V834 Centauri, this collimation effect was
clearly detected as a rotational modulation of the equivalent width of the Fe-K
emission line. Extremely strong emission lines mentioned above can be explained
by our interpretation consistently. Combing this effect with other X-ray
information, the geometry and plasma parameters in the accretion column were
determined.


title:
HST Imagery and CFHT Fabry-Perot 2-D Spectroscopy in Halpha of the
  Ejected Nebula M1-67: Turbulent Status

abstract:
  With HST-WFPC2 we have obtained a deep, H-alpha image of the WR nebula M1-67.
With the interferometer of the Universite Laval (Quebec, Canada), we have
obtained complementary Fabry-Perot H-alpha data using CFHT MOS/SIS. On the deep
H-alpha image we have performed continuous wavelet transforms to isolate
stochastic structures of different characteristic size and look for scaling
laws. Small-scale wavelet coefficients show that the density field of M1-67 is
remarkably structured in chaotically oriented filaments. We draw attention to a
short, marginally inertial range at the smallest scales (6.7-15.0 X 10^{-3}
pc), which can be attributed to turbulence in M1-67, and a strong scale break
at larger scales. Using our Fabry-Perot interferograms, we also present an
investigation of the statistical properties of fluctuating gas motions using
structure functions traced by H-alpha emission-line centroid velocities. We
find that there is a clear correlation at scales 0.02-0.22 pc between the mean
quadratic differences of radial velocities and distance over the surface of the
nebula. This implies that the velocity field shows an inertial range likely
related to turbulence. The first and second order moments of the velocity
increments are found to scale as <|Delta v(r)|> ~ r^{0.5} and <|Delta v(r)|^2>
\~ r^{0.9}. The former scaling law strongly suggests that compressible
turbulence is at play in M1-67, on the other hand, the latter scaling law
agrees very well with Larson-type laws for velocity turbulence. Examination of
the structure functions for different orders shows that the turbulent regime is
slightly intermittent and highly multifractal with universal multifractal
indexes alpha ~ 1.90-1.92 and C_1 ~ 0.04 \pm 0.01.


title:
Constraints on the radial mass distribution of dark matter halos from
  rotation curves

abstract: Stars have mass; halos lack cusps.


title: Globular Cluster Population of Hickson Compact Group 22a and 90c

abstract:
  We present the first measurement of the globular cluster populations of
galaxies in Hickson compact groups, in order to investigate the effect of these
high density environments on the formation and evolution of globular cluster
systems. Based on V and R band images that we obtained of HCG 22a and HCG 90c
with the ESO New Technology Telescope (NTT), we find a total globular cluster
population of $1590\pm 854$ for HCG 22a and $2136\pm 718$ for 90c. The specific
frequency for HCG 22a was found to be $S_{N}=1.9\pm 1.0$ and $S_{N}= 3.4\pm
1.1$ for HCG 90c. A power-law fit to the globular cluster radial profile of HCG
22a yields $\sigma\sim R^{-2.01\pm 0.30}$ and for HCG 90c we found $\sigma \sim
R^{-1.20\pm0.16}$. A comparison of the globular cluster radial profiles with
the surface brightness of the parent galaxy shows that the globular cluster
systems are at least as extended as the halo light. The measured values for the
specific frequency are consistent with a scenario in which the host galaxies
were in a low density ``field-like'' environment when they formed their
globular cluster systems.


title: Production of Lithium, Beryllium, and Boron by Hypernovae

abstract:
  We investigate a possible nucleosynthetic signature of highly energetic
explosions of C-O cores ("hypernovae," HNe) which might be associated with
gamma-ray bursts (GRBs). We note that the direct impact of C- and O-enriched
hypernova ejecta on the ambient hydrogen and helium leads to spallation
reactions which can produce large amounts of the light nuclides lithium,
beryllium, and boron (LiBeB). Using analytic velocity spectra of the hypernova
ejecta, we calculate the LiBeB yields of different exploding C-O cores
associated with observed hypernovae. The deduced yields are $\sim 10^3$ times
higher than those produced by similar (direct) means in normal Type II
supernovae, and are higher than the commonly used ones arising from shock wave
acceleration induced by Type II supernova (SN) explosions. To avoid
overproduction of these elements in our Galaxy, hypernovae should be rare
events, with $\la 10^{-3}$ hypernovae per supernova, assuming a constant HN/SN
ratio over time. This rate is in good agreement with that of long duration GRBs
if we assume that the gamma-ray emission is focussed with a beaming factor
$\Omega/4\pi \la 10^{-2}$. This encouraging result supports the possible HN-GRB
association. Thus, Galactic LiBeB abundance measurements offer a promising way
to probe the HN rate history and the possible HN-GRB correlation. On the other
hand, if hypernovae are associated to very massive pregalactic stars
(Population III) they would produce a LiBeB pre-enrichment in proto-galactic
gas, which could show up as a plateau in the lowest metallicities of the Be-Fe
relation in halo stars.


title: Untangling the X-ray Emission From the Sa Galaxy NGC1291 With Chandra

abstract:
  We present a Chandra ACIS-S observation of the nearby bulge-dominated Sa
galaxy NGC1291. The X-ray emission from the bulge resembles the X-ray emission
from a sub-class of elliptical and S0 galaxies with low L_X/L_B luminosity
ratios. The X-ray emission is composed of a central point-like nucleus, ~50
point sources that are most likely low mass X-ray binaries (LMXBs), and diffuse
gas detectable out to a radius of 120" (5.2 kpc). The diffuse gas has a global
temperature of 0.32^{+0.04}_{-0.03} keV and metallicity of 0.06 +/- 0.02 solar,
and both quantities marginally decrease with increasing radius. The hot gas
fills the hole in the HI distribution, and the softening of the spectrum of the
X-ray gas with radius might indicate a thermal coupling of the hot and cold
phases of the interstellar medium as previously suggested. The integrated X-ray
luminosity of the LMXBs, once normalized by the optical luminosity, is a factor
of 1.4 less than in the elliptical galaxy NGC4697 or S0 galaxy NGC1553. The
difference in L_{X,stellar}/L_B between the galaxies appears to be because of a
lack of very bright sources in NGC1291. No sources above 3 x 10^38 ergs/s were
found in NGC1291 when ~7 were expected from scaling from NGC4697 and NGC1553.
The cumulative L_{X,stellar}/L_B value including only sources below 1.0 x 10^38
ergs/s is remarkably similar between NGC1291 and NGC4697, if a recent surface
brightness fluctuation-determined distance is assumed for NGC4697. If this is a
common feature of the LMXB population in early-type systems, it might be used
as a distance indicator. Finally, a bright, variable (1.6-3.1 x 10^39 ergs/s)
source was detected at the optical center of the galaxy. Its spectrum shows
excess soft emission superimposed on a highly absorbed power law component,
similar to what has been found in several other low luminosity AGN (ABRIDGED).


title: Clumpy outer Galaxy molecular clouds and the steepening of the IMF

abstract:
  We report the results of high-resolution (~0.2 pc) CO(1-0) and CS(2-1)
observations of the central regions of three star-forming molecular clouds in
the far-outer Galaxy (~16 kpc from the Galactic Center): WB89 85 (Sh 2-127),
WB89 380, and WB89 437. We used the BIMA array in combination with IRAM 30-m
and NRAO 12-m observations. The GMC's in which the regions are embedded were
studied by means of KOSMA 3-m CO(2-1) observations. The properties the CO and
CS clumps are analyzed and compared with newly derived results of previously
published single-dish measurements of local clouds (OrionB South and Rosette).
We find that the slopes of the clump mass distributions (-1.28 and -1.49, for
WB89 85 and WB89 380, respectively) are somewhat less steep than found for most
local clouds, but similar to those of clouds which have been analyzed with the
same clumpfind program. We investigate the clump stability by using the virial
theorem, including all possible contributions (gravity, turbulence, magnetic
fields, and pressure due to the interclump gas). It appears that under
reasonable assumptions a combination of these forces would render most clumps
stable. Comparing only gravity and turbulence, we find that in the far-outer
Galaxy clouds, these forces are in equilibium (virial parameter alpha~1) for
clumps down to the lowest masses found (a few Msol). For clumps in the local
clouds alpha~1 only for clumps with masses larger than a few tens of Msol. Thus
it appears that in these outer Galaxy clumps gravity is the dominant force down
to a much lower mass than in local clouds, implying that gravitational collapse
and star formation may occur more readily even in the smallest clumps. Although
there are some caveats, due to the inhomogeneity of the data used, this might
explain the apparently steeper IMF found in the outer Galaxy.


title:
The Contribution of HI-Rich Galaxies to the Damped Absorber Population
  at z=0

abstract:
  We present a study of HI-rich galaxies in the local universe selected from
blind emission-line surveys. These galaxies represent the emission-line
counterparts of local damped Lyman-alpha systems. We find that the HI
cross-section of galaxies is drawn from a large range of galaxy masses below
M_star, 66% of the area comes from galaxies in the range 8.5 < Log M_star <
9.7. Both because of the low mass galaxy contribution, and because of the range
of galaxy types and luminosities at any given HI mass, the galaxies
contributing to the HI cross-section are not exclusively L_star spirals, as is
often expected. The optical and near infrared counterparts of these galaxies
cover a range of types (from spirals to irregulars), luminosities (from L_star
to <0.01 L_star), and surface brightnesses. The range of optical and near
infrared properties as well as the kinematics for this population are
consistent with the properties for the low-z damped Lyman-alpha absorbers. We
also show that the number of HI-rich galaxies in the local universe does not
preclude evolution of the low-z damped absorber population, but it is
consistent with no evolution.


title: Are High-Velocity Clouds the Building Blocks of the Local Group?

abstract:
  Motivated by the apparent order-of-magnitude discrepancy between the observed
number of Local Group satellite galaxies, and that predicted by Lambda-CDM
hierarchical clustering cosmologies, we explore an alternate suggestion -
perhaps the missing satellites are not actually ``missing'', but are instead
``in disguise''. The disguise we consider here is that of the classical HI
High-Velocity Clouds. Is it possible that what have been thought of
traditionally as a ``Galactic'' phenomenon, are actually the building blocks of
the Local Group? We discuss the strengths and weaknesses of this hypothesis,
and highlight avenues of future research which may provide an unequivocal
resolution to this contentious issue.


title:
Astrophysical constraints on a possible neutrino ball at the Galactic
  Center

abstract:
  The nature of the massive object at the Galactic Center (Sgr A$^{*}$) is
still unclear even if various observational campaigns led many authors to
believe that our Galaxy hosts a super-massive black hole with mass $M\simeq
2.6\times 10^6$ M$_{\odot}$. However, the black hole hypothesis, which
theoretically implies a luminosity $\simeq 10^{41}$ erg s$^{-1}$, runs into
problems if one takes into account that the observed luminosity, from radio to
$\gamma$-ray wavelengths, is below $10^{37}$ erg s$^{-1}$. In order to solve
this blackness problem, alternative models have been recently proposed. In
particular, it has been suggested that the Galactic Center hosts a ball made up
by non-baryonic matter (e.g. massive neutrinos and anti-neutrinos) in which the
degeneracy pressure of fermions balances their self-gravity. Requiring to be
consistent with all the available observations towards the Galactic Center
allows us to put severe astrophysical constraints on the neutrino ball
parameters. The presence of such an object in the Galactic Center may be
excluded if the constituent neutrino mass $m_{\nu}$ is $\ut> 24$ keV, while if
$m_{\nu}\ut< 24$ keV observations can not give a definite answer.


title: Molecular gas in nearby powerful radio galaxies

abstract:
  We report the detection of CO(1-0) and CO(2-1) emission from the central
region of nearby 3CR radio galaxies (z$<$ 0.03). Out of 21 galaxies, 8 have
been detected in, at least, one of the two CO transitions. The total molecular
gas content is below 10$^9$ \msun. Their individual CO emission exhibit, for 5
cases, a double-horned line profile that is characteristic of an inclined
rotating disk with a central depression at the rising part of its rotation
curve. The inferred disk or ring distributions of the molecular gas is
consistent with the observed presence of dust disks or rings detected optically
in the cores of the galaxies. We reason that if their gas originates from the
mergers of two gas-rich disk galaxies, as has been invoked to explain the
molecular gas in other radio galaxies, then these galaxies must have merged a
long time ago (few Gyr or more) but their remnant elliptical galaxies only
recently (last 10$^7$ years or less) become active radio galaxies. Instead, we
argue the the cannibalism of gas-rich galaxies provide a simpler explanation
for the origin of molecular gas in the elliptical hosts of radio galaxies (Lim
et al. 2000). Given the transient nature of their observed disturbances, these
galaxies probably become active in radio soon after the accretion event when
sufficient molecular gas agglomerates in their nuclei.


title: The nature of the progenitor of the Type II-P supernova 1999em

abstract:
  We present high quality ground-based VRI images of the site of the Type II-P
SN1999em (in NGC1637) taken before explosion, which were extracted from the
CFHT archive. We determine a precise position of the SN on these images to an
accuracy of 0.17''. The host galaxy is close enough (7.5 +/- 0.5 Mpc) that the
bright supergiants are resolved as individual objects, however we show that
there is no detection of an object at the SN position before explosion that
could be interpreted as the progenitor star. By determining the sensitivity
limits of the VRI data, we derive bolometric luminosity limits for the
progenitor. Comparing these to standard stellar evolutionary tracks which trace
evolution up to the point of core carbon ignition, we initially derive an upper
mass limit of approximately 12M_sol. However we present evolutionary
calculations that follow 7-12M_sol stars throughout their C-burning lifetime
and show that we can restrict the mass of the progenitor even further. Our
calculations indicate that progenitors initially of 8-10M_sol, undergoing
expected mass loss, can also be excluded because a second dredge up sends them
to somewhat higher luminosities than a star of initially 12M_sol. These results
limit the progenitor's initial main-sequence mass to a very narrow range of 12
+/- 1 M_sol. We discuss the similarities between the Type II-P SNe 1999em and
1999gi and their progenitor mass limits, and suggest that SN Type II-P
originate only in intermediate mass stars of 8-12M_sol, which are in the red
supergiant region and that higher mass stars produce the other Type II
sub-types. (Abridged).


title: Direct and indirect detection of WIMPs

abstract:
  We present here the principles of detection of Weakly Interacting Massive
Particles, which could represent a large contribution to Dark Matter. A status
of the experimental situation is given both for indirect and direct detection.
In particular, the DAMA claim for a WIMP signal is confronted to the recent
results of the CDMS and EDELWEISS experiments. We conclude by comparing direct
and indirect search sensitivities.


title:
HI absorption from the central kpc of radio galaxies: effect of
  orientation or interstellar medium?

abstract:
  We present a summary of recent studies of HI absorption in radio galaxies.
The results show how the absorption can be due to a variety of phenomena and
how they can help us in understanding more of what is happening in and around
AGNs.


title:
Adiabatic and Isocurvature Perturbations from Inflation: Power Spectra
  and Consistency Relations

abstract:
  We study adiabatic and isocurvature perturbations produced during a period of
cosmological inflation. We compute the power spectra and cross spectra of the
curvature and isocurvature modes, as well as the tensor perturbation spectrum
in terms of the slow-roll parameters. We provide two consistency relations for
the amplitudes and spectral indices of the corresponding power spectra. These
relations represent a definite prediction and a test of inflationary models
which should be adopted when studying cosmological perturbations through the
Cosmic Microwave Background in forthcoming satellite experiments.


title: Cosmological Constant from Conformal Fluctuations of the Metric

abstract:
  At the level of the Planck scale, the spacetime metric has to be considered a
quantum variable. Conformal quantum fluctuations of the metric tensor are
studied here. They lead to an extra term in the Einstein equations which can be
identified with a cosmological constant. Quantum fluctuations may therefore
contribute to an accelerated expansion of the universe, in accordance with
newer observational data.


title:
Discovery of depressions in the X-ray emission of the distant galaxy
  cluster RBS797 in a CHANDRA observation

abstract:
  We present CHANDRA observations of the X-ray luminous, distant galaxy cluster
RBS797 at z=0.35. In the central region the X-ray emission shows two pronounced
X-ray minima, which are located opposite to each other with respect to the
cluster centre. These depressions suggest an interaction between the central
radio galaxy and the intra-cluster medium, which would be the first detection
in such a distant cluster. The minima are symmetric relative to the cluster
centre and very deep compared to similar features found in a few other nearby
clusters. A spectral and morphological analysis of the overall cluster emission
shows that RBS797 is a hot cluster (T=7.7^{+1.2}_{-1.0} keV) with a total mass
of M_{tot}(r_{500})=6.5^{+1.6}_{-1.2} 10^{14} \msol.


title:
High Speed photometry of faint Cataclysmic Variables: I. V359 Cen, XZ
  Eri, HY Lup, V351 Pup, V630 Sgr, YY Tel, CQ Vel, CE-315

abstract:
  The first results of a photometric survey of faint Cataclysmic Variables are
presented. V359 Cen is an SU UMa star with a period of 112 min. Even though
observed at quiescence, the mass transfer rate in this old nova may be
sufficiently high that in such a short period system (with its implied small
mass ratio) the disc may be excited into an elliptical shape with the result
that the observed brightness modulation gives a superhump period rather than an
orbital period. XZ Eri is an eclipsing dwarf nova with an orbital period
(P(orb)) of 88.1 min. HY Lup has only slight variability. V351 Pup, the remnant
of Nova Puppis 1991, has P(orb) = 2.837 h and a light curve that strongly
resembles that of the magnetic Nova Cyg 1975. V630 Sgr is the first nova
remnant that has both positive superhumps (P(sh) = 2.980 h) and eclipses
(P(orb) = 2.831 h). The YY Tel identification is somewhat uncertain. The
correct identification for CQ Vel is provided from discovery of its flickering
activity. The light curve of CE-315, a recently discovered AM CVn star, shows
similarities to that of GP Com, with no apparent orbital modulation.


title: Multicolor observations of the Hubble Deep Field South

abstract:
  We present a deep multicolor (UBVIJsHKs) catalog of galaxies in the HDF-S,
based on observations obtained with the HST WFPC2 in 1998 and VLT-ISAAC in
1999. The photometric procedures were tuned to derive a catalog optimized for
the estimation of photometric redshifts. In particular we adopted a
``conservative'' detection threshold which resulted in a list of 1611 objects.
The behavior of the observed source counts is in general agreement with the
result of Casertano et al. (2000) in the HDF-S and Williams et al. (1996) in
the HDF-N, while the corresponding counts in the HDF-N provided by
Fernandez-Soto et al. (1999) are systematically lower by a factor 1.5 beyond
I_AB=26. After correcting for the incompleteness of the source counts, the
object surface density at I_AB<27.5 is estimated to be 220 per square arcmin,
providing an estimate of the Extragalactic Background Light in the I band
consistent with the work of Madau & Pozzetti(2000). The comparison between the
median V-I color in the HDF-North and South shows a significant difference
around I_AB~26, possibly due to the presence of large scale structure at z~1 in
the HDF-N. High-z galaxy candidates (90 U dropout and 17 B dropout) were
selected by means of color diagrams, down to a magnitude I_AB=27, with a
surface density of (21+-1) and (3.9+-0.9) per square arcmin, respectively. 11
EROs (with (I-K)_AB>2.7) were selected down to K_AB=24, plus 3 objects whose
upper limit to the Ks flux is still compatible with the selection criterion.
The corresponding surface density of EROs is (2.5+-0.8) per sq.arcmin
((3.2+-0.9) per sq.arcmin if we include the three Ks upper limits). They show a
remarkably non-uniform spatial distribution and are classified with roughly
equal fractions in the categories of elliptical and starburst galaxies.


title: Can Photoionization Squelching Resolve the Sub-structure Crisis?

abstract:
  Cold Dark Matter theory predicts that the Local Group should contain many
more dwarf-sized objects than the observed number of dwarf galaxies --- the
so-called sub-structure problem. We investigate whether the suppression of star
formation in these small objects due to the presence of a photoionizing
background can resolve the problem. We make use of results from recent
hydrodynamic simulations to build a recipe for the suppression of gas infall
into semi-analytic galaxy formation models, and use these to predict the
luminosity function of dwarf galaxies in the Local Group. In the models without
photoionization ``squelching'', we predict a large excess of faint dwarf
galaxies compared with the observed number in the Local Group --- thus, the
usual recipe for supernovae feedback used in semi-analytic models does not
solve the sub-structure problem on its own. When we include photoionization
squelching, we find good agreement with the observations. We have neglected
tidal destruction, which probably further reduces the number of dwarf galaxies.
We conclude that photoionizing squelching easily solves the sub-structure
problem. In fact, it is likely that once this effect is taken into account,
models with reduced small-scale power (e.g. Warm Dark Matter) would
underproduce dwarf galaxies.


title: Studies of Magnetic and Suspected-Magnetic Southern White Dwarfs

abstract:
  Optical spectrophotometry and circular spectropolarimetry are presented for
several candidate magnetic white dwarfs that were identified during the
Hamburg/ESO survey for bright QSOs. HE 1211-1707 and HE 1043-0502 are rare
examples of white dwarfs that show neutral helium lines in a high magnetic
field, in these cases ~50 MG and ~800 MG, respectively. The former is also
found to be rotating with a period of ~2 hr. HE 1045-0908 is a hydrogen-line
star at ~20 MG, spinning with a period in the range ~2-4 hr. Attempts at
modeling the limited amount of phase-resolved data that is available suggest
that the field structure departs substantially from a simple centered dipolar
geometry. Two rather cool white dwarfs with unidentified broad absorption
spectral features, HE 0236-2656 and HE 0330-0002, are confirmed to be magnetic.
Line identifications for these stars are not yet possible, but the atmospheres
are probably helium-rich, with spectral features formed by trace compounds of
hydrogen, carbon, and perhaps other metals. HE 0003-5701 and HE 0338-3853 were
proposed by Reimers et al. (1996) alongside two similar objects to be magnetic
DB white dwarfs, with magnetic fields all near 20 MG. However, our observations
show that the first two, and by extension all four, are nonmagnetic white dwarf
+ cool dwarf pairs, deserving of study in their own right as possible close
binaries. Finally, a lack of circular polarisation suggests that HE 0000-3430
and HE 0127-3110 are also nonmagnetic. HE 0000-3430 appears to be a featureless
DC white dwarf over the spectral range observed here, while the sole absorption
line near 5890A in HE 0127-3110 could be either He I 5876A or the Na I D
doublet, but there are difficulties with either interpretation.


title: Exploring Brown Dwarf Disks

abstract:
  We discuss the spectral energy distribution of three very low mass objects in
Chamaeleon I for which ground-based spectroscopy and photometry as well as ISO
measurements in the mid-infrared are available (Comeron et al. 2000; Persi et
al. 2000). One of these stars (Cha Halpha1) is a bona-fide brown dwarf, with
mass 0.04-0.05 Msun. We show that the observed emission is very well described
by models of circumstellar disks identical to those associated to T Tauri
stars, scaled down to keep the ratio of the disk-to-star mass constant and to
the appropriate stellar parameters. This result provides a first indication
that the formation mechanism of T Tauri stars (via core contraction and
formation of an accretion disk) extends to objects in the brown dwarf mass
range.


title:
Ubiquitous variability of X-ray absorbing column densities in Seyfert 2
  Galaxies

abstract:
  We present a study of the variations in the absorbing column density of 25
X-ray defined Seyfert 2 galaxies, as inferred from hard X-ray observations, on
timescales from months to several years. We show that a significant variation
of N$_H$ (from 20% to 80%) is observed in almost all (22/25) the sources with
multiple X-ray observations, although X-ray absorption never vanishes. For a
subsample of 11 sources observed at least five times the typical variation
time, as defined by a structure function, is less than one year for both
heavily absorbed (nH ~ 10^23 cm^(-2)) and moderately absorbed (nH ~ 10^22
cm^(-2)) sources. These variations rule out the simplest version of the unified
models, based on a homogeneous obscuring torus, and suggest the presence of
clumpy circumnuclear material on a scale well below a parsec. We propose a
modification of the torus model in which an overabundance of slightly dusty
BELR clouds obscures the BELR. The BELR needs, like the torus, to have an
axisymmetric structure. This model is closely related to that of Elvis (2000)
for type 1 AGN. For lightly obscured AGN (nH ~ 10^22 cm^(-2)) the structure
function shows an increase at a timescale of $\sim$5 yr, indicating a second
absorber, most probably on a 5-10 pc scale associated with the host galaxy.


title: Giant Pulses from the Millisecond Pulsar B1821-24

abstract:
  We have carried out a survey for `giant pulses' in 5 millisecond pulsars. We
detect individual pulses from the high Edot pulsar PSR B1821-24 with energies
exceeding 50x the mean pulse energy. These giant pulses are concentrated in a
narrow phase window coincident with the power-law non-thermal pulse seen in
hard X-rays. This is the third example of the giant pulse phenomenon. It
supports the idea that large B fields in the outer magnetosphere are critical
to the formation of such pulses and further suggests a direct connection
between giant pulses and high energy emission.


title: A Simple Varying-alpha Cosmology

abstract:
  We investigate the cosmological consequences of a simple theory in which the
electric charge $e$ is allowed to vary. The theory is locally gauge and Lorentz
invariant, and satisfies general covariance. We find that in this theory the
fine structure 'constant', $\alpha $, remains almost constant in the radiation
era, undergoes small increase in the matter era, but approaches a constant
value when the universe starts accelerating because of the presence of a
positive cosmological constant. This model satisfies geonuclear,
nucleosynthesis, and CMB constraints on time-variation in $\alpha $, while
fitting simultaneously the observed accelerating universe and the recent
high-redshift evidence for small $\alpha $ variations in quasar spectra. The
model also places specific restrictions on the nature of the dark matter.
Further tests, involving stellar spectra and the E\"otv\"os experiment, are
proposed.


title: Identification of the LMXB and Faint X-ray Sources in NGC 6652

abstract:
  We have detected three new x-ray point sources, in addition to the known
low-mass x-ray binary (LMXB) X1832-330, in the globular cluster NGC 6652 with a
Chandra 1.6 ksec HRC-I exposure. Star 49 (M_{V}~4.7), suggested by Deutsch et
al.(1998) as the optical candidate for the LMXB, is identified (<0.3") not with
the LMXB, but with another, newly detected source (B). Using archival HST
images, we identify (<0.3") the LMXB (A) and one of the remaining new sources
(C) with blue variable optical counterparts at M_{V}~3.7 and 5.3 respectively.
The other new source (D) remains unidentified in the crowded cluster core. In
the 0.5-2.5 keV range, assuming a 5 keV thermal bremsstrahlung spectrum and
N_{H}=5.5*10^{20}, source A has intrinsic luminosity L_{X}~5.3*10^{35} ergs/s.
Assuming a 1 keV thermal bremsstrahlung spectrum, B has L_{X}~4.1*10^{33}
ergs/s, while C and D have L_{X}~8*10^{32}$ ergs/s. Source B is probably a
quiescent LMXB, while source C may be either a luminous CV or quiescent LMXB.


title:
XMM-Newton Spectroscopy of the Galactic Microquasar GRS 1758-258 in the
  Peculiar Off/Soft State

abstract:
  We report on an XMM-Newton Reflection Grating Spectrometer observation of the
black hole candidate and Galactic microquasar GRS 1758-258. The source entered
a peculiar "off/soft" state in late February, 2001, in which the spectrum
softened while the X-ray flux -- and the inferred mass accretion rate --
steadily decreased. We find no clear evidence for emission or absorption lines
in the dispersed spectra, indicating that most of the observed soft flux is
likely from an accretion disk and not from a cool plasma. The accretion disk
strongly dominates the spectrum in this lower-luminosity state, and is only
mildly recessed from the marginally stable orbit. These findings may be
difficult to explain in terms of advection-dominated accretion flow, or "ADAF"
models. We discuss these results within the context of ADAF models,
simultaneous two-flow models, and observed correlations between hard X-ray flux
and jet production.


title:
A SCUBA Scanmap of the HDF: Measuring the bright end of the sub-mm
  source counts

abstract:
  Using the 850 micron SCUBA camera on the JCMT and a scanning technique
different from other sub-mm surveys, we have obtained a 125 square arcminute
map centered on the Hubble Deep Field. The one-sigma sensitivity to point
sources is roughly 3 mJy and thus our map probes the brighter end of the sub-mm
source counts. We find 6 sources with a flux greater than about 12 mJy (>4
sigma) and, after a careful accounting of incompleteness and flux bias,
estimate the integrated density of bright sources N(>12 mJy)= 164 (+77/-58) per
square degree (68 per cent confidence bounds).


title:
Small Scale Structure at High Redshift: III. The Clumpiness of the
  Intergalactic Medium on Sub-kpc Scales

abstract:
  Spectra from the Keck HIRES instrument of the Lyman alpha forests in the
lines of sight to the A and C components of the gravitationally lensed QSO
Q1422+231 were used to investigate the structure of the intergalactic medium at
mean redshift z = 3.3 on sub-kpc scales. We measured the cross-correlation
amplitude between the two Lyman alpha forests for a mean transverse separation
of 120 pc, and computed the RMS column density and velocity differences between
individual absorption systems seen in both lines of sight. The RMS differences
between the velocity centroids of the Lyman alpha forest lines were found to be
less than about 400 m/s, for unsaturated absorption lines with column densities
in the range 12<log N(HI)<14.13. The rate of energy transfer into the low
density IGM on a typical scale of 100 pc seems to be lower by 3-4 orders of
magnitude than the rate measured earlier for strong CIV metal absorption
systems. The tight correlation between HI column density and baryonic density
in the intergalactic medium was used to obtain an upper limit on the RMS
fluctuations of the baryonic density field. The fraction of the absorption
lines that are different across the lines of sight was used to determine the
filling factor of the universe for gas which has suffered recent hydrodynamic
disturbances. We thereby derived upper limits on the filling factor of galactic
outflows at high redshift. Short-lived, short-range ancient winds are
essentially unconstrained by this method but strong winds blowing for a
substantial fraction of a Hubble time (at z = 3.3) appear to fill less than 20
percent of the volume of the universe.


title:
Properties of Microlensing Light Curve Anomalies Induced by Multiple
  Planets

abstract:
  In this paper, we show that the pattern of microlensing light curve anomalies
induced by multiple planets are well described by the superposition of those of
the single-planet systems where the individual planet-primary binary pairs act
as independent lens systems. Since the outer deviation regions around the
planetary caustics of the individual planets occur in general at different
locations, we find that the pattern of anomalies in these regions are hardly
affected by the existence of other planet(s). This implies that even if an
event is caused by a multiple planetary system, a simple single-planet lensing
model is good enough for the description of most anomalies caused by the source
passage of the outer deviation regions. Detection of the anomalies resulting
from the source trajectory passing both the outer deviation regions caused by
more than two planets will provide a new channel of detecting multiple planets.


title:
Astrometric Method to Break the Photometric Degeneracy between
  Binary-source and Planetary Microlensing Perturbations

abstract:
  An extra-solar planet can be detected by microlensing because the planet can
perturb the smooth lensing light curve created by the primary lens. However, it
was shown by Gaudi that a subset of binary-source events can produce light
curves that closely resemble those produced by a significant fraction of
planet/star lens systems, causing serious contamination of a sample of
suspected planetary systems detected via microlensing. In this paper, we show
that if a lensing event is observed astrometrically, one can unambiguously
break the photometric degeneracy between binary-source and planetary lensing
perturbations. This is possible because while the planet-induced perturbation
in the trajectory of the lensed source image centroid shifts points away from
the opening of the unperturbed elliptical trajectory, while the perturbation
induced by the binary source companion points always towards the opening.
Therefore, astrometric microlensing observations by using future high-precision
interferometers will be important for solid confirmation of microlensing planet
detections.


title: Deep Radio, Optical, and Infrared Observations of SGR 1900+14

abstract:
  We present HST/STIS, Keck J- and K_s-band, and VLA 332-MHz and 1400-MHz
images of the region around the Soft Gamma-ray Repeater SGR 1900+14. No
non-stellar sources were detected at the position of SGR 1900+14, giving
3-sigma limits of S_{332 MHz} < 6 mJy, S_{1.4 GHz} < 1.5 mJy, m_{50CCD} > 29.0
mag, J > 22.8 mag, K_s > 20.8 mag (point sources), and S_{332 MHz} < 6.1 mJy
arcmin^{-2}, S_{1.4 GHz} < 6.2 mJy arcmin^{-2} (extended emission). Given the
very high extinction, the STIS and J-band non-detections are not constraining,
but the K_s-band limit rules out basic accretion disk models for SGR 1900+14,
and is almost comparable in depth to the K_s-band detection for the anomalous
X-ray pulsar 4U 0142+61. Finally, we report the detection in this field of
three new candidate supernova remnants, SNRs G043.5+00.6, G042.0-00.1, and
G041.5+00.4.


title:
High-Resolution X-ray and Radio Observations of SGR 1900+14 in the
  Immediate Aftermath of a Giant Flare

abstract:
  We present the results of Chandra, RXTE, and VLA observations of SGR 1900+14
in the immediate aftermath of its 2001 April 18 giant flare event. In the X-ray
band we find the source in a pulsating and bursting state, with time-averaged
2--10 keV flux initially elevated by 20% above the source's previous quiescent
periods. In the radio we establish upper limits on the strength of any
persistent post-flare emission of 0.7 and 0.1 mJy at 1.4 GHz and 8.0 GHz,
respectively. The position of the X-ray source is consistent, to approximately
1 arcsec precision, with the August 1998 VLA determination, and the
one-dimensional X-ray profile is consistent with that of a point source. The
X-ray spectrum is best-fit by a two component power-law plus blackbody model,
with fitted blackbody temperature kT_{BB} ~ 0.5 keV and radius R_{BB} ~ 1.5 km
for an assumed distance of 5 kpc. The spectral parameters of this thermal
component are consistent with those reported for the source in quiescence, and
the variations in the source flux we observe may be explained as variations in
the power-law component alone, providing support for magnetar models of SGR
1900+14.


title:
Existence of Large Scale Synchrotron X-ray Jets in Radio-loud Active
  Galactic Nuclei

abstract:
  In this paper, analytical arguments are presented that there exists a
synchrotron X-ray jet on large scales in most radio-loud AGNs, based on the
knowledge of the nature and physics of blazars. In blue blazars and
blue-blazar-like radio galaxies, the large scale X-ray jet may get faint along
the jet, while in most red blazars and red-blazar-like radio galaxies, the
X-ray jet is bright on 10 kpc scales whether the jet is highly relativistic on
large scales or not. In extreme red blazars in which the jet is still highly
relativistic on large scales and the synchrotron peak of the inner jet lies in
the infrared bands, the X-ray jet may get fainter along the jet from 10 kpc to
100 kpc scales while the optical and IR jet gets brighter. The predictions can
be tested with the ongoing observations of the Chandra X-ray Observatory.


title: Local Column Density Distribution Function from HI selected galaxies

abstract:
  The cross-section of sky occupied by a particular neutral hydrogen column
density provides insight into the nature of Lyman-alpha absorption systems. We
have measured this column density distribution at z=0 using 21-cm HI emission
from a blind survey. A subsample of HI Parkes All Sky Survey (HIPASS) galaxies
have been imaged with the Australia Telescope Compact Array (ATCA). The
contribution of low HI mass galaxies 10^7.5 to 10^8 M_solar is compared to that
of M_star (10^10 to 10^10.5 M_solar) galaxies. We find that the column density
distribution function is dominated by low HI mass galaxies with column
densities in the range 3x10^18 to 2x10^20 cm^-2. This result is not intuitively
obvious. M_star galaxies may contain the bulk of the HI gas, but the
cross-section presented by low HI mass galaxies 10^7.5 to 10^8 M_solar is
greater at moderate column densities. This result implies that moderate column
density Lyman-alpha absorption systems may be caused by a range of galaxy types
and not just large spiral galaxies as originally thought.


title:
2-Point Moments in Cosmological Large Scale Structure: I. Theory and
  Comparison with Simulations

abstract:
  We present new perturbation theory (PT) predictions in the Spherical Collapse
(SC) model for the 2-point moments of the large-scale distribution of dark
matter density in the universe. We assume that these fluctuations grow under
gravity from small Gaussian initial conditions. These predictions are compared
with numerical simulations and with previous PT results to assess their domain
of validity. We find that the SC model provides in practice a more accurate
description of 2-point moments than previous tree-level PT calculations. The
agreement with simulations is excellent for a wide range of scales (5-50 Mpc/h)
and fluctuations amplitudes (0.02-2 variance). When normalized to unit variance
these results are independent of the cosmological parameters and of the initial
amplitude of fluctuations. The 2-point moments provide a convenient tool to
study the statistical properties of gravitational clustering for fairly
non-linear scales and complicated survey geometries, such as those probing the
clustering of the Ly-alpha forest. In this context, the perturbative SC
predictions presented here, provide a simple and novel way to test the
gravitational instability paradigm.


title: Backwards galaxy formation with only a few parameters

abstract:
  We present a simple phenomenological model of star formation in galaxies that
describes the process with a set of a few parameters. The star formation
efficiency and the fraction of gas and metals ejected in outflows are assumed
to be the main drivers of star formation. The Tully-Fisher and the
Faber-Jackson relations in different passbands are used as constraints in the
analysis of disk and elliptical galaxies, respectively. We find that a steep
correlation between efficiency and maximum rotation velocity is needed in disk
systems, whereas elliptical galaxies can be explained by a high star formation
efficiency, uncorrelated with the central velocity dispersion. Gas outflows are
not important in disks, whereas a correlation of outflows with galaxy mass must
be invoked in early-type galaxies to account for the observed colors.


title: Structure Formation and the Time Dependence of Quintessence

abstract:
  We discuss the influence of dark energy on structure formation, especially
the effects on \sigma_8. Our interest is particularly focused on quintessence
models with time-dependent equation of state and non-negligible quintessence
component in the early universe. We obtain an analytic expression for \sigma_8
valid for a large class of dark energy models. We conclude that structure
formation is a good indicator for the history of dark energy and use our
results to set constraints on quintessence models.


title:
The Canada-France deep fields survey-I: 100,000 galaxies, 1 deg^2: a
  precise measurement of \omega(\theta) to IAB~25

abstract:
  (abridged) Using the UH8K mosaic camera, we have measured the angular
correlation function \omega(\theta) for 100,000 galaxies over four widely
separated fields totalling ~1\deg^2 and reaching IAB~25.5. With this sample we
investigate the dependence of \omega(\theta) at 1', A_\omega(1'), on sample
median IAB magnitude in the range 19.5<I(AB-med)<24. Our results show that
A_\omega(1') decreases monotonically to IAB~25. At bright magnitudes,
\omega(\theta) is consistent with a power-law of slope \delta = -0.8 for
0.2'<\theta<3.0' but at fainter magnitudes we find \delta ~ -0.6. At the
3\sigma level, our observations are still consistent with \delta=-0.8.
Furthermore, in the magnitude ranges 18.5<IAB<24.0 and 18.5<IAB<23.0 we find
galaxies with 2.6<(V-I)AB<2.9 have A_\omega(1')'s which are ~10x higher than
field values. We demonstrate that our model redshift distributions for the
faint galaxy population are in good agreement with current spectroscopic
observations. Using these predictions, we find that for low-omega cosmologies
and assuming r_0=4.3/h Mpc, in the range 19.5<I(AB-med)<22, the growth of
galaxy clustering is \epsilon~0. However, at 22<I(AB-med)<24.0, our
observations are consistent with \epsilon>1. Models with \epsilon~0 cannot
simultaneously match both bright and faint measurements of A_\omega(1`). We
show how this result is a natural consequence of the ``bias-free'' nature of
the \epsilon formalism and is consistent with the field galaxy population in
the range 22.0<IAB<24.0 being dominated by galaxies of low intrinsic
luminosity.


title:
The dependence of cosmological parameters estimated from the microwave
  background on non-gaussianity

abstract:
  The estimation of cosmological parameters from cosmic microwave experiments
has almost always been performed assuming gaussian data. In this paper the
sensitivity of the parameter estimation to different assumptions on the
probability distribution of the fluctuations is tested. Specifically, adopting
the Edgeworth expansion, I show how the cosmological parameters depend on the
skewness of the C_l spectrum. In the particular case of skewness independent of
the multipole number I find that the primordial slope, the baryon density and
the cosmological constant increase with the skewness.


title:
Black hole mass and velocity dispersion of narrow line region in active
  galactic nuclei and narrow line Seyfert 1 galaxies

abstract:
  Controversy regarding whether Narrow Line Seyfert 1 galaxies (NLS1s) follow
the same black hole --- galactic bulge relation as normal galaxies has been
raised by Mathur et al. (2001) and Ferrarese et al. (2001). In this paper, the
correlation between the black hole mass and the velocity dispersion of the
narrow line region, indicated by the [OIII] line width for a heterogeneous
sample of NLS1s, is examined. We show that the non-virial component subtracted
[OIII] width, which may well represent the stellar velocity dispersion
($\sigma$), correlates with the estimated black hole mass $M_bh$, made using
the empirical law for broad line region sizefrom the reverberation mapping
measurements. Considering measurement errors, we find that the relation between
$M_bh$ and the [OIII] width in active galactic nuclei (both NLS1s in this paper
and normal Seyferts in Nelson 2000) is consistent with that defined in nearby
hot galaxies (ellipticals and spiral bulge) but with more scatter. It seems
that there is no clear difference in the relation between $M_bh$ and $\sigma$
(represented by the [OIII] width for AGN) in both NLS1s and normal broad line
active galactic nuclei from the same relation defined by nearby hot galaxies.


title:
A comparison of DA white dwarf temperatures and gravities from Lyman and
  Balmer line studies

abstract:
  We present measurements of the effective temperatures and surface gravities
for a sample of hot DA white dwarfs, using the Lyman line data available from
the HUT, ORFEUS and FUSE far-UV space missions. Comparing the results with
those from the standard Balmer line technique, we find that there is a general
good overall agreement between the two methods. However, significant
differences are found for a number of stars, but not always of a consistent
nature in that sometimes the Balmer temperature exceeds that derived from the
Lyman lines and in other instances is lower. We conclude that, with the latest
model atmosphere calculations, these discrepancies probably do not arise from
an inadequate theoretical treatment of the Lyman lines but rather from
systematic effects in the observation and data reduction processes, which
dominate the statistical errors in these spectra. If these systematic data
reduction effects can be adequately controlled, the Lyman line temperature and
gravity measurements are consistent with those obtained from the Balmer lines
when allowance is made for reasonable observational uncertainties.


title:
Particle acceleration by ultra-relativistic shocks: theory and
  simulations

abstract:
  We consider the acceleration of charged particles near ultra-relativistic
shocks, with Lorentz factor Gamma_s >> 1. We present simulations of the
acceleration process and compare these with results from semi-analytical
calculations. We show that the spectrum that results from acceleration near
ultra-relativistic shocks is a power law, N(E) \propto E^{-s}, with a nearly
universal value s \approx 2.2 - 2.3 for the slope of this power law.
  We confirm that the ultra-relativistic equivalent of Fermi acceleration at a
shock differs from its non-relativistic counterpart by the occurence of large
anisotropies in the distribution of the accelerated particles near the shock.
In the rest frame of the upstream fluid, particles can only outrun the shock
when their direction of motion lies within a small loss cone of opening angle
theta_c \approx 1/Gamma_s around the shock normal.
  We also show that all physically plausible deflection or scattering
mechanisms can change the upstream flight direction of relativistic particles
originating from downstream by only a small amount: Delta theta ~ 1/Gamma_s.
This limits the energy change per shock crossing cycle to Delta E ~ E, except
for the first cycle where particles originate upstream. In that case the
upstream energy is boosted by a factor ~ Gamma_s^2 for those particles that are
scattered back across the shock into the upstream region.


title: Hard X-ray tails and cyclotron features in X-ray pulsars

abstract:
  We review the physical processes occurring in the magnetosphere of accreting
X-ray pulsars, with emphasis on those processes that give rise to observable
effects in their high (E>10 keV) energy spectra. In the second part we compare
the empirical spectral laws used to fit the observed spectra with theoretical
models, at the light of the BeppoSAX results on the broad-band characterization
of the X-ray pulsar continuum, and the discovery of new (multiple) cyclotron
resonance features.


title: Relativistic Beaming and Flux Variability in Active Galactic Nuclei

abstract:
  We discuss the impact of special relativistic effects on the observed light
curves and variability duty cycles of AGNs. We model the properties of AGN
light curves at radio wavelengths using a simulated shot noise process in which
the occurrence of major flaring events in a relativistic jet is governed by
Poisson statistics. We show that flaring sources whose radiation is highly
beamed toward us are able to reach very high flux levels, but will in fact
spend most of their time in relatively low flaring states due to relativistic
contraction of flare time scales in the observer frame. The fact that highly
beamed AGNs do not return to a steady-state quiescent level between flares
implies that their weakly beamed counterparts should have highly stable flux
densities that result from a superposition of many long-term, low-amplitude
flares. The ``apparent'' quiescent flux levels of these weakly beamed AGNs
(identified in many unified models as radio galaxies) will be significantly
higher than their ''true'' quiescent (i.e., non-flaring) levels. We use Monte
Carlo simulations to investigate flux variability bias in the selection
statistics of flat-spectrum AGN samples. In the case of the Caltech-Jodrell
Flat-spectrum survey, the predicted orientation bias towards jets seen end-on
is weakened if the parent population is variable, since the highly beamed
sources have a stronger tendency to be found in low flaring states. This effect
is small, however, since highly beamed sources are relatively rare, and their
fluxes tend to be boosted sufficiently above the survey limit such that they
are selected regardless of their flaring level. We find that for larger
flat-spectrum AGN surveys with fainter flux cutoffs, variability should not be
an appreciable source of selection bias.


title:
The star cluster system of the 3 Gyr old merger remnant NGC 1316: Clues
  from optical and near-infrared photometry

abstract:
  The giant merger remnant galaxy NGC 1316 (Fornax A) is an ideal probe for
studying the long-term effects of a past major merger on star cluster systems,
given its spectroscopically derived merger age of ~3 Gyr which we reported in a
recent paper. Here we present new ground-based, large-area optical and near-IR
imaging of star clusters in NGC 1316, complemented with deep HST/WFPC2 imaging.
We find that the optical-near-IR colours and luminosities of the brightest ~10
clusters in NGC 1316 are consistent with those of intermediate-age (2-3 Gyr)
populations. Unlike `normal' giant ellipticals, the B-I colour distribution of
clusters in NGC 1316 is not clearly bimodal. However, the luminosity functions
(LFs) of the blue and red parts of the cluster colour distribution are
different: The red cluster LF is well represented by a power law with index
-1.2 +/- 0.3, extending to about 1.5 mag brighter (in B) than those of typical
giant ellipticals. In contrast, the shape of the blue cluster LF is consistent
with that of `normal' spiral and elliptical galaxies. We conclude that the star
cluster system of NGC 1316 is a combination of a population of age ~3 Gyr
having roughly solar metallicity and a population of old, metal-poor clusters
which probably belonged to the pre-merger galaxies. After the 3 Gyr old,
metal-rich clusters fade to an age of 10 Gyr, they will form a red `peak' in a
bimodal cluster colour distribution. This `red peak' will have a colour
consistent with that found in `normal, old' giant ellipticals of the same
galaxy luminosity (taking age dimming into account). These features of the star
cluster system of NGC 1316 are fully consistent with scenarios for forming
`normal' giant elliptical galaxies through gas-rich mergers at look-back times
$\ga$ 10 Gyr.


title:
A New Observational Upper Limit to the Low Redshift Ionizing Background
  Radiation

abstract:
  We report a new Fabry-Perot search for Halpha emission from the intergalactic
cloud HI 1225+01 in an attempt to measure the low redshift ionizing background
radiation. We set a new 2 sigma upper limit on Halpha emission of 8 mR (5 x
10^{-20} ergs cm^{-2} s^{-1} arcsec^{-2}). Conversion of this limit to limits
on the strength of the ionizing background requires knowledge of the ratio of
the projected to total surface area of this cloud, which is uncertain. We
discuss the plausible range of this ratio, and within this range find that the
strength of the ionizing backround is in the lower range of, but consistent
with, previous observational and theoretical estimates.


title: The GRB Host Galaxies and Redshifts

abstract:
  Observations of GRB host galaxies and their environments in general can
provide valuable clues about the nature of progenitors. Bursts are associated
with faint, <R> ~ 25 mag, galaxies at cosmological redshifts, <z> ~ 1. The host
galaxies span a range of luminosities and morphologies, and appear to be
broadly typical for the normal, evolving, actively star-forming galaxy
populations at comparable redshifts and magnitudes, but may have somewhat
elevated SFR per unit luminosity. There are also spectroscopic hints of massive
star formation, from the ratios of [Ne III] and [O II] lines. The observed,
unobscured star formation rates are typically a few M_sun/yr, but a
considerable fraction of the total star formation in the hosts may be obscured
by dust. A census of detected optical afterglows provides a powerful new handle
on the obscured fraction of star formation in the universe; the current results
suggest that at most a half of the massive star formation was hidden by dust.


title:
Ultraviolet Broad Absorption Features and the Spectral Energy
  Distribution of the QSO PG 1351+64

abstract:
  We present a moderate-resolution (~20 km/s) spectrum of the mini
broad-absorption-line QSO PG1351+64 between 915-1180 A, obtained with the Far
Ultraviolet Spectroscopic Explorer (FUSE). Additional spectra at longer
wavelengths were also obtained with the HST and ground-based telescopes. Broad
absorption is present on the blue wings of CIII 977, Ly-beta, OVI 1032,1038,
Ly-alpha, NV 1238,1242, SiIV 1393,1402, and CIV 1548,1450. The absorption
profile can be fitted with five components at velocities of ~ -780, -1049,
-1629, -1833, and -3054 km/s with respect to the emission-line redshift of z =
0.088. All the absorption components cover a large fraction of the continuum
source as well as the broad-line region. The OVI emission feature is very weak,
and the OVI/Lyalpha flux ratio is 0.08, one of the lowest among low-redshift
active galaxies and QSOs. The UV continuum shows a significant change in slope
near 1050 A in the restframe. The steeper continuum shortward of the Lyman
limit extrapolates well to the observed weak X-ray flux level. The absorbers'
properties are similar to those of high-redshift broad absorption-line QSOs.
The derived total column density of the UV absorbers is on the order of 10^21
cm^-2, unlikely to produce significant opacity above 1 keV in the X-ray. Unless
there is a separate, high-ionization X-ray absorber, the QSO's weak X-ray flux
may be intrinsic. The ionization level of the absorbing components is
comparable to that anticipated in the broad-line region, therefore the
absorbers may be related to broad-line clouds along the line of sight.


title: Power Spectra for Galaxy Shape Correlations

abstract:
  It has recently been argued that the observed ellipticities of galaxies may
be determined at least in part by the primordial tidal gravitational field in
which the galaxy formed. Long-range correlations in the tidal field could thus
lead to an ellipticity-ellipticity correlation for widely separated galaxies. I
present results of a calculation of the angular power spectrum of intrinsic
galaxy shape correlations using a new model relating ellipticity to angular
momentum. I show that for low redshift galaxy surveys, the model predicts that
intrinsic correlations will dominate correlations induced by weak lensing, in
good agreement with previous theoretical work and observations. The model also
produces `E-mode' correlations enhanced by a factor of 3.5 over `B-modes' on
small scales, making it harder to disentangle intrinsic correlations from weak
lensing.


title: Squelched Galaxies and Dark Halos

abstract:
  There is accumulating evidence that the faint end of the galaxy luminosity
function might be very different in different locations. The luminosity
function might be rising in rich clusters and flat or declining in regions of
low density. If galaxies form according to the model of hierarchical clustering
then there should be many small halos compared to the number of big halos. If
this theory is valid then there must be a mechanism that eliminates at least
the visible component of galaxies in low density regions. A plausible mechanism
is photoionization of the intergalactic medium at a time before the epoch that
most dwarf galaxies form in low density regions but after the epoch of
formation for similar systems that ultimately end up in rich clusters. The
dynamical timescales are found to accommodate this hypothesis in a flat
universe with Omega_m < 0.4.
  If small halos exist but simply cannot be located because they have never
become the sites of significant star formation, they still might have dynamical
manifestations. These manifestations are hard to identify in normal groups of
galaxies because small halos do not make a significant contribution to the
global mass budget. However, it could be entertained that there are clusters of
halos where there are only small systems, clusters that are at the low mass end
of the hierarchical tree. There may be places where only a few small galaxies
managed to form, enough for us to identify and use as test probes of the
potential. It turns out that such environments might be common. Four probable
groups of dwarfs are identified within 5 Mpc and the assumption they are
gravitationally bound suggests M/L_B ~ 300 - 1200 M_sun/L_sun, 6 +/- factor 2
times higher than typical values for groups with luminous galaxies.


title: The Afterglow and the Host Galaxy of the Dark Burst GRB 970828

abstract:
  GRB 970828 was the first well-localized gamma-ray burst (GRB) X-ray afterglow
for which no optical afterglow was found despite a prompt, deep search down to
R_lim ~ 24.5 mag. We report the discovery of a short-lived radio flare within
the X-ray localization error circle of this burst. Such radio flares are seen
in about 25% of GRB afterglows and their origin is not well understood. The
precise radio position enabled us to identify the likely host galaxy of this
burst, and to measure its redshift, z = 0.9578. The host appears to be an
interacting/merging system. Under the assumption that the X-ray afterglow is
mainly due to synchrotron mechanism we infer the optical afterglow flux. The
observed upper limits to the optical flux are easily explained by invoking an
intervening dusty cloud within the host galaxy. These observations support the
idea that GRBs with no detectable optical afterglows or "dark GRBs" can be due
to dust extinction within the host galaxies. The census of dark GRBs can then
be used to constrain the fraction of the obscured star formation in the
universe. We argue that the existing data already indicate that the obscured
star-formation rate is no more than 1/2 of that seen at UV and optical
wavelengths.


title:
X-Ray Synchrotron Emission from 10-100 TeV Cosmic-Ray Electrons in the
  Supernova Remnant SN 1006

abstract:
  We present the results of a joint spectral analysis of RXTE PCA, ASCA SIS,
and ROSAT PSPC data of the supernova remnant SN 1006. This work represents the
first attempt to model both the thermal and nonthermal X-ray emission over the
entire X-ray energy band from 0.12 to 17 keV. The thermal flux is described by
a nonequilibrium ionization model with an electron temperature kT = 0.6 keV, an
ionization timescale n0t = 9 x 10^9 s / cm^3, and a relative elemental
abundance of silicon that is 10-18 times larger than the solar abundance. The
nonthermal X-ray spectrum is described by a broken power law model with low-
and high-energy photon indices Gamma_1 = 2.1 and Gamma_2 = 3.0, respectively.
Since the nonthermal X-ray spectrum steepens with increasing energy, the
results of the present analysis corroborate previous claims that the nonthermal
X-ray emission is produced by synchrotron radiation. We argue that the magnetic
field strength is significantly larger than previous estimates of about 1 x
10^-5 G and arbitrarily use a value of 4 x 10^-5 G to estimate the parameters
of the cosmic-ray electron, proton, and helium spectra of the remnant. The
results for the ratio of the number densities of protons and electrons (R = 160
at 1 GeV), the total energy in cosmic rays (E_cr = 1 x 10^50 ergs), and the
spectral index of the electrons at 1 GeV (Gamma_e = 2.14 +/- 0.12) are
consistent with the hypothesis that Galactic cosmic rays are accelerated
predominantly in the shocks of supernova remnants. Yet, the remnant may or may
not accelerate nuclei to energies as high as the energy of the "knee,"
depending on the reason why the maximum energy of the electrons is only 10 TeV.