14,202 research outputs found
A search for radio pulsars and fast transients in M31 using the WSRT
We present the results of the most sensitive and comprehensive survey yet
undertaken for radio pulsars and fast transients in the Andromeda galaxy (M31)
and its satellites, using the Westerbork Synthesis Radio Telescope (WSRT) at a
central frequency of 328 MHz. We used the WSRT in a special configuration
called 8gr8 (eight-grate) mode, which provides a large instantaneous
field-of-view, about 5 square degrees per pointing, with good sensitivity, long
dwell times (up to 8 hours per pointing), and good spatial resolution (a few
arc minutes) for locating sources. We have searched for both periodicities and
single pulses in our data, aiming to detect bright, persistent radio pulsars
and rotating radio transients (RRATs) of either Galactic or extragalactic
origin. Our searches did not reveal any confirmed periodic signals or bright
single bursts from (potentially) cosmological distances. However, we do report
the detection of several single pulse events, some repeating at the same
dispersion measure, which could potentially originate from neutron stars in
M31. One in particular was seen multiple times, including a burst of six pulses
in 2000 seconds, at a dispersion measure of 54.7 pc cm^-3, which potentially
places the origin of this source outside of our Galaxy. Our results are
compared to a range of hypothetical populations of pulsars and RRATs in M31 and
allow us to constrain the luminosity function of pulsars in M31. They also show
that, unless the pulsar population in M31 is much dimmer than in our Galaxy,
there is no need to invoke any violation of the inverse square law of the
distance for pulsar fluxes.Comment: 18 pages, 14 figures, 8 tables. Accepted for publication in the main
journal of MNRA
Static cylindrical symmetry and conformal flatness
We present the whole set of equations with regularity and matching conditions
required for the description of physically meaningful static cylindrically
symmmetric distributions of matter, smoothly matched to Levi-Civita vacuum
spacetime. It is shown that the conformally flat solution with equal principal
stresses represents an incompressible fluid. It is also proved that any
conformally flat cylindrically symmetric static source cannot be matched
through Darmois conditions to the Levi-Civita spacetime. Further evidence is
given that when the Newtonian mass per unit length reaches 1/2 the spacetime
has plane symmetry.Comment: 13 pages, Late
The eta' in baryon chiral perturbation theory
We include in a systematic way the eta' in baryon chiral perturbation theory.
The most general relativistic effective Lagrangian describing the interaction
of the lowest lying baryon octet with the Goldstone boson octet and the eta' is
presented up to linear order in the derivative expansion and its heavy baryon
limit is obtained. As explicit examples, we calculate the baryon masses and the
pi N sigma-term up to one-loop order in the heavy baryon formulation. A
systematic expansion in the meson masses is possible, and appearing divergences
are renormalized.Comment: 16 pages, 2 figure
Quantum singularity of Levi-Civita spacetimes
Quantum singularities in general relativistic spacetimes are determined by
the behavior of quantum test particles. A static spacetime is quantum
mechanically singular if the spatial portion of the wave operator is not
essentially self-adjoint. Here Weyl's limit point-limit circle criterion is
used to determine whether a wave operator is essentially self-adjoint. This
test is then applied to scalar wave packets in Levi-Civita spacetimes to help
elucidate the physical properties of the spacetimes in terms of their metric
parameters
Stellar Populations and Star Cluster Formation in Interacting Galaxies with the Advanced Camera for Surveys
Pixel-by-pixel colour-magnitude and colour-colour diagrams - based on a
subset of the Hubble Space Telescope Advanced Camera for Surveys Early Release
Observations - provide a powerful technique to explore and deduce the star and
star cluster formation histories of the Mice and the Tadpole interacting
galaxies. In each interacting system we find some 40 bright young star clusters
(20 <= F606W (mag) <= 25, with a characteristic mass of ~3 x 10^6 Msun), which
are spatially coincident with blue regions of active star formation in their
tidal tails and spiral arms. We estimate that the main events triggering the
formation of these clusters occurred ~(1.5-2.0) x 10^8 yr ago. We show that
star cluster formation is a major mode of star formation in galaxy
interactions, with >= 35% of the active star formation in encounters occurring
in star clusters. This is the first time that young star clusters have been
detected along the tidal tails in interacting galaxies. The tidal tail of the
Tadpole system is dominated by blue star forming regions, which occupy some 60%
of the total area covered by the tail and contribute ~70% of the total flux in
the F475W filter (decreasing to ~40% in F814W). The remaining pixels in the
tail have colours consistent with those of the main disk. The tidally triggered
burst of star formation in the Mice is of similar strength in both interacting
galaxies, but it has affected only relatively small, spatially coherent areas.Comment: 23 pages in preprint form, 6 (encapsulated) postscript figures;
accepted for publication in New Astronomy; ALL figures (even the grey-scale
ones) need to be printed on a colour printer style files included; for
full-resolution paper, see http://www.ast.cam.ac.uk/STELLARPOPS/ACSpaper
Phase mapping of aging process in InN nanostructures: oxygen incorporation and the role of the zincblende phase
Uncapped InN nanostructures undergo a deleterious natural aging process at
ambient conditions by oxygen incorporation. The phases involved in this process
and their localization is mapped by Transmission Electron Microscopy (TEM)
related techniques. The parent wurtzite InN (InN-w) phase disappears from the
surface and gradually forms a highly textured cubic layer that completely wraps
up a InN-w nucleus which still remains from original single-crystalline quantum
dots. The good reticular relationships between the different crystals generate
low misfit strains and explain the apparent easiness for phase transformations
at room temperature and pressure conditions, but also disable the classical
methods to identify phases and grains from TEM images. The application of the
geometrical phase algorithm in order to form numerical moire mappings, and RGB
multilayered image reconstructions allows to discern among the different phases
and grains formed inside these nanostructures. Samples aged for shorter times
reveal the presence of metastable InN:O zincblende (zb) volumes, which acts as
the intermediate phase between the initial InN-w and the most stable cubic
In2O3 end phase. These cubic phases are highly twinned with a proportion of
50:50 between both orientations. We suggest that the existence of the
intermediate InN:O-zb phase should be seriously considered to understand the
reason of the widely scattered reported fundamental properties of thought to be
InN-w, as its bandgap or superconductivity.Comment: 18 pages 7 figure
Intestinal volvulus in the pump twin of a twin reversed arterial perfusion (TRAP) sequence after laser therapy at 18 weeks: a case report
Background: Twin reversed arterial perfusion sequence is a rare and potentially lethal condition affecting approximately 1% of monochorionic twin pregnancies and 1 in 35,000 pregnancies overall. An apparently normal (pump) twin perfuses its severely malformed cotwin with deoxygenated blood via retrograde flow in direct arterioarterial anastomoses between the umbilical arteries of each twin. Fetal intestinal volvulus is a rare condition usually manifesting after birth. We report a unique case of twin reversed arterial perfusion sequence in association with intestinal volvulus in the surviving pump twin. Case presentation: A 32-year-old Hispanic primigravida was referred to our clinic after a fetoscopy procedure of laser photocoagulation of anastomoses at 18 weeks of gestation. Follow up scans in the ex-pump twin revealed dilated bowel loops and a typical “whirlpool sign” at 26 weeks of gestation, and intrauterine intestinal volvulus was suspected. At 29 weeks of gestation, preterm premature rupture of membranes occurred, and an emergency cesarean section was performed. The newborn was diagnosed in the early neonatal period with intestinal perforation. The diagnosis was postnatally confirmed by surgery and histopathology. Conclusions: The type of fetal intervention and late gestational age of the procedure increase the risk of complications. This case alerts health providers to be vigilant in the follow-up of patients with complicated monochorionic pregnanciesBackground: Twin reversed arterial perfusion sequence is a rare and potentially lethal condition affecting approximately 1% of monochorionic twin pregnancies and 1 in 35,000 pregnancies overall. An apparently normal (pump) twin perfuses its severely malformed cotwin with deoxygenated blood via retrograde flow in direct arterioarterial anastomoses between the umbilical arteries of each twin. Fetal intestinal volvulus is a rare condition usually manifesting after birth. We report a unique case of twin reversed arterial perfusion sequence in association with intestinal volvulus in the surviving pump twin. Case presentation: A 32-year-old Hispanic primigravida was referred to our clinic after a fetoscopy procedure of laser photocoagulation of anastomoses at 18 weeks of gestation. Follow up scans in the ex-pump twin revealed dilated bowel loops and a typical “whirlpool sign” at 26 weeks of gestation, and intrauterine intestinal volvulus was suspected. At 29 weeks of gestation, preterm premature rupture of membranes occurred, and an emergency cesarean section was performed. The newborn was diagnosed in the early neonatal period with intestinal perforation. The diagnosis was postnatally confirmed by surgery and histopathology. Conclusions: The type of fetal intervention and late gestational age of the procedure increase the risk of complications. This case alerts health providers to be vigilant in the follow-up of patients with complicated monochorionic pregnancie
Spherically symmetric dissipative anisotropic fluids: A general study
The full set of equations governing the evolution of self--gravitating
spherically symmetric dissipative fluids with anisotropic stresses is deployed
and used to carry out a general study on the behaviour of such systems, in the
context of general relativity. Emphasis is given to the link between the Weyl
tensor, the shear tensor, the anisotropy of the pressure and the density
inhomogeneity. In particular we provide the general, necessary and sufficient,
condition for the vanishing of the spatial gradients of energy density, which
in turn suggests a possible definition of a gravitational arrow of time. Some
solutions are also exhibited to illustrate the discussion.Comment: 28 pages Latex. To appear in Phys.Rev.
Levi-Civita Solutions Coupled with Electromagnetic Fields
The local and global properties of the Levi-Civita (LC) solutions coupled
with an electromagnetic field are studied and some limits to the vacuum LC
solutions are given. By doing such limits, the physical and geometrical
interpretations of the free parameters involved in the solutions are made
clear. Sources for both the LC vacuum solutions and the LC solutions coupled
with an electromagnetic field are studied, and in particular it is found that
all the LC vacuum solutions with can be produced by
cylindrically symmetric thin shells that satisfy all the energy conditions,
weak, dominant, and strong. When the electromagnetic field is present, the
situation changes dramatically. In the case of a purely magnetic field, all the
solutions with or can be
produced by physically acceptable cylindrical thin shells, while in the case of
a purely electric field, no such shells are found for any value of .Comment: Typed in Revtex, including two figure
Exact relativistic stellar models with liquid surface. I. Generalizing Buchdahl's polytrope
A family of exact relativistic stellar models is described. The family
generalizes Buchdahl's n=1 polytropic solution. The matter content is a perfect
fluid and, excluding Buchdahl's original model, it behaves as a liquid at low
pressures in the sense that the energy density is non-zero in the zero pressure
limit. The equation of state has two free parameters, a scaling and a stiffness
parameter. Depending on the value of the stiffness parameter the fluid
behaviour can be divided in four different types. Physical quantities such as
masses, radii and surface redshifts as well as density and pressure profiles
are calculated and displayed graphically. Leaving the details to a later
publication, it is noted that one of the equation of state types can quite
accurately approximate the equation of state of real cold matter in the outer
regions of neutron stars. Finally, it is observed that the given equation of
state does not admit models with a conical singularity at the center.Comment: 19 pages, 12 figures (16 eps files), LaTeX2e with the standard
packages amssymb, amsmath, graphicx, subfigure, psfra
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