3,200 research outputs found
Implications of the Measured Image Size for the Radio Afterglow of GRB 030329
We use data on the image size of the radio afterglow of GRB 030329 (Taylor et
al. 2004) to constrain the physical parameters of this explosion. Together with
the observed broad band spectrum, this data over-constrains the physical
parameters, thus enabling to test different GRB jet models for consistency. We
consider two extreme models for the lateral spreading of the jet: model 1 with
relativistic expansion in the local rest frame, and model 2 with little lateral
expansion as long as the jet is highly relativistic. We find that both models
are consistent with the data for a uniform external medium, while for a stellar
wind environment model 1 is consistent with the data but model 2 is disfavored
by the data. Our derivations can be used to place tighter constraints on the
dynamics and structure of GRB jets in future afterglows, following a denser
monitoring campaign for the temporal evolution of their image size.Comment: 12 pages, 6 figues; submitted to Ap
Afterglow Observations Shed New Light on the Nature of X-ray Flashes
X-ray flashes (XRFs) and X-ray rich gamma-ray bursts (XRGRBs) share many
observational characteristics with long duration GRBs, but the reason for which
their prompt emission peaks at lower photon energies, , is still under
debate. Although many different models have been invoked in order to explain
the lower values, their implications for the afterglow emission were not
considered in most cases, mainly because observations of XRF afterglows have
become available only recently. Here we examine the predictions of the various
XRF models for the afterglow emission, and test them against the observations
of XRF 030723 and XRGRB 041006, the events with the best monitored afterglow
light curves in their respective class. We show that most existing XRF models
are hard to reconcile with the observed afterglow light curves, which are very
flat at early times. Such light curves are, however, naturally produced by a
roughly uniform jet with relatively sharp edges that is viewed off-axis (i.e.
from outside of the jet aperture). This type of model self consistently
accommodates both the observed prompt emission and the afterglow light curves
of XRGRB 041006 and XRF 030723, implying viewing angles from the
jet axis of and ,
respectively, where deg is the jet half-opening angle. This
suggests that GRBs, XRGRBs and XRFs are intrinsically similar relativistic jets
viewed from different angles, corresponding to
of less than 1, between 1 and a few, and more than a few, respectively, where
is the Lorentz factor. Future observations with Swift could help test
this unification scheme in which GRBs, XRGRBs and XRFs share the same basic
physics and differ only by their orientation relative to our line of sight.Comment: some references added, small typos corrected, and the important role
of HETE II emphasize
Simulations of GRB Jets in a Stratified External Medium: Dynamics, Afterglow Lightcurves, Jet Breaks and Radio Calorimetry
The dynamics of GRB jets during the afterglow phase is most reliably and
accurately modelled using hydrodynamic simulations. All published simulations,
however, have considered only a uniform external medium, while a stratified
external medium is expected around long duration GRB progenitors. Here we
present simulations of the dynamics of GRB jets and the resulting afterglow
emission for both uniform and stratified external media with for k = 0, 1, 2. The simulations are performed in 2D using the special
relativistic version of the Mezcal code. The dynamics for stratified external
media are broadly similar to those derived for expansion into a uniform
external medium. The jet half-opening angle start increasing logarithmically
with time once the Lorentz factor drops below 1/theta_0. For larger k values
the lateral expansion is faster at early times and slower at late times with
the jet expansion becoming Newtonian and slowly approaching spherical symmetry
over progressively longer timescales. We find that contrary to analytic
expectations, there is a reasonably sharp jet break in the lightcurve for k = 2
although the shape of the break is affected more by the viewing angle than by
the slope of the external density profile. Steeper density profiles are found
to produce more gradual jet breaks while larger viewing angles cause smoother
and later appearing jet breaks. The counter-jet becomes visible as it becomes
sub-relativistic, and for k=0 this results in a clear bump-like feature in the
light curve. However, for larger k values the jet decelerates more gradually,
causing only a mild flattening in the radio light curve that might be hard to
discern when k=2. Late time radio calorimetry is likely to consistently
over-estimate the true energy by up to a factor of a few for k=2, and either
over-predict or under-predict it by a smaller factor for k = 0,1.Comment: 10 pages, 13 figures, submitted to Ap
Health, Healthcare Access, and Use of Traditional Versus Modern Medicine in Remote Peruvian Amazon Communities: A Descriptive Study of Knowledge, Attitudes, and Practices
There is an urgent need for healthcare research, funding, and infrastructure in the Peruvian Amazon. We performed a descriptive study of health, health knowledge and practice, and healthcare access of 13 remote communities of the Manatí and Amazon Rivers in northeastern Peru. Eighty-five adults attending a medical boat service were interviewed to collect data on socioeconomic position, health, diagnosed illnesses, pain, healthcare access, and traditional versus modern medicine use. In this setting, poverty and gender inequality were prevalent, and healthcare access was limited by long distances to the health post and long waiting times. There was a high burden of reported pain (mainly head and musculoskeletal) and chronic non-communicable diseases, such as hypertension (19%). Nearly all participants felt that they did not completely understand their diagnosed illnesses and wanted to know more. Participants preferred modern over traditional medicine, predominantly because of mistrust or lack of belief in traditional medicine. Our findings provide novel evidence concerning transitional health beliefs, hidden pain, and chronic non-communicable disease prevalence in marginalized communities of the Peruvian Amazon. Healthcare provision was limited by a breach between health education, knowledge, and access. Additional participatory research with similar rural populations is required to inform regional healthcare policy and decision-making
An Off-Axis Model for GRB 031203
The low luminosity radio emission of the unusually faint GRB 031203 has been
argued to support the idea of a class of intrinsically sub-energetic gamma-ray
bursts (GRBs), currently comprising two members. While low energy GRBs probably
exist, we show that the collective prompt and multiwavelength observations of
the afterglow of GRB 031203 do not necessarily require a sub-energetic nature
for that event. In fact, the data are more consistent with a typical, powerful
GRB seen at an angle of about twice the opening angle of the central jet. The
(redshift corrected) peak energy, E_p, of GRB 031203 then becomes ~ 2 MeV,
similar to many other GRBs.Comment: 4 pages, 3 figures; ApJL in pres
Masses of a Fourth Generation with Two Higgs Doublets
We use sampling techniques to find robust constraints on the masses of a
possible fourth sequential fermion generation from electroweak oblique
variables. We find that in the case of a light (115 GeV) Higgs from a single
electroweak symmetry breaking doublet, inverted mass hierarchies are possible
for both quarks and leptons, but a mass splitting more than M(W) in the quark
sector is unlikely. We also find constraints in the case of a heavy (600 GeV)
Higgs in a single doublet model. As recent data from the Large Hadron Collider
hints at the existence of a resonance at 124.5 GeV and a single Higgs doublet
at that mass is inconsistent with a fourth fermion generation, we examine a
type II two Higgs doublet model. In this model, there are ranges of parameter
space where the Higgs sector can potentially counteract the effects of the
fourth generation. Even so, we find that such scenarios produce qualitatively
similar fermion mass distributions.Comment: v2: 9 pages, 7 figures, improved analysis of Higgs decay constraints,
typos corrected and reference adde
Manejo de heridas crónicas con terapia de presión negativa ambulatoria por profesionales de enfermería en el servicio de clínica de heridas del H.G.R N° 251
El empleo de la terapia de presión negativa es una opción ampliamente extendida
para el manejo de heridas crónicas, promueve la curación mediante la aplicación
de presión negativa, de manera que el uso de niveles controlados de presión subatmosférica
y succión controlada, acelera la resolución de las mismas
favoreciendo la vascularización y el desbridamiento. Actualmente, este sistema se
considera una intervención fundamental en el tratamiento de las heridas
complejas, las cuales suponen un reto terapéutico importante y muchas veces
generan un largo y costoso egreso hospitalario. Gracias a la terapia de vacío se
puede disminuir el tamaño de dichas heridas a la par que se protegen de la
infección, haciendo innecesario el uso prioritario de colgajos libres y grandes
intervenciones. La terapia de presión negativa se remonta a miles de años atrás,
se utilizó por primera vez como tratamiento complementario; a nivel básico, la
presión negativa constituye un apósito sofisticado, estéril y cerrado con
propiedades que hacen que el entorno para la cicatrización de la herida sea
húmedo
Identificación de bacterias halófilas tolerantes a metales pesados
Los metales pesados son materiales empleados en procesos tecnológicos y actividades antropogénicas, algunas veces son liberados al medio ambiente provocando deterioro de flora, fauna y salud humana por lo que se han implementado diferentes estrategias para eliminarlos o reducirlos del medio ambiente como los tratamientos químicos, los cuales utilizan las propiedades físicas y químicas de los contaminantes o del medio contaminado para transformar, separar o inmovilizarlos pero muchas veces estos tratamientos presentan como desventaja el requerir un tratamiento secundario o disposición final del contaminante, lo que genera un costo económico y ambiental. También existen los métodos biológicos (biorremediación) que utilizan principalmente plantas, hongos y bacterias principalmente. Existen ambientes que presentan parámetros fisicoquímicos extremos ya sea de temperatura, pH, presión, salinidad etc., en estos ambientes se desarrollan microorganismos extremófilos los cuales requieren de condiciones extremas para desarrollarse, tal es el caso de las bacterias halófilas, las cuales requieren cierta concentración de salinidad para desarrollarse
Covariance Estimation from Compressive Data Partitions using a Projected Gradient-based Algorithm
Covariance matrix estimation techniques require high acquisition costs that
challenge the sampling systems' storing and transmission capabilities. For this
reason, various acquisition approaches have been developed to simultaneously
sense and compress the relevant information of the signal using random
projections. However, estimating the covariance matrix from the random
projections is an ill-posed problem that requires further information about the
data, such as sparsity, low rank, or stationary behavior. Furthermore, this
approach fails using high compression ratios. Therefore, this paper proposes an
algorithm based on the projected gradient method to recover a low-rank or
Toeplitz approximation of the covariance matrix. The proposed algorithm divides
the data into subsets projected onto different subspaces, assuming that each
subset contains an approximation of the signal statistics, improving the
inverse problem's condition. The error induced by this assumption is
analytically derived along with the convergence guarantees of the proposed
method. Extensive simulations show that the proposed algorithm can effectively
recover the covariance matrix of hyperspectral images with high compression
ratios (8-15% approx) in noisy scenarios. Additionally, simulations and
theoretical results show that filtering the gradient reduces the estimator's
error recovering up to twice the number of eigenvectors.Comment: submitted to IEEE Transactions on Image Processin
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