186 research outputs found
No visible optical variability from a relativistic blast wave encountering a wind-termination shock
Gamma-ray burst afterglow flares and rebrightenings of the optical and X-ray
light curve have been attributed to both late time inner engine activity and
density changes in the medium surrounding the burster. To test the latter, we
study the encounter between the relativistic blast wave from a gamma-ray
burster and a stellar wind termination shock. The blast wave is simulated using
a high performance adaptive mesh relativistic hydrodynamics code, AMRVAC, and
the synchrotron emission is analyzed in detail with a separate radiation code.
We find no bump in the resulting light curve, not even for very high density
jumps. Furthermore, by analyzing the contributions from the different shock
wave regions we are able to establish that it is essential to resolve the blast
wave structure in order to make qualitatively correct predictions on the
observed output and that the contribution from the reverse shock region will
not stand out, even when the magnetic field is increased in this region by
repeated shocks. This study resolves a controversy in recent literature.Comment: 4 figures, submitted to MNRAS letter
Shocks in relativistic transverse stratified jets, a new paradigm for radio-loud AGN
The transverse stratification of active galactic nuclei (AGN) jets is
suggested by observations and theoretical arguments, as a consequence of
intrinsic properties of the central engine (accretion disc + black hole) and
external medium. On the other hand, the one-component jet approaches are
heavily challenged by the various observed properties of plasmoids in radio
jets (knots), often associated with internal shocks. Given that such a
transverse stratification plays an important role on the jets acceleration,
stability, and interaction with the external medium, it should also induce
internal shocks with various strengths and configurations, able to describe the
observed knots behaviours. By establishing a relation between the transverse
stratification of the jets, the internal shock properties, and the multiple
observed AGN jet morphologies and behaviours, our aim is to provide a
consistent global scheme of the various AGN jet structures. Working on a large
sample of AGN radio jets monitored in very long baseline interferometry (VLBI)
by the MOJAVE collaboration, we determined the consistency of a systematic
association of the multiple knots with successive re-collimation shocks. We
then investigated the re-collimation shock formation and the influence of
different transverse stratified structures by parametrically exploring the two
relativistic outflow components with the specific relativistic hydrodynamic
(SRHD) code AMRVAC. We were able to link the different spectral classes of AGN
with specific stratified jet characteristics, in good accordance with their
VLBI radio properties and their accretion regimes.Comment: 16 pages, 12 figures, accepted for publication in A&
Two-flow magnetohydrodynamical jets around young stellar objects
We present the first-ever simulations of non-ideal magnetohydrodynamical
(MHD) stellar winds coupled with disc-driven jets where the resistive and
viscous accretion disc is self-consistently described. The transmagnetosonic,
collimated MHD outflows are investigated numerically using the VAC code. Our
simulations show that the inner outflow is accelerated from the central object
hot corona thanks to both the thermal pressure and the Lorentz force. In our
framework, the thermal acceleration is sustained by the heating produced by the
dissipated magnetic energy due to the turbulence. Conversely, the outflow
launched from the resistive accretion disc is mainly accelerated by the
magneto-centrifugal force. We also show that when a dense inner stellar wind
occurs, the resulting disc-driven jet have a different structure, namely a
magnetic structure where poloidal magnetic field lines are more inclined
because of the pressure caused by the stellar wind. This modification leads to
both an enhanced mass ejection rate in the disc-driven jet and a larger radial
extension which is in better agreement with the observations besides being more
consistent.Comment: Accepted for publication in Astrophysics & Space Science. Referred
proceeding of the fifth Mont Stromlo Symposium Dec. 1-8 2006, Canberra,
Australia. 5 pages, 3 figures. For high resolution version of the paper,
please click here http://www.apc.univ-paris7.fr/~fcasse/publications.htm
Gamma-Ray Burst afterglow scaling coefficients for general density profile
Gamma-ray burst (GRB) afterglows are well described by synchrotron emission
originating from the interaction between a relativistic blast wave and the
external medium surrounding the GRB progenitor. We introduce a code to
reconstruct spectra and light curves from arbitrary fluid configurations,
making it especially suited to study the effects of fluid flows beyond those
that can be described using analytical approximations. As a check and first
application of our code we use it to fit the scaling coefficients of
theoretical models of afterglow spectra. We extend earlier results of other
authors to general circumburst density profiles. We rederive the physical
parameters of GRB 970508 and compare with other authorsComment: 11 pages, 5 figures. Revised edition removes references to unphysical
chromatic break and adds appendix on hot region directly behind shoc
Thin shell morphology in the circumstellar medium of massive binaries
We investigate the morphology of the collision front between the stellar
winds of binary components in two long-period binary systems, one consisting of
a hydrogen rich Wolf-Rayet star (WNL) and an O-star and the other of a Luminous
Blue Variable (LBV) and an O-star. Specifically, we follow the development and
evolution of instabilities that form in such a shell, if it is sufficiently
compressed, due to both the wind interaction and the orbital motion. We use
MPI-AMRVAC to time-integrate the equations of hydrodynamics, combined with
optically thin radiative cooling, on an adaptive mesh 3D grid. Using parameters
for generic binary systems, we simulate the interaction between the winds of
the two stars. The WNL+O star binary shows a typical example of an adiabatic
wind collision. The resulting shell is thick and smooth, showing no
instabilities. On the other hand, the shell created by the collision of the O
star wind with the LBV wind, combined with the orbital motion of the binary
components, is susceptible to thin shell instabilities, which create a highly
structured morphology. We identify the nature of the instabilities as both
linear and non-linear thin-shell instabilities, with distinct differences
between the leading and the trailing parts of the collision front. We also find
that for binaries containing a star with a (relatively) slow wind, the global
shape of the shell is determined more by the slow wind velocity and the orbital
motion of the binary, than the ram pressure balance between the two winds. The
interaction between massive binary winds needs further parametric exploration,
to identify the role and dynamical importance of multiple instabilities at the
collision front, as shown here for an LBV+O star system.Comment: 10 pages, 13 figures. Accepted for publication in A&
Experimental creep test sensibility of mechanical properties of LDPE films used as greenhouse roof.
Plastic films are widely used in agriculture throughout the Mediterranean area and the rest of the world, but their impact on the sustainability of agricultural production and the environment is a major concern. One of the factors limit-ing the recycling of agricultural plastics is the loss of mechanical properties due to ageing from exposure to exter-nal agents. This research analysed the degradation of the mechanical properties of LDPE films used for greenhouse roofs by naturally weathering them for nine months in harsh climatic conditions in the Tiaret region of Algeria. Tensile and creep-recovery tests were conducted to measure changes in the mechanical properties of both monolayer and tri-layer films, and the anisotropic character of the films was preserved during ageing. Cross linking improved the creep resistance for both types of films but affected different defor-mations. Results show that the degradation performance of the new tri-layer films is significantly better than that of the monolayer film with regard to mechanical properties. There-fore, it can be concluded that the erosion of greenhouse LDPE film by temperature, water, sand particles, wind, and humidity results in an extreme lifetime limit. LDPE film may have a longer period of use in Mediterranean Europe (with lower temperatures and no sand-wind effects) than in North Africa (with high temperature and adverse sand-wind effects)
The association of APOE ε4 with cognitive function over the adult life course and incidence of dementia: 20 years follow-up of the Whitehall II study
Background: Approximately 25% of the general population carries at least one ε4 allele of the Apolipoprotein E
(APOE ε4), the strongest genetic risk factor for late onset Alzheimer’s disease. Beyond its association with late-onset
dementia, the association between APOE ε4 and change in cognition over the adult life course remains uncertain.
This study aims to examine whether the association between Apolipoprotein E (APOE) ε4 zygosity and cognition
function is modified between midlife and old age.
Methods: A cohort study of 5561 participants (mean age 55.5 (SD = 5.9) years, 27.1% women) with APOE
genotyping and repeated cognitive tests for reasoning, memory, and semantic and phonemic fluency, during a
mean (SD) follow-up of 20.2 (2.8) years (the Whitehall II study). We used joint models to examine the association of
APOE genotype with cognitive function trajectories between 45 and 85 years taking drop-out, dementia, and death
into account and Fine and Gray models to examine associations with dementia.
Results: Compared to non-carriers, heterozygote (prevalence 25%) and homozygote (prevalence 2%) APOE ε4
carriers had increased risk of dementia, sub-distribution hazard ratios 2.19 (95% CI 1.73, 2.77) and 5.97 (95% CI 3.85,
9.28) respectively. Using data spanning 45–85 years with non-ε4 carriers as the reference, ε4 homozygotes had
poorer global cognitive score starting from 65 years; ε4 heterozygotes had better scores between 45 and 55 years,
then no difference until poorer cognitive scores from 75 years onwards. In analysis of individual cognitive tests,
better cognitive performance in the younger ε4 heterozygotes was primarily attributable to executive function.
Conclusions: Both heterozygous and homozygous ε4 carriers had poorer cognition and greater risk of dementia at
older ages. Our findings show some support for a complex antagonist pleiotropic effect of APOE ε4 heterozygosity
over the adult life course, characterized by cognitive advantage in midlife
On the plerionic rectangular supernova remnants of static progenitors
Pulsar wind nebulae are a possible final stage of the circumstellar evolution
of massive stars, where a fast rotating, magnetised neutron star produces a
powerful wind that interacts with the supernova ejecta. The shape of these so
called plerionic supernova remnants is influenced by the distribution of
circumstellar matter at the time of the explosion, itself impacted by the
magnetic field of the ambient medium responsible for the expansion of the
circumstellar bubble of the progenitor star. To understand the effects of
magnetization on the circumstellar medium and resulting pulsar nebulae, we
conduct 2D magnetohydrodynamical simulations. Our models explore the impact of
the interstellar medium magnetic field on the morphology of a supernova remnant
and pulsar wind nebula that develop in the circumstellar medium of massive star
progenitor in the warm phase of the Milky Ways interstellar medium. Our
simulations reveal that the jet like structures formed on both sides
perpendicularly to the equatorial plane of the pulsar, creating complex radio
synthetic synchrotron emissions. This morphology is characterized by a
rectangular like remnant, which is typical of the circumstellar medium of
massive stars in a magnetized medium, along with the appearance of a spinning
top structure within the projected rectangle. We suggest that this mechanism
may be partially responsible for the complex morphologies observed in pulsar
wind nebulae that do not conform to the typical torus, jet or bow shock, tail
shapes observed in most cases.Comment: Accepted at MNRA
Gamma-ray burst afterglows from trans-relativistic blast wave simulations
We present a study of the intermediate regime between ultra-relativistic and
nonrelativistic flow for gamma-ray burst afterglows. The hydrodynamics of
spherically symmetric blast waves is numerically calculated using the AMRVAC
adaptive mesh refinement code. Spectra and light curves are calculated using a
separate radiation code that, for the first time, links a parametrisation of
the microphysics of shock acceleration, synchrotron self-absorption and
electron cooling to a high-performance hydrodynamics simulation. For the
dynamics we find that the transition to the nonrelativistic regime generally
occurs later than expected, that the Sedov-Taylor solution overpredicts the
late time blast wave radius and that the analytical formula for the blast wave
velocity from Huang (1999) overpredicts the late time velocity by a factor 4/3.
For the radiation we find that the flux may differ up to an order of magnitude
depending on the equation of state that is used for the fluid and that the
counterjet leads to a clear rebrightening at late times for hard-edged jets.
Simulating GRB030329 using predictions for its physical parameters from the
literature leads to spectra and light curves that may differ significantly from
the actual data, emphasizing the need for very accurate modelling. Predicted
light curves at low radio frequencies for a hard-edged jet model of GRB030329
with opening angle 22 degrees show typically two distinct peaks, due to the
combined effect of jet break, non relativistic break and counterjet.Comment: 18 pages, 19 figures. Replaced with MNRAS accepted version, minor
changes onl
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