1,527 research outputs found
The minijets-in-a-jet statistical model and the RMS-flux correlation
The flux variability of blazars at very high energies does not have a clear
origin. Flux variations on time scales down to the minute suggest that
variability originates in the jet, where a relativistic boost can shorten the
observed time scale, while the linear relation between the flux and its RMS or
the skewness of the flux distribution suggests that the variability stems from
multiplicative processes, which are associated in some models with the
accretion disk. We study the RMS-flux relation and emphasize its link to Pareto
distributions, characterized by a power-law probability density function. Such
distributions are naturally generated within a minijets- in-a-jet statistical
model, in which boosted emitting regions are isotropically oriented within the
bulk relativistic flow of a jet. We prove that, within this model, the flux of
a single minijet is proportional to its RMS. This relation still holds when
considering a large number of emitting regions, for which the distribution of
the total flux is skewed and could be interpreted as being log-normal. The
minijets-in-a-jet statistical model reconciles the fast variations and the
statistical properties of the flux of blazars at very high energies.Comment: 6 pages, 6 figures, accepted in A
Slit/Robo Signaling Regulates Cell Fate Decisions in the Intestinal Stem Cell Lineage of Drosophila
SummaryIn order to maintain tissue homeostasis, cell fate decisions within stem cell lineages have to respond to the needs of the tissue. This coordination of lineage choices with regenerative demand remains poorly characterized. Here, we identify a signal from enteroendocrine cells (EEs) that controls lineage specification in the Drosophila intestine. We find that EEs secrete Slit, a ligand for the Robo2 receptor in intestinal stem cells (ISCs) that limits ISC commitment to the endocrine lineage, establishing negative feedback control of EE regeneration. Furthermore, we show that this lineage decision is made within ISCs and requires induction of the transcription factor Prospero in ISCs. Our work identifies a function for the conserved Slit/Robo pathway in the regulation of adult stem cells, establishing negative feedback control of ISC lineage specification as a critical strategy to preserve tissue homeostasis. Our results further amend the current understanding of cell fate commitment within the Drosophila ISC lineage
Impact of Galaxy Clusters on UHECR propagation
Galaxy clusters are the universe's largest objects in the universe kept
together by gravity. Most of their baryonic content is made of a magnetized
diffuse plasma. We investigate the impact of such magnetized environment on
ultra-high-energy-cosmic-ray (UHECR) propagation. The intracluster medium is
described according to the self-similar assumption, in which the gas density
and pressure profiles are fully determined by the cluster mass and redshift.
The magnetic field is scaled to the thermal components of the intracluster
medium under different assumptions. We model the propagation of UHECRs in the
intracluster medium using a modified version of the Monte Carlo code {\it
SimProp}, where hadronic processes and diffusion in the turbulent magnetic
field are implemented. We provide a universal parametrization that approximates
the UHECR fluxes escaping from the environment as a function of the most
relevant quantities, such as the mass of the cluster, the position of the
source with respect to the center of the cluster and the nature of the
accelerated particles. We show that galaxy clusters are an opaque environment
especially for UHECR nuclei. The role of the most massive nearby clusters in
the context of the emerging UHECR astronomy is finally discussed.Comment: 12 pages, 6 figures, resived by Ap
Thermal and chemical behaviour of an energetic material and a heat release rate issue
Energetic materials encompass a wide range of chemical compounds all associated with a
significant risk of fire and explosion. They include explosives, fireworks, pyrotechnics, powders,
propellants and other unsteady chemicals. These materials store a high level of chemical
energy and are able to release it rapidly without external contribution of oxygen or any other
oxidizer. The behaviour of these materials in case of explosive detonations is relatively wellknown
from empirical and practical points of view. However, fundamental scientific questions
remain unanswered related to the mechanisms of heat release. The current understanding of
these mechanisms lacks appropriate thermochemical characterisation. The aim of the study is
the analysis of thermal and chemical characteristics of energetic materials under conditions
that exclude detonations. Detonation is excluded in order to better isolate the thermal and
chemical mechanisms involved in the burning process. The experimental work has been
conducted using the FM Global Fire Propagation Apparatus (FPA) [ASTM E2058‐03]. One of the
benefits of using this experimental apparatus rather than the Cone Calorimeter is that it allows
controlling the feed of heat and oxidizer to the reaction zone.
The material chosen to conduct experiments on is a ternary smoke powder based on a mixture
of starch and lactose as fuel components and potassium nitrate as oxidizer. This product is
currently used by fire brigades to assess smoke venting systems efficiency of buildings. The
kinetics associated with the combustion of the material was assessed slow enough to allow
measuring instruments to capture the thermal and chemical evolution during combustion
reaction. Thermal analysis has first been carried out by means of DSC, TGA, DTA, MS and FTIR
data in order to understand the decomposition of the material and its energetic evolution
when undergoing heating. However, if the latter methods help defining the decomposing path
of the material, they do not provide an integral view of its combustion behaviour, in particular,
the emissions of toxics which are kinetic path dependent. Subsequently, combustion tests
have been carried out using the FPA. Its ability to capture the evolution of gases emissions
formed during the reaction has been proved. The influence of two configuration parameters
on the combustion behaviour and on the gaseous emissions of the material has been
investigated. The proportion fuel/oxidizer has been varied as well as the composition of the
reacting atmosphere. Results shows that the quantity of oxidizer in the material affects the kinetics of the reactions taking place in the condense phase. Increasing the concentration of
potassium nitrate in the mixture enhanced the reaction rate of the smouldering combustion.
Higher quantity of volatiles is released which favoured the initiation of a diffusion flame
regime in the gaseous phase, above the sample. While the kinetics of the condense phase is
governed by the oxidizer concentration, experiments show that the flaming regime is
influenced by the concentration of oxygen (O2) in the reacting atmosphere. A transition from
diffusion to premixed flame is found when the concentration of O2 surrounding the sample is
reduced below 18%. An analytical model has been used to explain the existence of a transition
for a critical O2 concentration. Finally, thermal and combustion analyses have allowed to
characterise the behaviour of the material under critical conditions, in terms of decomposition
taking place in the condense phase but also potential toxic emissions that can be released.
Toxicity, kinetics, temperature evolution do not provide a complete view of the combustion
phenomenon. Beside these elements that characterise the behaviour of a material for given
conditions as well as also the degree of fire hazard encountered, the energetic issue holds as
an essential feature that cannot be neglected. The heat release rate (HRR) is a critical
parameter that defines a fire. It does not constitute an intrinsic material property but it
describes the energetic response of the couple formed by the material and its environment.
Oxygen Consumption calorimetry (OC) and Carbon Dioxide Generation calorimetry (CDG) are
widespread methods to calculate the HRR resulting from a combustion reaction. Apparatuses
such as the FPA or the cone calorimeter have already proved their potential to qualify the
burning behaviour of common fuels in addition to polymers when their data are combined
with an adapted calorimetric procedure.
The same approach has been applied to energetic materials. However, prior to using these
techniques, it is fundamental to have identified their restrictions. These techniques provide
approximate estimations of the HRR. Results are affected by the propagation of uncertainties.
Several sources of uncertainties can be found. One can cite:
1. Uncertainties associated with the sample material;
2. Uncertainties associated with the test conditions;
3. Uncertainties associated with the measurements;
4. Uncertainties associated with calculation assumptions.
If uncertainties cannot always be estimated, the three first sources cited have received
attention in the past from the scientific community, alike the last one. The restrictions
associated with the assumptions developed for using the OC and CDG principles have to be
clarified. The limits of validity of the hypotheses have to be clearly defined. In particular, the
present dissertation questions the relevance of the energy constants that have been specified
for OC and CDG as well as their related uncertainties. One of the purposes of the research
deals with the ability to estimate accurate error bars for the calculation of the HRR. Once
uncertainties related to the calorimetric methods are assessed, a method adapted from the
basic OC and CDG principles is introduced that allows estimating the HRR of energetic
materials. The approach is based on considering the chemical decomposition of the burning
compound and defining a fictitious molecule for which energy coefficients can be calculated.
Nevertheless, it requires the material to be known. Finally, the question of the advantage
brought by these techniques over others, in terms of accuracy, is discussed within the
framework of unconventional products, such as energetic materials or compounds whose
composition is ignored. The results from this work will contribute to the development of fireanalysis
methodologies and validate their use with energetic materials
Pink1 and Parkin regulate Drosophila intestinal stem cell proliferation during stress and aging.
Intestinal stem cells (ISCs) maintain the midgut epithelium in Drosophila melanogaster Proper cellular turnover and tissue function rely on tightly regulated rates of ISC division and appropriate differentiation of daughter cells. However, aging and epithelial injury cause elevated ISC proliferation and decreased capacity for terminal differentiation of daughter enteroblasts (EBs). The mechanisms causing functional decline of stem cells with age remain elusive; however, recent findings suggest that stem cell metabolism plays an important role in the regulation of stem cell activity. Here, we investigate how alterations in mitochondrial homeostasis modulate stem cell behavior in vivo via RNA interference-mediated knockdown of factors involved in mitochondrial dynamics. ISC/EB-specific knockdown of the mitophagy-related genes Pink1 or Parkin suppresses the age-related loss of tissue homeostasis, despite dramatic changes in mitochondrial ultrastructure and mitochondrial damage in ISCs/EBs. Maintenance of tissue homeostasis upon reduction of Pink1 or Parkin appears to result from reduction of age- and stress-induced ISC proliferation, in part, through induction of ISC senescence. Our results indicate an uncoupling of cellular, tissue, and organismal aging through inhibition of ISC proliferation and provide insight into strategies used by stem cells to maintain tissue homeostasis despite severe damage to organelles
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