1,467 research outputs found
Constraints on particle acceleration sites in the Crab Nebula from relativistic MHD simulations
The Crab Nebula is one of the most efficient accelerators in the Galaxy and
the only galactic source showing direct evidence of PeV particles. In spite of
this, the physical process behind such effective acceleration is still a deep
mystery. While particle acceleration, at least at the highest energies, is
commonly thought to occur at the pulsar wind termination shock, the properties
of the upstream flow are thought to be non-uniform along the shock surface, and
important constraints on the mechanism at work come from exact knowledge of
where along this surface particles are being accelerated. Here we use
axisymmetric relativistic MHD simulations to obtain constraints on the
acceleration site(s) of particles of different energies in the Crab Nebula.
Various scenarios are considered for the injection of particles responsible for
synchrotron radiation in the different frequency bands, radio, optical and
X-rays. The resulting emission properties are compared with available data on
the multi wavelength time variability of the inner nebula. Our main result is
that the X-ray emitting particles are accelerated in the equatorial region of
the pulsar wind. Possible implications on the nature of the acceleration
mechanism are discussed.Comment: 12 pages, 7 figures, 2 table
Simulated synchrotron and Inverse Compton emission from Pulsar Wind Nebulae
We present a complete set of diagnostic tools aimed at reproducing synthetic
non-thermal (synchrotron and/or Inverse Compton, IC) emissivity, integrated
flux energy, polarization and spectral index simulated maps in comparison to
observations. The time dependent relativistic magnetohydrodynamic (RMHD)
equations are solved with a shock capturing code together with the evolution of
the maximum particles energy. Applications to Pulsar Wind Nebulae (PWNe) are
shown.Comment: 3 pages, 7 figures, proceeding of the conference "40 Years of Pulsars
", 12-17 August 2007, Montreal, Canada, submitted to AI
Application of Graph Theory to the elaboration of personal genomic data for genealogical research
In this communication a representation of the links between DNA-relatives based on Graph Theory is applied to the analysis of personal genomic data to obtain genealogical information. The method is tested on both simulated and real data and its applicability to the field of genealogical research is discussed. We envisage the proposed approach as a valid tool for a streamlined application to the publicly available data generated by many online personal genomic companies. In this way, anonymized matrices of pairwise genome sharing counts can help to improve the retrieval of genetic relationships between customers who provide explicit consent to the treatment of their data
Unsupervised Vehicle Counting via Multiple Camera Domain Adaptation
Monitoring vehicle flows in cities is crucial to improve the urban
environment and quality of life of citizens. Images are the best sensing
modality to perceive and assess the flow of vehicles in large areas. Current
technologies for vehicle counting in images hinge on large quantities of
annotated data, preventing their scalability to city-scale as new cameras are
added to the system. This is a recurrent problem when dealing with physical
systems and a key research area in Machine Learning and AI. We propose and
discuss a new methodology to design image-based vehicle density estimators with
few labeled data via multiple camera domain adaptations.Comment: 1st International Workshop on New Foundations for Human-Centered AI
(NeHuAI) at ECAI-202
The β3 adrenoceptor in proliferative retinopathies: "Cinderella" steps out of its family shadow
: In the retina, hypoxic condition leads to overgrowing leaky vessels resulting in altered metabolic supply that may cause impaired visual function. Hypoxia-inducible factor-1 (HIF-1) is a central regulator of the retinal response to hypoxia by activating the transcription of numerous target genes, including vascular endothelium growth factor, which acts as a major player in retinal angiogenesis. In the present review, oxygen urge by the retina and its oxygen sensing systems including HIF-1 are discussed in respect to the role of the beta-adrenergic receptors (β-ARs) and their pharmacologic manipulation in the vascular response to hypoxia. In the β-AR family, β1- and β2-AR have long been attracting attention because their pharmacology is intensely used for human health, while β3-AR, the third and last cloned receptor is no longer increasingly emerging as an attractive target for drug discovery. Here, β3-AR, a main character in several organs including the heart, the adipose tissue and the urinary bladder, but so far a supporting actor in the retina, has been thoroughly examined in respect to its function in retinal response to hypoxia. In particular, its oxygen dependence has been taken as a key indicator of β3-AR involvement in HIF-1-mediated responses to oxygen. Hence, the possibility of β3-AR transcription by HIF-1 has been discussed from early circumstantial evidence to the recent demonstration that β3-AR acts as a novel HIF-1 target gene by playing like a putative intermediary between oxygen levels and retinal vessel proliferation. Thus, targeting β3-AR may implement the therapeutic armamentarium against neovascular pathologies of the eye
On the magnetohydrodynamic modelling of the Crab nebula radio emission
In recent years, it has become a well-established paradigm that many aspects
of the physics of Pulsar Wind Nebulae (PWNe) can be fully accounted for within
a relativistic MHD description. Numerical simulations have proven extremely
successful in reproducing the X-ray morphology of the Crab Nebula, down to very
fine detail. Radio emission, instead, is currently one of the most obscure
aspects of the physics of these objects, and one that holds important
information about pulsar properties and their role as antimatter factories.
Here we address the question of radio emission morphology and integrated
spectrum from the Crab Nebula, by using for the first time an axisymmetric
dynamical model with parameters chosen to best reproduce its X-ray morphology.
Based on our findings we discuss constraints on the origin of the radio
emitting particles.Comment: 8 pages, 5 figures, 1 tabl
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