Multi-D magnetohydrodynamic modelling of pulsar wind nebulae: recent progress and open questions

In the last decade, the relativistic magnetohydrodynamic (MHD) modelling of pulsar wind nebulae, and of the Crab nebula in particular, has been highly successful, with many of the observed dynamical and emission properties reproduced down to the finest detail. Here, we critically discuss the results of some of the most recent studies: namely the investigation of the origin of the radio emitting particles and the quest for the acceleration sites of particles of different energies along the termination shock, by using wisps motion as a diagnostic tool; the study of the magnetic dissipation process in high magnetization nebulae by means of new long-term three-dimensional simulations of the pulsar wind nebula evolution; the investigation of the relativistic tearing instability in thinning current sheets, leading to fast reconnection events that might be at the origin of the Crab nebula gamma-ray flares.Comment: 30 pages, 12 figure

Relativistic MHD modeling of magnetized neutron stars, pulsar winds, and their nebulae

Neutron stars are among the most fascinating astrophysical sources, being characterized by strong gravity, densities about the nuclear one or even above, and huge magnetic fields. Their observational signatures can be extremely diverse across the electromagnetic spectrum, ranging from the periodic and low-frequency signals of radio pulsars, up to the abrupt high-energy gamma-ray flares of magnetars, where energies of ~10^46 erg are released in a few seconds. Fast-rotating and highly magnetized neutron stars are expected to launch powerful relativistic winds, whose interaction with the supernova remnants gives rise to the non-thermal emission of pulsar wind nebulae, which are known cosmic accelerators of electrons and positrons up to PeV energies. In the extreme cases of proto-magnetars (magnetic fields of ~10^15 G and millisecond periods), a similar mechanism is likely to provide a viable engine for the still mysterious gamma-ray bursts. The key ingredient in all these spectacular manifestations of neutron stars is the presence of strong magnetic fields in their constituent plasma. Here we will present recent updates of a couple of state-of-the-art numerical investigations by the high-energy astrophysics group in Arcetri: a comprehensive modeling of the steady-state axisymmetric structure of rotating magnetized neutron stars in general relativity, and dynamical 3-D MHD simulations of relativistic pulsar winds and their associated nebulae.Comment: EPS 44th Conference on Plasma Physics (June 2017, Belfast), paper accepted for publication on Plasma Physics and Controlled Fusio

The epidermal ‘excretory’ syncytial plates in species of Temnocephala (Platyhelminthes, Temnocephalida): Proposal of a new methodology

The epidermis of temnocephalids is formed by a mosaic of syncytial plates as revealed by electron microscopy and/or 5% silver nitrate impregnation. The variable form and size have been observed but the value of the epidermal ‘excretory’ syncytial plates (EPs) in the identification of Temnocephala species never were proper discuss. Ten species of Temnocephala were studied using images taken with a scanning electron microscope (SEM), describing a morphometry method to allow the comparison, thus, evaluating the EPs as a specific character in the identification of species of Temnocephala. The measurements (length of the anterior and posterior portions of the plate relative to the excretory pore, width of the internal and external limits, length of the portion exceeding the tentacles, total width, and total length) were obtained using AxioVision® Zeiss LWE 4.7.2 (AVZ) software. The EPs of the species tested were different revealing unique characteristics thus allowing their use as specific characters within Temnocephala. There was a pattern for each host group (Mollusca, Crustacea, Insecta, and Chelonia); the exceptions were the crustacean species and one insect species. The intra-specific variation study in Temnocephala trapeziformis demonstrated that this character varied minimally within the species. The SEM images and the AVZ software measurements were positive for the characterization of species of Temnocephala. The cirrus remained the most important specific character within Temnocephala. The traditional morphometric method was sufficient to demonstrate several specific EP characteristics, but new EPs should always be clearly described and illustrated following the now proposed methodology for Temnocephala species identification.(As placas epidérmicas sinciciais ‘excretoras’ em espécies de Temnocephala (Platyhelminthes, Temnocephalida): Proposta de uma nova metodologia.) A epiderme dos representantes da ordem Temnocephalida é formada por um mosaico de placas epidérmicas sinciciais evidenciado através da microscopia eletrônica e da impregnação com nitrato de prata 5%. A variação de forma e tamanho do par de sincícios pós-tentaculares, ou ‘placas excretoras’ (PEs), entre as espécies já foram observados, mas o valor das PEs na identificação de espécies de Temnocephala nunca foi adequadamente discutido. Dez espécies de Temnocephala foram analisadas através de imagens obtidas em microscopia eletrônica de varredura (MEV), descrevendo um método morfométrico que permite a comparação, e avaliação, das PEs como carácter específico. As medidas (comprimento da porção anterior e posterior, largura do limite interno e externo, comprimento da porção da placa acima do limite dos tentáculos, comprimento e largura totais) foram obtidas no programa AxioVision Zeiss LE 4.7.2 (AVZ). As PEs das espécies estudadas diferiram entre si e possuem características únicas que fazem delas bons caracteres diagnósticos específicos. Possuem um padrão por grupo hospedeiro; as exceções são as espécies epibiontes em crustáceos e uma espécie epibionte em insetos. O estudo da variação intraespecífica de Temnocephala trapeziformis mostrou que este caráter tem pouca variação dentro de uma mesma espécie. A visualização das PEs em MEV e as medidas obtidas através do programa AVZ produziram bons resultados na caracterização das espécies. O estudo morfométrico tradicional foi suficiente para evidenciar as características específicas das PEs. Estas devem ser claramente descritas e ilustradas, seguindo a metodologia agora proposta, para permitir a utilização deste caráter diagnóstico na identificação das espécies de Temnocephala

On Shepard–Gupta-type operators

A Gupta-type variant of Shepard operators is introduced and convergence results and pointwise and uniform direct and converse approximation results are given. An application to image compression improving a previous algorithm is also discussed

Macroscopic and Local Magnetic Moments in Si-doped CuGeO3_3 with Neutron and μ\muSR Studies

The temperature-concentration phase diagram of the Si-doped spin-Peierls compound CuGeO$_{3}$ is investigated by means of neutron scattering and muon spin rotation spectroscopy in order to determine the microscopic distribution of the magnetic and lattice dimerised regions as a function of doping. The analysis of the zero-field muon spectra has confirmed the spatial inhomogeneity of the staggered magnetisation that characterises the antiferromagnetic superlattice peaks observed with neutrons. In addition, the variation of the macroscopic order parameter with doping can be understood by considering the evolution of the local magnetic moment as well as of the various regions contributing to the muon signal

Determination of the zero-field magnetic structure of the helimagnet MnSi at low temperature

Below a temperature of approximately 29 K the manganese magnetic moments of the cubic binary compound MnSi order to a long-range incommensurate helical magnetic structure. Here, we quantitatively analyze a high-statistic zero-field muon spin rotation spectrum recorded in the magnetically ordered phase of MnSi by exploiting the result of representation theory as applied to the determination of magnetic structures. Instead of a gradual rotation of the magnetic moments when moving along a axis, we find that the angle of rotation between the moments of certain subsequent planes is essentially quenched. It is the magnetization of pairs of planes which rotates when moving along a axis, thus preserving the overall helical structure.Comment: 10 pages, 4 figure

Evidence for spin liquid ground state in SrDy2_2O4_4 frustrated magnet probed by muSR

Muon spin relaxation ($\mu$SR) measurements were carried out on SrDy$_2$O$_4$, a frustrated magnet featuring short range magnetic correlations at low temperatures. Zero-field muon spin depolarization measurements demonstrate that fast magnetic fluctuations are present from $T=300$ K down to 20 mK. The coexistence of short range magnetic correlations and fluctuations at $T=20$ mK indicates that SrDy$_2$O$_4$ features a spin liquid ground state. Large longitudinal fields affect weakly the muon spin depolarization, also suggesting the presence of fast fluctuations. For a longitudinal field of $\mu_0H=2$ T, a non-relaxing asymmetry contribution appears below $T=6$ K, indicating considerable slowing down of the magnetic fluctuations as field-induced magnetically-ordered phases are approached.Comment: 6 pages, 4 figures, to be published as a proceeding of HFM2016 in Journal of Physics: Conference Series (JPCS