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Thursday, 11.03.2010, 16:00, 31 Fitzwilliam Place

Dr. Gareth Murphy

DIAS

Magnetic field amplification and electron acceleration to near-energy equipartition with ions by a mildly relativistic quasi-parallel plasma protoshock.

Abstract: We explore with two-dimensional relativistic particle-in-cell simulations the transition layer of a shock, evolving from the collision of two plasma clouds with a shock speed of 0.9c,in the presence of a quasi-parallel magnetic field.The ratio of the cloud densities is 10. The number densities of ions and electrons are equal in each cloud, and the reduced mass ratio is 250. The peak Lorentz factor of the electrons is determined,along with the orientation and the strength of the magnetic field at the interface of the two colliding clouds. We find a strong plasma filamentation behind the otherwise planar shock front as well as signatures of orthogonal magnetic field striping, indicative of the filamentation instability (FI). The magnetic field component orthogonal to the initial field direction is amplified to values that exceed those expected from the shock compression by over an order of magnitude. The forming shock is quasi-perpendicular due to this amplification. These powerful magnetic fields convect away from the shock boundary and their energy density exceeds by far the thermal pressure of the plasma. Localized magnetic bubbles form. The relativistic masses of the electrons and ions close to the shock transition layer are comparable. The simultaneous presence of highly relativistic electrons and strong magnetic fields will give rise to synchrotron (jitter) emissions at a significant intensity.

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Friday, 26.02.2010, 14:00, 31 Fitzwilliam Place

Dr. Athina Meli

Erlangen Center for Astroparticle Physics University of Erlangen-Nuremberg, Germany

Cosmic rays and relativistic shock acceleration

Abstract: The diffusive shock acceleration mechanism is invoked to explain non-thermal cosmic rays in Supernova Remnants, Active Galactic Nuclei Jets, Gamma ray Bursts and various large scale cosmic structures. Especially, the importance of achieving the highest observed cosmic ray energies by such a mechanism in a given astrophysical situation is a recurring theme. In this presentation, Monte Carlo simulation studies of relativistic subluminal and superluminal shocks will be presented, with an emphasis on the acquired diffuse cosmic ray signal at the highest energies observed, and the role of the efficiency of the relativistic shocks will be discussed.

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Thursday, 25.02.2010, 16:00, 31 Fitzwilliam Place

Dr Jorik Vink

Armagh Observatory

Is star formation universal across the stellar mass range?

Abstract: I discuss the geometry of accretion flows and disks around a variety of young stars, including T Tauri stars, Herbig Ae/Be stars, and more massive young stellar objects. For this purpose, I focus on the technique of linear line polarimetry, a most powerful tool to constrain the geometry and kinematics of the circumstellar material around young stars.

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Thursday, 18.02.2010, 16:00, 31 Fitzwilliam Place

Dr Julien Morin

DIAS

Dynamo processes in fully-convective stars : a spectropolarimetric study

Abstract: In solar-type stars the generation of magnetic fields through dynamo processes apparently concentrates in the tachocline, a thin layer of strong shear located at the interface between the inner radiative zone and the outer convective envelope. At the low-mass end of the main sequence, stars are fully convective and therefore do not posses such a tachocline. And yet observations reveal that some fully convective stars are very active and magnetic. Non- solar dynamo mechanisms are thus expected to operate in these objects. Theoretical and numerical studies need observational guidance to build up a consistent picture of dynamo in cool stars. I will present spectropolarimetric methods and their interest to provide dynamo theorists with relevant observational constraints. Then, I will outline the main results of a first spectropolarimetric survey of a small sample of active M dwarfs.

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Friday, 12.02.2010, 16:00, 31 Fitzwilliam Place

Dr Sinead Mc Glynn

KTH School of Engineering Sciences, Sweden

High energy emission from gamma ray bursts detected by the Fermi Gamma ray Space Telescope

Abstract: The Fermi Gamma ray Space Telescope has regularly observed emission from gamma ray bursts (GRBs) with its two instruments. The Large Area Telescope (LAT) and Gamma ray Burst Monitor (GBM) on board Fermi provide spectral coverage from 8 keV to 300 GeV in total. Several of these bursts show evidence for an extra component at GeV energies, which had previously not been confirmed. I will present some results from the first 18 months of Fermi observations, with particular emphasis on new phenomena such as the extra high energy component, and evidence for a multicoloured thermal component in GRB 090902b.

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JOINT SEMINAR - Thursday, 4.02.2010, 16:00, 31 Fitzwilliam Place

Denis Malyshev

DIAS

A simple model for electron plasma heating in supernova remnants.

Abstract: Multiwavelength observations of supernova remnants can be explained within the framework of diffusive shock acceleration theory, which allows effective conversion of the explosion energy into cosmic rays. Although the models of nonlinear shocks describe reasonably well the nonthermal component of emission, certain issues, including the heating of the thermal electron plasma and the related X-ray emission, still remain open. In my talk I will review our study of electron heating in supernova remnants in the Sedov phase due to Coulomb exchange between protons and electrons. We have calculated the electron temperature and the X-ray thermal Bremsstrahlung emission from supernova remnants as functions of the relevant parameters. Since only the Coulomb mechanism was considered for electron heating, the values obtained for the electron temperatures should be treated as lower limits. Results from this work can be useful to constrain model parameters for observed SNRs.

Seán Delaney

DIAS

Time Dependent Shock Acceleration.

Abstract: For about 30 years diffusive shock acceleration has been the dominant theory used to explain the origin of cosmic rays. In that time, a great deal of progress has been made in determining the approximate form of the particle distribution function close to shocks in the steady state. In the case of non-relativistic shocks, codes are now available to calculate time dependent solutions to the transport equation for very general diffusion and loss processes. Unfortunately, no such code has been devised for relativistic shocks. We investigate numerical methods for time dependent shock acceleration with the intention of extending them to relativistic shocks.

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Thursday, 28.01.2010, 16:00, 31 Fitzwilliam Place

Dr Mark Eric Dieckmann

Linköping University, Sweden

PIC simulation of a mildly relativistic collision of an electron-ion plasma carrying a quasi-parallel magnetic field

Abstract: Plasma processes close to SNR shocks result in the amplification of magnetic fields and in the acceleration of electrons, injecting them into the diffusive acceleration mechanism. The acceleration of electrons and the B field amplification by the collision of two plasma clouds, each consisting of electrons and ions, at a speed of 0.5c is investigated. A quasi-parallel guiding magnetic field, a cloud density ratio of 10 and a plasma temperature of 25 keV are considered. A quasi-planar shock forms at the front of the dense plasma cloud. It is mediated by a circularly left-hand polarized electromagnetic wave with an electric field component along the guiding magnetic field. Its propagation direction is close to that of the guiding field and orthogonal to the collision boundary. It has a low frequency and a wavelength that equals several times the ion inertial length, which would be indicative of a dispersive Alfven wave close to the ion cyclotron resonance frequency of the left-handed mode (ion whistler), provided that the frequency is appropriate. However, it moves with the super-alfvenic plasma collision speed, suggesting that it is an Alfven precursor or a nonlinear MHD wave such as a Short Large-Amplitude Magnetic Structure (SLAMS). The growth of the magnetic amplitude of this wave to values well in excess of those of the quasi-parallel guiding field and of the filamentation modes results in a quasi-perpendicular shock. The waves developing upstream of the dense cloud give rise to electron acceleration ahead of the collision boundary. Energy equipartition between the ions and the electrons is established at the shock and the electrons are accelerated to relativistic speeds.

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