In-situ Particle Acceleration in Collisionless Shocks

The outflows from gamma ray bursts, active galactic nuclei and relativistic jets in general interact with the surrounding media through collisionless shocks. With three dimensional relativistic particle-in-cell simulations we investigate such shocks. The results from these experiments show that small--scale magnetic filaments with strengths of up to percents of equipartition are generated and that electrons are accelerated to power law distributions N(E)~E^{-p} in the vicinity of the filaments through a new acceleration mechanism. The acceleration is locally confined, instantaneous and differs from recursive acceleration processes such as Fermi acceleration. We find that the proposed acceleration mechanism competes with thermalization and becomes important at high Lorentz factors.Comment: 4 pages, 2 figures, submitted to Il nuovo cimento (4th Workshop Gamma-Ray Bursts in the Afterglow Era, Rome, 18-22 October 2004

Non-Fermi Power law Acceleration in Astrophysical Plasma Shocks

Collisionless plasma shock theory, which applies for example to the afterglow of gamma ray bursts, still contains key issues that are poorly understood. In this paper we study charged particle dynamics in a highly relativistic collisionless shock numerically using ~10^9 particles. We find a power law distribution of accelerated electrons, which upon detailed investigation turns out to originate from an acceleration mechanism that is decidedly different from Fermi acceleration. Electrons are accelerated by strong filamentation instabilities in the shocked interpenetrating plasmas and coincide spatially with the power law distributed current filamentary structures. These structures are an inevitable consequence of the now well established Weibel-like two-stream instability that operates in relativistic collisionless shocks. The electrons are accelerated and decelerated instantaneously and locally; a scenery that differs qualitatively from recursive acceleration mechanisms such as Fermi acceleration. The slopes of the electron distribution power laws are in concordance with the particle power law spectra inferred from observed afterglow synchrotron radiation in gamma ray bursts, and the mechanism can possibly explain more generally the origin of non-thermal radiation from shocked inter- and circum-stellar regions and from relativistic jets.Comment: 4 pages accepted for publication in ApJ Letters. High resolution figures are available online at http://www.astro.ku.dk/users/hededal/040855

Observational manifestations of solar magneto-convection -- center-to-limb variation

We present the first center-to-limb G-band images synthesized from high resolution simulations of solar magneto-convection. Towards the limb the simulations show "hilly" granulation with dark bands on the far side, bright granulation walls and striated faculae, similar to observations. At disk center G-band bright points are flanked by dark lanes. The increased brightness in magnetic elements is due to their lower density compared with the surrounding intergranular medium. One thus sees deeper layers where the temperature is higher. At a given geometric height, the magnetic elements are cooler than the surrounding medium. In the G-band, the contrast is further increased by the destruction of CH in the low density magnetic elements. The optical depth unity surface is very corrugated. Bright granules have their continuum optical depth unity 80 km above the mean surface, the magnetic elements 200-300 km below. The horizontal temperature gradient is especially large next to flux concentrations. When viewed at an angle, the deep magnetic elements optical surface is hidden by the granules and the bright points are no longer visible, except where the "magnetic valleys" are aligned with the line of sight. Towards the limb, the low density in the strong magnetic elements causes unit line-of-sight optical depth to occur deeper in the granule walls behind than for rays not going through magnetic elements and variations in the field strength produce a striated appearance in the bright granule walls.Comment: To appear in ApJL. 6 pages 4 figure

Identification of gravity waves in hydrodynamical simulations

The excitation of internal gravity waves by an entropy bubble oscillating in an isothermal atmosphere is investigated using direct two-dimensional numerical simulations. The oscillation field is measured by a projection of the simulated velocity field onto the anelastic solutions of the linear eigenvalue problem for the perturbations. This facilitates a quantitative study of both the spectrum and the amplitudes of excited g-modes.Comment: 12 pages, 11 figures, Appendices only available onlin

Solar Oscillations and Convection: II. Excitation of Radial Oscillations

Solar p-mode oscillations are excited by the work of stochastic, non-adiabatic, pressure fluctuations on the compressive modes. We evaluate the expression for the radial mode excitation rate derived by Nordlund and Stein (Paper I) using numerical simulations of near surface solar convection. We first apply this expression to the three radial modes of the simulation and obtain good agreement between the predicted excitation rate and the actual mode damping rates as determined from their energies and the widths of their resolved spectral profiles. We then apply this expression for the mode excitation rate to the solar modes and obtain excellent agreement with the low l damping rates determined from GOLF data. Excitation occurs close to the surface, mainly in the intergranular lanes and near the boundaries of granules (where turbulence and radiative cooling are large). The non-adiabatic pressure fluctuations near the surface are produced by small instantaneous local imbalances between the divergence of the radiative and convective fluxes near the solar surface. Below the surface, the non-adiabatic pressure fluctuations are produced primarily by turbulent pressure fluctuations (Reynolds stresses). The frequency dependence of the mode excitation is due to effects of the mode structure and the pressure fluctuation spectrum. Excitation is small at low frequencies due to mode properties -- the mode compression decreases and the mode mass increases at low frequency. Excitation is small at high frequencies due to the pressure fluctuation spectrum -- pressure fluctuations become small at high frequencies because they are due to convection which is a long time scale phenomena compared to the dominant p-mode periods.Comment: Accepted for publication in ApJ (scheduled for Dec 10, 2000 issue). 17 pages, 27 figures, some with reduced resolution -- high resolution versions available at http://www.astro.ku.dk/~aake/astro-ph/0008048

Phase-Dependent Properties of Extrasolar Planet Atmospheres

Recently the Spitzer Space Telescope observed the transiting extrasolar planets, TrES-1 and HD209458b. These observations have provided the first estimates of the day side thermal flux from two extrasolar planets orbiting Sun-like stars. In this paper, synthetic spectra from atmospheric models are compared to these observations. The day-night temperature difference is explored and phase-dependent flux densities are predicted for both planets. For HD209458b and TrES-1, models with significant day-to-night energy redistribution are required to reproduce the observations. However, the observational error bars are large and a range of models remains viable.Comment: 8 pages, 7 figures, accepted for publication in the Astrophysical Journa