arXiv subjects

Felix Spanier

Publications and source records attributed to Felix Spanier.

At least 37 records · Page 2Linked to original sources

arXiv2014

Multi-band implications of external-IC flares

Very fast variability on scales of minutes is regularly observed in Blazars. The assumption that these flares are emerging from the dominant emission zone of the very high energy (VHE) radiation within the jet challenges current acceleration and radiation models. In this work we use a spatially resolved and time dependent synchrotron-self-Compton (SSC) model that includes the full time dependence of Fermi-I acceleration. We use the (apparent) orphan $γ$-ray flare of \textit{Mrk501} during MJD 54952 and test various flare scenarios against the observed data. We find that a rapidly variable external radiation field can reproduce the high energy lightcurve best. However, the effect of the strong inverse Compton (IC) cooling on other bands and the X-ray observations are constraining the parameters to rather extreme ranges. Then again other scenarios would require parameters even more extreme or stronger physical constraints on the rise and decay of the source of the variability which might be in contradiction with constraints derived from the size of the black hole's ergosphere.

astro-ph.HE↗

arXiv2014

Wave-Particle-Interaction in Kinetic Plasmas

Resonant scattering of energetic protons off magnetic irregularities is the main process in cosmic ray diffusion. The typical theoretical description uses Alfven waves in the low frequency limit. We demonstrate that the usage of Particle-in-Cell (PiC) simulations for particle scattering is feasible. The simulation of plasma waves is performed with the relativistic electro-magnetic PiC code ACRONYM and the tracks of test particles are evaluated in order to study particle diffusion. Results for the low frequency limit are equivalent to those obtained with an MHD description, but only for high frequencies results can be obtained with reasonable effort. PiC codes have the potential to be a useful tool to study particle diffusion in kinetic turbulence.

astro-ph.SR↗

arXiv2014

Fundamental and harmonic plasma emission in different plasma environments

Aims: Emission of radio waves from plasmas through plasma emission with fundamental and harmonic frequencies is a familiar process known from solar type II radio bursts. Current models assume the existence of counterstreaming electron beam populations excited at shocks as sources for these emission features, which limits the plasma parameters to reasonable heliospheric shock conditions. However, situations in which counterstreaming electron beams are present can also occur with different plasma parameters, such as higher magnetisation, including but not limited to our Sun. Similar radio emissions might also occur from these situations. Methods: We used particle-in-cell simulations, to compare plasma microphysics of radio emission processes from counterstreaming beams in different plasma environments that differed in density and magnetization. Results: Although large differences in wave populations are evident, the emission process of type II bursts appears to be qualitatively unaffected and shows the same behaviour in all environments.

astro-ph.SR↗

arXiv2014

The radio morphology of a spatially resolved SSC model

One of the main, unresolved questions about the nature of quasars is the position of the acceleration site responsible for the highest energies. The attempt to investigate this question in the energy regime with the highest resolution, the radio band, has the downside that no theoretical model exists that can connect these two regimes. The model in this work tries to shrink this gap by extending the general synchrotron self Compton (SSC) model up to length scales in the order of the resolution of radio observations. The resulting spectral energy distributions (SED) show a qualitative improvement in the representation of the radio spectrum. Furthermore the obtained emission morphology shows similar properties to the radio structures observed in jets of quasars. A complete and quantitative connection will however need either much higher numerical effort or an improved methodology.

astro-ph.HE↗

arXiv2013

Particle scattering in turbulent plasmas with amplified wave modes

High-energy particles stream during coronal mass ejections or flares through the plasma of the solar wind. This causes instabilities, which lead to wave growth at specific resonant wave numbers, especially within shock regions. These amplified wave modes influence the turbulent scattering process significantly. In this paper, results of particle transport and scattering in turbulent plasmas with excited wave modes are presented. The method used is a hybrid simulation code, which treats the heliospheric turbulence by an incompressible magnetohydrodynamic approach separately from a kinetic particle description. Furthermore, a semi-analytical model using quasilinear theory (QLT) is compared to the numerical results. This paper aims at a more fundamental understanding and interpretation of the pitch-angle scattering coefficients. Our calculations show a good agreement of particle simulations and the QLT for broad-band turbulent spectra; for higher turbulence levels and particle beam driven plasmas, the QLT approximation gets worse. Especially the resonance gap at $μ=0$ poses a well-known problem for QLT for steep turbulence spectra, whereas test-particle computations show no problems for the particles to scatter across this region. The reason is that the sharp resonant wave--particle interactions in QLT are an oversimplification of the broader resonances in test-particle calculations, which result from nonlinear effects not included in the QLT. We emphasise the importance of these results for both numerical simulations and analytical particle transport approaches, especially the validity of the QLT.

astro-ph.SR↗

arXiv2012

Note on the use of Yee-lattices in (semi-) implicit Particle-in-cell codes

A modification of the implicit algorithm for particle-in-cell simulations proposed by Petrov and Davis [2011] is presented. The original lattice arrangement is not inherently divergence-free, possibly leading to unphysical results. This arrangement is replaced by a staggered mesh resulting in a reduction of the divergence of the magnetic field by several orders of magnitude.

physics.comp-ph↗

arXiv2012

Evolution of plasma turbulence excited with particle beams

Particles ejected from the Sun that stream through the surrounding plasma of the solar wind are causing instabilities. These generate wavemodes in a certain frequency range especially within shock regions, where particles are accelerated. The aim of this paper is to investigate of amplified Alfvenic wavemodes in driven incompressible magnetohydrodynamic turbulence. Results of different heliospheric scenarios from isotropic and anisotropic plasmas, as well as turbulence near the critical balance are shown. The energy transport of the amplified wavemode is governed by the mechanisms of diffusion, convection and dissipation of energy in wavenumber space. The strength of these effects varies with energy and wavenumber of the mode in question. Two-dimensional energy spectra of spherical k-space integration that permit detailed insight into the parallel and perpendicular development are presented. The evolution of energy injected through driving shows a strong energy transfer to perpendicular wavemodes. The main process at parallel wavemodes is the dissipation of energy in wavenumber space. The generation of higher harmonics along the parallel wavenumber axis is observed. We find evidence for a critical balance in our simulations.

astro-ph.SR↗

arXiv2012

Numerical challenges in kinetic simulations of three-wave interactions

Generation of radio bursts in CME foreshock regions and turbulent cascades in the solar wind are assumed to be results of three-wave interaction processes of dispersive plasma modes. Using our Particle in Cell code ACRONYM, we have studied the behaviour of kinetic wavemodes in the presence of beamed electron populations, with a focus on type II radio burst emission processes. We discuss the numerical challenges in generating and analyzing self-consistently evolving wave coupling processes with a PiC-Code and present preliminary results of said project.

astro-ph.SR↗

arXiv2012

A spatially resolved SSC Shock-in-Jet model

In this paper a spatially resolved, fully self-consistent SSC model is presented. The observable spectral energy distribution (SED) evolves entirely from a low energetic delta distribution of injected electrons by means of the implemented microphysics of the jet. These are in particular the properties of the shock and the ambient plasma, which can be varied along the jet axis. Hence a large variety of scenarios can be computed, e.g. the acceleration of particles via multiple shocks. Two acceleration processes, shock acceleration and stochastic acceleration, are taken into account. From the resulting electron distribution the SED is calculated taking into account synchrotron radiation, inverse Compton scattering (full cross section) and synchrotron self absorption. The model can explain SEDs where cooling processes are crucial. It can verify high variability results from acausal simulations and produce variability not only via injection of particles, but due to the presence of multiple shocks. Furthermore a fit of the data, obtained in the 2010 multi-frequency campaign of Mrk501, is presented.

astro-ph.HE↗

arXiv2012

Emission of Type II Radio Bursts - Single-Beam versus Two-Beam Scenario

The foreshock region of a CME shock front, where shock accelerated electrons form a beam population in the otherwise quiescent plasma is generally assumed to be the source region of type II radio bursts. Nonlinear wave interaction of electrostatic waves excited by the beamed electrons are the prime candidates for the radio waves' emission. To address the question whether a single, or two counterpropagating beam populations are a requirement for this process, we have conducted 2.5D particle in cell simulations using the fully relativistic ACRONYM code. Results show indications of three wave interaction leading to electromagnetic emission at the fundamental and harmonic frequency for the two-beam case. For the single-beam case, no such signatures were detectable.

astro-ph.SR↗

arXiv2012

Nonlinear Wave Interactions as Emission Process of Type II Radio Bursts

The emission of fundamental and harmonic frequency radio waves of type II radio bursts are assumed to be products of three-wave interaction processes of beam-excited Langmuir waves. Using a particle-in-cell code, we have performed simulations of the assumed emission region, a CME foreshock with two counterstreaming electron beams. Analysis of wavemodes within the simulation shows self-consistent excitation of beam driven modes, which yield interaction products at both fundamental and harmonic emission frequencies. Through variation of the beam strength, we have investigated the dependence of energy transfer into electrostatic and electromagnetic modes, confirming the quadratic dependence of electromagnetic emission on electron beam strength.

astro-ph.SR↗

arXiv2011

Variability along the Blazar-Sequence - Hints for extragalactic Cosmic Rays?

The spectral energy distribution and variability of several blazars (Mkn 501, 1 ES 2344+51.4, PKS 2155-30.4, 1 ES 1218+30.4, 3C 454.3) along the blazar sequence is investigated using a selfconsistent and timedependent lepto-hadronic hybrid emission model. The BL Lac objects in the list are successfully modelled with purely leptonic radiation processes (essentially Synchrotron Self-Compton emission), while the Flat Spectrum Radio Quasar requires highly relativistic hadrons to be present within the jet. Variability is exploited as well with our model to distinguish between Self-Compton and hadronic gamma radiation making use of their different signatures in lightcurves. As a consequence active galactic nuclei with z > 0.5 are the best candidates as sources of extragalactic consmic rays, since High-Peaked BL Lac objects do not seem to accelerate protons to energies above thermal. Furthermore the parameters found during the modelling of the objects agree very well with obervations of e.g. superluminal motion or typical variability timescales.

astro-ph.HE↗

arXiv2011

The beaming of external Compton emission

We consider a relativistically moving blob consisting of an isotropic electron distribution that Compton-scatters photons from an external isotropic radiation field. We compute the resulting beaming pattern, i.e. the distribution of the scattered photons, in the blob frame as well as in the observer's frame by using the full Klein-Nishina cross section and the exact incident photon distribution. In the Thomson regime the comparison of our approach with Dermer 1995 results in concurrent characteristics but different absolute number of the scattered photons by a factor of f_corr = 3.09. Additionally, our calculation yields a slightly lower boost factor which varies the more from the corresponding value in Dermer 1995 the higher the spectral index p of the electron distribution gets.

astro-ph.HE↗

arXiv2011

Semi-analytical model of cosmic ray electron transport

We present a numerical extension to the analytical propagation model introduced in Hein and Spanier (2008) to describe the leptonic population in the galactic disc. The model is used to derive a possible identification of the components that contribute to the leptonic cosmic ray spectrum, as measured by PAMELA, Fermi and HESS, with an emphasis on secondary e+ - e- production in collisions of cosmic ray particles with ambient interstellar medium (ISM). We find that besides secondaries, an additional source symmetric in e+ and e- production is needed to explain both the PAMELA anomaly and the Fermi bump, assuming a power-law primary electron spectrum. Our model also allows us to derive constraints for some properties of the ISM.

astro-ph.HE↗

arXiv2010

Simplified models for photohadronic interactions in cosmic accelerators

We discuss simplified models for photo-meson production in cosmic accelerators, such as Active Galactic Nuclei and Gamma-Ray Bursts. Our self-consistent models are directly based on the underlying physics used in the SOPHIA software, and can be easily adapted if new data are included. They allow for the efficient computation of neutrino and photon spectra (from pi^0 decays), as a major requirement of modern time-dependent simulations of the astrophysical sources and parameter studies. In addition, the secondaries (pions and muons) are explicitely generated, a necessity if cooling processes are to be included. For the neutrino production, we include the helicity dependence of the muon decays which in fact leads to larger corrections than the details of the interaction model. The separate computation of the pi^0, pi^+, and pi^- fluxes allows, for instance, for flavor ratio predictions of the neutrinos at the source, which are a requirement of many tests of neutrino properties using astrophysical sources. We confirm that for charged pion generation, the often used production by the Delta(1232)-resonance is typically not the dominant process in Active Galactic Nuclei and Gamma-Ray Bursts, and we show, for arbitrary input spectra, that the number of neutrinos are underestimated by at least a factor of two if they are obtained from the neutral to charged pion ratio. We compare our results for several levels of simplification using isotropic synchrotron and thermal spectra, and we demonstrate that they are sufficiently close to the SOPHIA software.

astro-ph.HE↗

arXiv2010

The influence of the mass-ratio on the acceleration of particles by filamentation instabilities

Almost all sources of high energy particles and photons are associated with jet phenomena. Prominent sources of such highly relativistic outflows are pulsar winds and Active Galactic Nuclei. The current understanding of these jets assumes diluted plasmas which are best described as kinetic phenomena. In this kinetic description particle acceleration to ultra-relativistic speeds can occur in completely unmagnetized and neutral plasmas through insetting effects of instabilities. Even though the morphology and nature of particle spectra are understood to a certain extent, the composition of the jets is not known yet. While Poynting-flux dominated jets are certainly composed of electron-positron plasmas, the understanding of the governing physics in AGN jets is mostly unclear. In this article we investigate how the constituting elements of an electron-positron-proton plasma behave differently under the variation of the fundamental mass-ratio m_p/m_e. We studied initially unmagnetized counterstreaming plasmas using fully relativistic three-dimensional particle-in-cell simulations to investigate the influence of the mass-ratio on particle acceleration and magnetic field generation in electron-positron-proton plasmas. We covered a range of mass-ratios m_p/m_e between 1 and 100 with a particle number composition of n_{p^+}/n_{e^+} of 1 in one stream, only protons are injected in the other, whereas electrons are present in both to guarantee charge neutrality in the simulation box. We find that with increasing proton mass the instability takes longer to develop and for mass-ratios > 20 the particles seem to be accelerated in two phases which can be accounted to the individual instabilities of the different species. This means that for high mass ratios the coupling between electrons/positrons and the heavier protons, which occurs in low mass-ratios, disappears.

astro-ph.HE↗

arXiv2010

Modelling the emission from blazar jets - the case of PKS 2155-304

A time-dependent Synchrotron Self Compton model (SSC) which is able to motivate the used electron spectra of many SSC models as a balance of acceleration and radiative losses is introduced. Using stochastic acceleration as well as Fermi-I processes even electron spectra with a rising part can be explained, which are mandatory to fit the lowstate spectral energy distribution (SED) of PKS 2155-304 as constrained from Fermi LAT observations. Due to the time resolution the outburst of PKS 2155-304 observed by H.E.S.S. in 2006 can be modelled selfconsistently as fluctuations along the jet axis without introducing new sets of parameters. The model makes the time evolution of the SED also accessible. Hence giving new insights into the flaring behavior of blazars.

astro-ph.HE↗

arXiv2010

Modelling the variability of 1ES1218+30.4

The blazar 1ES1218+30.4 has been previously detected by the VERITAS and MAGIC telescopes in the very high energies. The new detection of VERITAS from December 2008 to April 2009 proves that 1ES1218+30.4 is not static, but shows short-time variability. We show that the time variability may be explained in the context of a self-consistent synchrotron-self Compton model, while the long time observation do not necessarily require a time-resolved treatment. The kinetic equations for electrons and photons in a plasma blob are solved numerically including Fermi acceleration for electrons as well as synchrotron radiation and Compton scattering. The light curve observed by VERITAS can be reproduced in our model by assuming a changing level of electron injection compared to the constant state of 1ES1218+30.4. The multiwavelength behaviour during an outburst becomes comprehensible by the model. The long time measurements of VERITAS are still explainable via a constant emission in the SSC context, but the short outbursts each require a time-resolved treatment.

astro-ph.HE↗