Searcharxiv⌕ Search

arXiv subjects

Martin Pohl

Publications and source records attributed to Martin Pohl.

At least 55 records · Page 3Linked to original sources
arXiv2020

Mildly relativistic magnetized shocks in electron-ion plasmas I. Electromagnetic shock structure

Mildly relativistic shocks in magnetized electron-ion plasmas are investigated with 2D kinetic particle-in-cell simulations of unprecedentedly high resolution and large scale for conditions that may be found at internal shocks in blazar cores. Ion-scale effects cause corrugations along the shock surface whose properties somewhat depend on the configuration of the mean perpendicular magnetic field, that is either in or out of the simulation plane. We show that the synchrotron maser instability persists to operate in mildly relativistic shocks in agreement with theoretical predictions and produces coherent emission of upstream-propagating electromagnetic waves. Shock front ripples are excited in both mean-field configurations and they engender effective wave amplification. The interaction of these waves with upstream plasma generates electrostatic wakefields.

astro-ph.HE↗
arXiv2020

Kinetic simulation of nonrelativistic perpendicular shocks of young supernova remnants. IV. Electron heating

High Mach number collisionless shocks are found in planetary systems and supernova remnants (SNRs). Electrons are heated at these shocks to the temperature well above the Rankine-Hugoniot prediction. However processes responsible for electron heating are still not well understood. We use a set of large-scale Particle-In-Cell simulations of non-relativistic shocks in high Mach number regime to clarify the electron heating processes. The physics of these shocks is defined by ion reflection at the shock ramp. Further interaction of the reflected ions and the upstream plasma excites electrostatic Buneman and two-stream ion-ion Weibel instabilities. Electrons are heated via shock surfing acceleration, the shock potential, magnetic reconnection, stochastic Fermi scattering and the shock compression. The main contributor is the shock potential. Magnetic field lines are tangled due to the Weibel instability, which allows the parallel electron heating by the shock potential. The constrained model of the electron heating predicts the ion-to-electron temperature ratio within observed values at SNR shocks and in Saturn's bow shock.

astro-ph.HE↗
arXiv2020

Production of secondary particles in heavy nuclei interactions in supernova remnants

Depending on their type, supernova remnants may have ejecta material with high abundance of heavy elements such as carbon or oxygen. In addition, core-collapse supernovae explode in the wind material of their progenitor star that may also have a high abundance of heavy elements. Hadronic collisions in these enriched media spawn the production of gamma rays, neutrinos, and secondary electrons with spectra that cannot be scaled from those calculated for pp collisions, potentially leading to erroneous results. We used Monte-Carlo event generators to calculate the differential production rate of particles such as gamma rays, neutrinos, and secondary electrons for H, He, C, and O nuclei as projectiles and as target material. The cross sections and the multiplicity spectra are separately computed for each of the 16 combinations of projectile and target. We describe characteristic effects of heavy nuclei in the shape and normalization of the spectra of various particles produced.

astro-ph.HE↗
arXiv2020

Kinetic simulations of nonrelativistic perpendicular shocks of young supernova remnants. III. Magnetic reconnection

Fully kinetic two-dimensional particle-in-cell simulations are used to study electron acceleration at high-Mach-number nonrelativistic perpendicular shocks. SNR shocks are mediated by the Weibel instability which is excited because of an interaction between shock-reflected and upstream ions. Nonlinear evolution of the Weibel instability leads to the formation of current sheets. At the turbulent shock ramp the current sheets decay through magnetic reconnection. The number of reconnection sites strongly depends on the ion-to-electron mass ratio and the Alfvénic Mach number of the simulated shock. Electron acceleration is observed at locations where magnetic reconnection operates. For the highest mass ratios almost all electrons are involved in magnetic reconnection, which makes the magnetic reconnection the dominant acceleration process for electrons at these shocks. We discuss the relevance of our results for 3D systems with realistic ion-to-electron mass ratio.

astro-ph.HE↗
arXiv2020

Rapid Particle Acceleration due to Recollimation Shocks and Turbulent Magnetic Fields in Injected Jets with Helical Magnetic Fields

One of the key questions in the study of relativistic jets is how magnetic reconnection occurs and whether it can effectively accelerate electrons in the jet. We performed 3D particle-in-cell (PIC) simulations of a relativistic electron-proton jet of relatively large radius that carries a helical magnetic field. We focussed our investigation on the interaction between the jet and the ambient plasma and explore how the helical magnetic field affects the excitation of kinetic instabilities such as the Weibel instability (WI), the kinetic Kelvin-Helmholtz instability (kKHI), and the mushroom instability (MI). In our simulations these kinetic instabilities are indeed excited, and particles are accelerated. At the linear stage we observe recollimation shocks near the center of the jet. As the electron-proton jet evolves into the deep nonlinear stage, the helical magnetic field becomes untangled due to reconnection-like phenomena, and electrons are repeatedly accelerated as they encounter magnetic-reconnection events in the turbulent magnetic field.

astro-ph.HE↗
arXiv2019

PIC Simulation Methods for Cosmic Radiation and Plasma Instabilities

Particle acceleration in collisionless plasma systems is a central question in astroplasma and astroparticle physics. The structure of the acceleration regions, electron-ion energy equilibration, preacceleration of particles at shocks to permit further energization by diffusive shock acceleration, require knowledge of the distribution function of particles besides the structure and dynamic of electromagnetic fields, and hence a kinetic description is desirable. Particle-in-cell simulations offer an appropriate, if computationally expensive method of essentially conducting numerical experiments that explore kinetic phenomena in collisionless plasma. We review recent results of PIC simulations of astrophysical plasma systems, particle acceleration, and the instabilities that shape them.

astro-ph.HE↗
arXiv2019

Cosmic-ray acceleration and escape from post-adiabatic Supernova remnants

Supernova remnants are known to accelerate cosmic rays on account of their non-thermal emission of radio waves, X-rays, and gamma rays. Although there are many models for the acceleration of cosmic rays in Supernova remnants, the escape of cosmic rays from these sources is yet understudied. We use our time-dependent acceleration code RATPaC to study the acceleration of cosmic rays and their escape in post-adiabatic Supernova remnants and calculate the subsequent gamma-ray emission from inverse-Compton scattering and Pion decay. We performed spherically symmetric 1-D simulations in which we simultaneously solve the transport equations for CRs, magnetic turbulence, and the hydrodynamical flow of the thermal plasma in a volume large enough to keep all CRs in the simulation. The transport equations for cosmic-rays and magnetic turbulence are coupled via the cosmic-ray gradient and the spatial diffusion coefficient of the cosmic rays, while the cosmic-ray feedback onto the shock structure can be ignored. Our simulations span 100kyrs, thus covering the remnants evolution until the beginning of the post-adiabatic phase. At later stages of the evolution cosmic rays over a wide range of energy can reside outside of the remnant, creating spectra that are softer than predicted by standard DSA and feature breaks in the 10-100 GeV-range. The total spectrum of cosmic rays released into the interstellar medium has a spectral index of s~2.4 above roughly 10 GeV which is close to that required by Galactic propagation models. We further find the gamma-ray luminosity to peak around an age of 4,000 years for inverse-Compton-dominated high-energy emission. Remnants expanding in low-density media emit generally more inverse-Compton radiation matching the fact that the brightest known supernova remnants - RCW86, Vela Jr, HESSJ1731-347 and RXJ1713.7-3946 - are all expanding in low density environments.

astro-ph.HE↗
arXiv2019

A note on the experiment parameters for the non-resonant streaming instability: competition between left and right circularly polarized modes

A non-resonant streaming instability driven by cosmic-ray currents, also called Bell's instability, is proposed as a candidate for providing the required magnetic turbulence of efficient diffusive shock accelerations. To demonstrate the saturation level and mechanism of the non-resonant streaming instability in a laboratory environment, we attempt to develop an experiment at the Photo Injector Test Facility at DESY, Zeuthen site (PITZ). As an electron beam is used to replace the proton beam to carry the cosmic-ray current in our experiment, the polarization of the non-resonant streaming instability will be modified from the left-handed (LH) mode to the right-handed (RH) mode. The theoretical instability analysis shows that the growth rate of this RH non-resonant mode may be smaller than it of the LH resonant mode. However the LH resonant mode can be ignored in our experiment while the expected wavelength is longer than the used plasma cell. The results of PIC simulations will also support this contention and the occurrence of non-resonant streaming instability in our experiment.

physics.plasm-ph↗
arXiv2019

Multi-messenger interpretation of the neutrinos from TXS 0506+056

We discuss possible interpretations of the neutrinos observed from the AGN blazar TXS 0506+056 in the multi-messenger and multi-wavelength context, including both the 2014-15 and 2017 neutrino flares. While the neutrino observed in September 2017 has to describe contemporary data in e.g. the X-ray and VHE gamma-ray ranges, data at the 2014-15 excess are much sparser. We demonstrate that in both cases the simplest possible one-zone AGN blazar models face challenges. While the 2017 flare can be well interpreted by considering more sophisticated source geometries, the 2014-15 flare is much harder to describe with conventional models. One challenge is the energy injected into the electromagnetic cascade coming together with the neutrino production, which cannot be reconciled with the 13 observed neutrino events. We also speculate if a common interpretation of both flares is feasible.

astro-ph.HE↗
arXiv2019

Kinetic simulations of nonrelativistic perpendicular shocks of young supernova remnants. II. Influence of shock-surfing acceleration on downstream electron spectra

We explore electron pre-acceleration at high Mach-number nonrelativistic perpendicular shocks at, e.g., young supernova remnants, which are a prerequisite of further acceleration to very high energies via diffusive shock acceleration. Using fully kinetic particle-in-cell simulations of shocks and electron dynamics in them, we investigate the influence of shock-surfing acceleration at the shock foot on the nonthermal population of electrons downstream of the shock. The shock-surfing acceleration is followed by further energization at the shock ramp where the Weibel instability spawns a type of second-order Fermi acceleration. The combination of these two processes leads to the formation of a nonthermal electron population, but the importance of shock-surfing acceleration becomes smaller for larger ion-to-electron mass ratio in the simulation. We discuss the resulting electron spectra and the relevance of our results to the physics of systems with real ion-to-electron mass ratio and fully three-dimensional behavior.

astro-ph.HE↗
arXiv2019

Precursor Wave Amplification by Ion-Electron Coupling through Wakefield in Relativistic Shocks

We investigated electromagnetic precursor wave emission in relativistic shocks by using two-dimensional particle-in-cell simulations. We found that the wave amplitude is significantly enhanced by a positive feedback process associated with ion-electron coupling through the wakefields for high magnetization. The wakefields collapse during the nonlinear process of the parametric decay instability in the near-upstream region, where nonthermal electrons and ions are generated. The intense coherent emission and the particle acceleration may opperate in high-energy astrophysical objects.

astro-ph.HE↗
arXiv2019

Modelling the coincident observation of a high-energy neutrino and a bright blazar flare

In September 2017, the IceCube Neutrino Observatory recorded a very-high-energy neutrino in directional coincidence with a blazar in an unusually bright gamma-ray state, TXS0506+056. Blazars are prominent photon sources in the universe because they harbor a relativistic jet whose radiation is strongly collimated and amplified. High-energy atomic nuclei known as cosmic rays can produce neutrinos; thus the recent detection may help identifying the sources of the diffuse neutrino flux and the energetic cosmic rays. Here we report on a self-consistent analysis of the physical relation between the observed neutrino and the blazar, in particular the time evolution and spectral behavior of neutrino and photon emission. We demonstrate that a moderate enhancement in the number of cosmic rays during the flare can yield a very strong increase of the neutrino flux which is limited by co-produced hard X-rays and TeV gamma rays. We also test typical radiation models for compatibility and identify several model classes as incompatible with the observations. We investigate to what degree the findings can be generalized to the entire population of blazars, to determine the relation between their output in photons, neutrinos, and cosmic rays, and suggest how to optimize the strategy of future observations.

astro-ph.HE↗
arXiv2019

Kinetic simulations of nonrelativistic perpendicular shocks of young supernova remnants. I. Electron shock-surfing acceleration

Electron injection at high Mach-number nonrelativistic perpendicular shocks is studied here for parameters that are applicable to young SNR shocks. Using high-resolution large-scale two-dimensional fully kinetic particle-in-cell (PIC) simulations and tracing individual particles we in detail analyze the shock surfing acceleration (SSA) of electrons at the leading edge of the shock foot. The central question is to what degree the process can be captured in 2D3V simulations. We find that the energy gain in SSA always arises from the electrostatic field of a Buneman wave. Electron energization is more efficient in the out-of-plane orientation of the large-scale magnetic field because both the phase speed and the amplitude of the waves are higher than for the in-plane scenario. Also, a larger number of electrons is trapped by the waves compared to the in-plane configuration. We conclude that significant modifications of the simulation parameters are needed to reach the same level of SSA efficiency as in simulations with out-of-plane magnetic field or 3D simulations.

astro-ph.HE↗
arXiv2019

Detailed polarization measurements of the prompt emission of five Gamma-Ray Bursts

Gamma-ray bursts are the strongest explosions in the Universe since the Big Bang, believed to be produced either in forming black holes at the end of massive star evolution or merging of compact objects. Spectral and timing properties of gamma-ray bursts suggest that the observed bright gamma-rays are produced in the most relativistic jets in the Universe; however, the physical properties, especially the structure and magnetic topologies in the jets are still not well known, despite several decades of studies. It is widely believed that precise measurements of the polarization properties of gamma-ray bursts should provide crucial information on the highly relativistic jets. As a result there have been many reports of gamma-ray burst polarization measurements with diverse results, see, however many such measurements suffered from substantial uncertainties, mostly systematic. After the first successful measurements by the GAP and COSI instruments, here we report a statistically meaningful sample of precise polarization measurements, obtained with the dedicated gamma-ray burst polarimeter, POLAR onboard China's Tiangong-2 spacelab. Our results suggest that the gamma-ray emission is at most polarized at a level lower than some popular models have predicted; although our results also show intrapulse evolution of the polarization angle. This indicates that the low polarization degrees could be due to an evolving polarization angle during a gamma-ray burst.

astro-ph.HE↗
arXiv2018

Time variability of TeV cosmic ray sky map

The variation in the intensity of cosmic rays at small angular scales is attributed to the interstellar turbulence in the vicinity of the Solar system. We show that {a turbulent origin of the small-scale structures implies that} the morphology of the observed cosmic-ray intensity skymap varies with our location in the interstellar turbulence. The gyroradius of cosmic rays is shown to be the length scale associated with an observable change in the skymap over a radian angular scale. The extent to which the intensity \mpo{at a certain} angular scale varies is proportional to the change in our location with a maximum change of about the amplitude of intensity variation at that scale in the existing skymap.} We suggest that for TeV cosmic rays a measurable variation could occur over a time scale of a decade due to the Earth's motion through the interstellar medium, if interstellar turbulence persists down to the gyroradius, \mpo{about $00\ μ\mathrm{pc}$ for TeV-ish cosmic rays}. Observational evidence of the variability, or an absence of it, could provide a useful insight into the physical origin of the small-scale anisotropy.

astro-ph.HE↗
arXiv2018

Galactic Bulge Preferred Over Dark Matter for the Galactic Center Gamma-Ray Excess

An anomalous gamma-ray excess emission has been found in Fermi Large Area Telescope data covering the centre of the Galaxy. Several theories have been proposed for this `Galactic Centre Excess'. They include self-annihilation of dark matter particles, an unresolved population of millisecond pulsars, an unresolved population of young pulsars, or a series of burst events. Here we report on a new analysis that exploits hydrodynamical modelling to register the position of interstellar gas associated with diffuse Galactic gamma-ray emission. We find evidence that the Galactic Centre Excess gamma rays are statistically better described by the stellar over-density in the Galactic bulge and the nuclear stellar bulge, rather than a spherical excess. Given its non-spherical nature, we argue that the Galactic Centre Excess is not a dark matter phenomenon but rather associated with the stellar population of the Galactic bulge and nuclear bulge.

astro-ph.HE↗
arXiv2018

The Large Area Detector onboard the eXTP mission

The eXTP (enhanced X-ray Timing and Polarimetry) mission is a major project of the Chinese Academy of Sciences (CAS) and China National Space Administration (CNSA) currently performing an extended phase A study and proposed for a launch by 2025 in a low-earth orbit. The eXTP scientific payload envisages a suite of instruments (Spectroscopy Focusing Array, Polarimetry Focusing Array, Large Area Detector and Wide Field Monitor) offering unprecedented simultaneous wide-band X-ray spectral, timing and polarimetry sensitivity. A large European consortium is contributing to the eXTP study and it is expected to provide key hardware elements, including a Large Area Detector (LAD). The LAD instrument for eXTP is based on the design originally proposed for the LOFT mission within the ESA context. The eXTP/LAD envisages a deployed 3.4 m2 effective area in the 2-30 keV energy range, achieved through the technology of the large-area Silicon Drift Detectors - offering a spectral resolution of up to 200 eV FWHM at 6 keV - and of capillary plate collimators - limiting the field of view to about 1 degree. In this paper we provide an overview of the LAD instrument design, including new elements with respect to the earlier LOFT configuration.

astro-ph.IM↗