arXiv subjects

H. Lesch

Publications and source records attributed to H. Lesch.

At least 37 records · Page 2Linked to original sources

arXiv2002

Emission Mechanisms in High Energy Pulsars: From Gamma Rays to Infrared

We present a model for the gamma-ray emission of pulsars in terms of curvature radiation by highly relativistic particles. It is shown that the injection of power-law primary particles from an outer gap and subsequent cooling by curvature radiation losses reproduces the high energy spectrum and the luminosity of pulsars. As a result a spectral break is to be expected at a photon energy of $\sim 1\thinspace{\rm GeV}$. This value does not depend on the surface magnetic field strength or on the period of the neutron star, but only involves a geometric factor. The predicted change of the spectral index by 1 explains in particular the spectral shape of PSR B1706--44. We find that according to this model the luminosity of pulsars in the high energy band varies according to $L_γ\propto B_0/P^{5/3}$, which is in good agreement with the observations. A model for the infrared, optical and soft X-ray emission of pulsar is also presented. It is based on anisotropic synchrotron emission by relativistic particles in an outer gap scenario with a single energy distribution $N(γ)\propto γ^{-2}$. It is shown that this synchrotron model is able to reproduce the spectral shape between the infrared and soft X-rays and also the corresponding luminosities for the Crab pulsar. In particular, the long standing problem of the rapid spectral decline towards infrared frequencies is understandable as emission at very small pitch angles from low energy particles with $γ\ga 10$. It is also shown that the scaling of the synchrotron model explains the observed correlation between the X-ray luminosity and the spin-down luminosity of the neutron star $L_{\rm x} \sim 10^{-3} {\dot E}$.

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arXiv2001

Hard radio spectra from reconnection regions in galactic nuclei

Extremely flat and inverted radio spectra as observed in galactic nuclei and BL Lac sources are still a challenge for fast particle acceleration models. Continuous acceleration by electric fields in reconnection regions can result in almost constant particle distributions and thereby in inverted synchrotron spectra independent from the details of the injected spectrum. These spectra are calculated from a spatially averaged diffusion transport equation in momentum space that includes systematic momentum gains and losses as well as finite particle escape lifetimes.

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arXiv2000

Shear-Flow Driven Current Filamentation: Two-Dimensional Magnetohydrodynamic Simulations

The process of current filamentation in permanently externally driven, initially globally ideal plasmas is investigated by means of two-dimensional Magnetohydrodynamic (MHD)-simulations. This situation is typical for astrophysical systems like jets, the interstellar and intergalactic medium where the dynamics is dominated by external forces. Two different cases are studied. In one case, the system is ideal permanently and dissipative processes are excluded. In the second case, a system with a current density dependent resistivity is considered. This resistivity is switched on self-consistently in current filaments and allows for local dissipation due to magnetic reconnection. Thus one finds tearing of current filaments and, besides, merging of filaments due to coalescence instabilities. Energy input and dissipation finally balance each other and the system reaches a state of constant magnetic energy in time.

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arXiv2000

The critical velocity effect as a cause for the Hαemission from the Magellanic stream

Observations show significant Hα-emissions in the Galactic halo near the edges of cold gas clouds of the Magellanic Stream. The source for the ionization of the cold gas is still a widely open question. In our paper we discuss the critical velocity effect as a possible explanation for the observed Hα-emission. The critical velocity effect can yield a fast ionization of cold gas if this neutral gas passes through a magnetized plasma under suitable conditions. We show that for parameters that are typical for the Magellanic Stream the critical velocity effect has to be considered as a possible ionization source of high relevance.

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arXiv2000

Charge densities above pulsar polar caps

The ability of the neutron star surface to supply all or only part of the charges filling the pulsar magnetosphere is crucial for the physics prevailing within it, with direct consequences for the possible formation of pair creation regions. We evaluate the conditions for $e^-$ emission from pulsar surfaces for a simple Goldreich-Julian geometry taking both thermal and field emission processes into account. Using recently published estimates for the equation of state at the neutron star's surface, we show, that for a large range of $T_{\rm surf}, B$ and $P$, the liberated charges will fully screen the accelerating B-parallel electric field ${\rm E_\|}$. For surface temperatures $T_{\rm surf}<2\cdot 10^5 $K a balance between field emission of electrons and shielding of the field will occur. Even in the overidealised case of $T_{\rm surf}=0$ one can expect a prodigious supply of electrons which will weaken the accelerating ${\rm E_\|}$. We calculated the motion of electrons along selected polar field lines numerically for the low temperature, field emission scenario yielding their Lorentz factors as well as the produced radiation densities. Inverse Compton and later curvature losses are seen to balance the acceleration by the residual electric fields. We found that the conditions for magnetic pair production are not met anywhere along the field lines up to a height of 1500 pulsar radii. We did not {\it a priori} assume an "inner gap", and our calculations did not indicate the formation of one under realistic physical conditions without the introduction of further assumptions.

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arXiv2000

Synchrotron Model for the Infrared, Optical and X-Ray Emission of the Crab Pulsar

We develop a model for the infrared, optical and soft X-ray emission of the Crab pulsar in terms of anisotropic synchrotron emission by relativistic particles in an outer gap scenario with a single energy distribution. It is shown that such a distribution is naturally produced in an efficient pair cascade and that the energy of the primary particles are limited by synchrotron radiation. It is further shown that this synchrotron model is able to reproduce the spectral shape between the infrared and soft X-rays and also the corresponding luminosities. In particular, the long standing problem of the rapid spectral decline towards infrared frequencies is understandable as emission at very small pitch angles from low energy particles. Finally, we show that the scaling of our synchrotron model explains the observed correlation between the X-ray luminosity and the spin-down luminosity of the neutron star.

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arXiv2000

On the Physics of Primordial Magnetic Fields

There are at present more then 30 theories about the origin of cosmic magnetic fields at galactic and intergalactic scales. Most of them rely on concepts of elementary particle physics, like phase transitions in the early Universe, string theory and processes during the inflationary epoch. Here we present some more astrophysical arguments to provide some guidance through this large number and variety of models. Especially the fact that the evolution of magnetic fields depends on the spatial coherence scale of the fields leds to some interesting conclusions, which may rule out the majority of the theoretical scenarios. In principle one has to distinguish between the large-scale and small-scale magnetic fields. Large scale fields are defined as those as becoming sub-horizon at that redshift at which the mass energy density becomes equal to the photon energy density, which we name as equality. Small scale fields which are sub-horizon even before equality, i.e. with scales lower than (present) few Mpc cannot survive the radiation era and cannot reach recombination, because of the effects of magnetic diffusion and photon diffusion. Therefore mechanisms based on phase transitions become unlike, as they provide magnetic fields on scales smaller than the horizon. Thus, the observed galactic and intergalactic fields, which are small scales in our terminology must be created after recombination by normal plasma processes during the protogalactic evolution. The large scale fields instead were produced during inflation and may have noticeable implications for the formation of the large scale structure of the Universe. The inclusion of large scale magnetic fields may improve present Cold Dark Matter theories of structure formation.

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arXiv1999

Stimulated Raman scattering of water maser lines in astrophysical plasmas

Radiative transfer equations are derived and solved for the stimulated Raman scattering of water maser lines in the astrophysical plasmas with electron density of about 10^6 - 10^7 cm-3. In stimulated Raman scattering, the energy of water maser line is transferred to the side band modes: Stokes mode and anti-Stokes mode. The Stokes mode is easily produced by backward Raman scattering while the anti-Stokes mode is created by the interacting intersecting masers in the plasma. The intensity of the Stokes mode is higher than that of the anti-Stokes mode. These side band modes are proposed as explanation for the extreme velocity features observed in the galaxy NGC 4258. The threshold value of the brightness temperature for the Raman scattering is about 10^16 - 10^19 K, and it is satisfied in the case of NGC 4258.

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arXiv1999

Propagation effect on polarization of pulsar radio emission

We consider the role of a propagation effect such as the stimulated Raman scattering on the polarization of radio pulses. When an intense electromagnetic wave with frequency close to the plasma frequency interacts with the plasma in the pulsar magnetosphere, the incident wave undergoes stimulated Raman scattering. Using typical plasma and magnetic field parameters, we compute the growth rate and estimate the polarization properties of the scattered mode. At some conditions, we find that the polarization properties of the scattered mode can become significantly different from those of the incident wave. The frequencies, at which strong Raman scattering occurs in the outer parts of magnetosphere, fall into the observed radio band. In some pulsars, for example, PSR B0628-28 and 1914+13, a large and symmetric type of circular polarization has been observed. We propose that such an unusual circular polarization is produced by the propagation effects.

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arXiv1998

Relativistic electron beams in IDV blazars

The observed variability of BL Lac objects and Quasars on timescales <1 day (intraday variability, IDV) have revealed radio brightness temperatures up to 10^{16}-10^{20} K. These values challenge the beaming model with isotropic comoving radio emission beyond its limits, requiring bulk relativistic motion with Lorentz factors >100. We argue in favor of a model where an anisotropic distribution of relativistic electrons streams out along the field lines. When this relativistic beam is scattered in pitch angle and/or hits a magnetic field with components perpendicular to the beam velocity it starts to emit synchrotron radiation and redistribute in momentum space. The propagation of relativistic electrons with Lorentz factor gamma=10^2-10^4 reduces the intrinsic variability timescale by a factor gamma^{-1}, so that the intrinsic brightness temperature is reduced by a factor of order gamma^{-2}, easily below the Inverse Compton limit of 10^{12} K. When looking at a single event we expect the variability time scales to be independent of frequency for a monoenergetic electron beam. The production of variable X- and gamma-ray flux is briefly discussed.

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arXiv1998

Magnetic Reconnection and Particle Acceleration in Active Galactic Nuclei

Magnetic field-aligned electric fields are characteristic features of magnetic reconnection processes operating in externally agitated magnetized plasmas. An especially interesting environment for such a process are the coronae of accretion disks in active galactic nuclei (AGN). There, Keplerian shear flows perturb the quite strong disk magnetic field leading to intense current sheets. It was previously shown that given field strengths of 200 G in a shear flow, reconnection driven magnetic field aligned electric fields can accelerate electrons up to Lorentz factors of about 2000 in those objects thus providing us with a possible solution of the injection (pre-acceleration) problem. However, whereas in the framework of magnetohydrodynamics the formation of the field-aligned electric fields can be described consistently, the question has to be addressed whether the charged particles can really be accelerated up to the maximum energy supplied by the field-aligned electric potentials, since the accelerated particles undergo energy losses either by synchrotron or inverse Compton mechanisms. We present relativistic particle simulations starting from electric and magnetic fields obtained from magnetohydrodynamic simulations of magnetic reconnection in an idealized AGN configuration including nonthermal radiative losses. The numerical results prove that the relativistic electrons can be effectively accelerated even in the presence of an intense radiation bath. Energies from 50 MeV up to 40 GeV can be reached easily, depending on the energy density of the photon bath.

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arXiv1998

Magnetic fields and rotation of spiral galaxies

We present a simplified model in which we suggest that two important galactic problems -the magnetic field configuration at large scales and the flat rotation curve- may be simultaneously explained. A highly convective disc produces a high turbulent magnetic diffusion in the vertical direction, stablishing a merging of extragalactic and galactic magnetic fields. The outer disc may then adquire a magnetic energy gradient very close to the gradient required to explain the rotation curve, without the hypothesis of galactic dark matter. Our model predicts symmetries of the galactic field in noticeable agreement with the large scale structure of our galaxy.

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arXiv1998

Global three-dimensional simulations of magnetic field evolution in a galactic disk. II. Gas rich galaxies

A fully three-dimensional computation of induction processes in a disk is performed to investigate the evolution of a large-scale magnetic field in gas-rich barred and spiral galaxies in the presence of field diffusion. As input parameters we use time dependent velocity fields obtained from self-consistent 3D N-body calculations of galactic dynamics. Our present work primarily analyzes the influence of the gas flows in non-axisymmetric gravitational disturbances (like spiral arms and bars) on the behavior of the magnetic field. The magnetic field is found to evolve towards structures resembling dynamical spiral arms and bars, however its distribution is often much more complicated than the velocity field itself. We show that a random configuration of seed magnetic fields quickly dissipates, and is therefore unable to explain the magnetic field strength observed in nearby galaxies. The detailed comparison between the simulated velocity and magnetic fields allows us to establish that the reversals of magnetic vectors appear in the vicinity of the corotation radius of non-axisymmetric disk structures, where the velocity field shows rather abrupt changes. We argue that this explains the observed field reversals in our Galaxy.

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arXiv1998

On the Possibility of Curvature Radiation from Radio Pulsars

We consider the widespread hypothesis that coherent curvature radiation is responsible for the radio emission of pulsars. The comparison of energy conservation and the published data and luminosities explicitely proves that coherent curvature radiation cannot be the source for the radio emission of pulsars for frequencies below a few GHz. At higher frequencies coherent curvature radiation can be ruled out because neither the observationally deduced emission heights nor the observed radius to frequency mapping can be reproduced by this mechanism. Our argumentation is in accordance with the more general critics (e.g. Melrose 1992) that no adequate bunching mechanism has been identified for coherent curvature radiation. We present 5 examples (0329+29, 0355+54, 0540+23, 1133+16, 1916+10) of pulsars whose high frequency (larger than 1.4. GHz, up to 32 GHz) luminosities are well known, and as a low frequency example the faintest radio pulsar 0655+64 of the Taylor et al (1993) sample.

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arXiv1997

The galactic dynamo effect due to Parker-shearing instability of magnetic flux tubes. III. The fast dynamo model

We present a new fast dynamo model for galactic magnetic fields, which is based on the Parker-shearing instability and magnetic reconnection, in the spirit of the model proposed by Parker (1992). We introduce a new scenario of flux tube interactions and estimate the dynamo transport coefficient basing on simple geometrical arguments. The obtained expressions are equivalent to the formally derived helicity $α_d$ and diffusivity $η_d$ in the first paper of this series. The model we propose predicts that the $α$-effect in galactic discs has opposite sign with respect to that resulting directly from the sign of the Coriolis force. We estimate the rate of magnetic heating due to the reconnection of magnetic flux tubes, which plays an important role in our dynamo model. The corresponding luminosities of the diffuse X-ray emission are consistent with the ROSAT observations of nearby galaxies. The present considerations synthesize the ideas of Parker with our own results presented in the preceding papers (Hanasz & Lesch 1993, 1997; Hanasz 1997).

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arXiv1997

Global three-dimensional simulations of magnetic field evolution in a galactic disk

The evolution of three-dimensional, large-scale, magnetic fields, in a galactic disk is investigated numerically. The N-body simulations of galactic dynamics are incorporated into the kinematic calculations of induction equations to study the influence of non-axisymmetric gas flows on the galactic magnetic field. The time-dependent gas velocity fields are introduced as input parameters for the MHD-simulations. Our principal concern is to check how dynamical evolution of the galactic gas affects the global magnetic field structure and intensity. We have found that the magnetic field responds sensitively to changes in the gas velocity field, and even slight variations of the dynamical parameters, such as the gas mass/total mass ratio results in nonuniform intensity structures, i.e. magnetic arms. The magnetic lines of force are well aligned with spiral arms and bar due to compressional flows in these features. In the inter-arm regions the areas with magnetic vectors going opposite to the main magnetic spirals are present.

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arXiv1997

Localized magnetic reconnection as a cause of extraplanar diffuse ionised in the halo

Many observations indicate the occurrence of ionised gas in the distant halos of galaxies (including our own). Since photoionisation by stars (mainly O stars, young or evolved low-mass stars depending on the kind of galaxy) does not seem to be exclusively responsible for the ionisation of the hydrogen filaments that should otherwise cool fast and recombine quickly, the question arises which extra energy source can produce the quasistationary ionisation. We show that stationary localized magnetic reconnection in current filaments may contribute to the ionisation of the extraplanar halo gas. In these filaments magnetic energy is dissipated. Consequently, the ionised as well as the neutral component are heated and re-ionised on a time scale significantly shorter than the recombination time scale. The amount of energy required for efficient re-ionisation can in principle easily be provided by the free magnetic energy. We present quasi-static models that are characterized by plasma temperatures and densities that agree well with the observed values for the diffuse ionised gas component of the interstellar medium. Plasma-neutral gas fluid simulations are made to show that the recombination induced dynamical reconnection process indeed works in a self-regulatory way.

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arXiv1997

The galactic dynamo effect due to Parker-shearing instability of magnetic flux tubes. I. General formalism and the linear approximation

In this paper we investigate the idea of Hanasz & Lesch 1993 that the galactic dynamo effect is due to the Parker instability of magnetic flux tubes. In addition to the former approach, we take into account more general physical conditions in this paper, by incorporating cosmic rays and differential forces due to the axisymmetric differential rotation and the density waves as well. We present the theory of slender magnetic flux tube dynamics in the thin flux tube approximation and the Lagrange description. This is the application of the formalism obtained for solar magnetic flux tubes by Spruit (1981), to the galactic conditions. We perform a linear stability analysis for the Parker-shearing instability of magnetic flux tubes in galactic discs and then calculate the dynamo coefficients. We present a number of new effects which are very essential for cosmological and contemporary evolution of galactic magnetic fields. First of all we demonstrate that a very strong dynamo $α$-effect is possible in the limit of weak magnetic fields in presence of cosmic rays. Second, we show that the differential force resulting from axisymmetric differential rotation and the linear density waves causes that the $α$-effect is essentially magnified in galactic arms and switched off in the interarm regions. Moreover, we predict a non-uniform magnetic field in spiral arms and well aligned one in interarm regions. These properties are well confirmed by recent observational results by Beck & Hoernes (1996)

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