arXiv subjects

R. Schlickeiser

Publications and source records attributed to R. Schlickeiser.

At least 37 records · Page 2Linked to original sources

arXiv2004

The impact of thermal gas in AGN jets on the low-frequency emission

We study the effect of non-relativistic, thermal matter in the jets of active galaxies (AGN) on the low-frequency non-thermal emission and the variability thereof. In matter-dominated jets, sizable quantities of gas should exist, in particular in the compression zones near the collision fronts that are an implicit ingredient of Fermi-type particle acceleration scenarios. Non-relativistic thermal gas in AGN jets noticably contributes to the optical depth at radio to infrared frequencies, and much less to the emission, with an efficiency that is strongly temperature-dependent. The observable flux of low-frequency emission is thus modulated by the temperature evolution of the thermal gas, and it can therefore display very complicated variability. For a particular particle energisation scenario we calculate the temperature evolution of the thermal plasma as well as the radiation transport of low-frequency emission, and thus derive simulated light curves at different frequencies and their typical correlation properties.

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arXiv2004

The Coma Cluster at gamma-ray energies: multifrequency constraints

The Coma cluster exhibits evidence for a high-energetic non-thermal particle population. At frequencies >1 GHz recent radio halo observations confirm a significant spectral steepening of the volume-integrated emission. We calculate the volume-averaged high-energy spectrum due to inverse Compton scattering off the CMB radiation field and non-thermal bremsstrahlung according to an exponential cutoff in the electron spectrum as deduced from the radio observations. The synchrotron radiation from secondary pairs, created from the decay of charged mesons produced in hadronic pp-interactions, is found to set significant constraints on the energy content of relativistic hadrons in Coma. This limits the maximum flux at high energies. Our findings support a low ratio of relativistic hadron to thermal energy density. Predictions for Coma's high energy emission are discussed in the light of current and expected abilities of upcoming gamma-ray instruments.

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arXiv2002

Kinetic theory of interstellar density and magnetic field fluctuations. Relations between interstellar density and magnetic field fluctuations

Using linear kinetic plasma theory the relation between electron density and magnetic field fluctuations for low-frequency plasma waves for Maxwellian background distribution functions of arbitrary temperatures in an uniform magnetic field is derived. By taking the non-relativistic temperature limit this relation is calculated for the diffuse intercloud medium in our Galaxy. The diffuse intercloud medium is the dominant phase of the interstellar medium with respect to radio wave propagation, dispersion and rotation measure studies. The differences between the relation of electron density and magnetic field fluctuations from the linear kinetic theory as compared to the classical MHD theory are established and discussed.

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arXiv2002

Conversion of relativistic pair energy into radiation in the jets of active galactic nuclei

It is generally accepted that relativistic jet outflows power the nonthermal emission from active galactic nuclei (AGN). The composition of these jets -- leptonic versus hadronic -- is still under debate. We investigate the microphysical details of the conversion process of the kinetic energy in collimated relativistic pair outflows into radiation through interactions with the ambient interstellar medium. Viewed from the coordinate system comoving with the pair outflow, the interstellar protons and electrons represent a proton-electron beam propagating with relativistic speed in the pair plasma. We demonstrate that the beam excites both electrostatic and low-frequency magnetohydrodynamic Alfven-type waves via a two-stream instability in the pair background plasma, and we calculate the time evolution of the distribution functions of the beam particles and the generated plasma wave turbulence power spectra. For standard AGN jet outflow and environment parameters we show that the initial beam distributions of interstellar protons and electrons quickly relax to plateau-distributions in parallel momentum, transferring thereby one-half of the initial energy density of the beam particles to electric field fluctuations of the generated electrostatic turbulence. On considerably longer time scales, the plateaued interstellar electrons and protons will isotropise by their self-generated transverse turbulence and thus be picked-up in the outflow pair plasma. These longer time scales are also characteristic for the development of transverse hydromagnetic turbulence from the plateaued electrons and protons. This hydromagnetic turbulence upstream and downstream is crucial for diffusive shock acceleration to operate at external or internal shocks associated with pair outflows.

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arXiv2002

Transformation Properties of External Radiation Fields, Energy-Loss Rates and Scattered Spectra, and a Model for Blazar Variability

We treat transformation properties of external radiation fields in the proper frame of a plasma moving with constant speed. The specific spectral energy densities of external isotropic and accretion-disk radiation fields are derived in the comoving frame of relativistic outflows, such as those thought to be found near black-hole jet and gamma-ray burst sources. Nonthermal electrons and positrons Compton-scatter this radiation field, and high-energy protons and ions interact with this field through photomeson and photopair production. We revisit the problem of the Compton-scattered spectrum associated with an external accretion-disk radiation field, and clarify a past treatment by the authors. Simple expressions for energy-loss rates and Thomson-scattered spectra are given for ambient soft photon fields consisting either of a surrounding external isotropic monochromatic radiation field, or of an azimuthally symmetric, geometrically thin accretion-disk radiation field. A model for blazar emission is presented that displays a characteristic spectral and variability behavior due to the presence of a direct accretion-disk component. The disk component and distinct flaring behavior can be bright enough to be detected from flat spectrum radio quasars with {\it GLAST}. Spectral states of blazars are characterized by the relative importance of the accretion-disk and scattered radiation fields and, in the extended jet, by the accretion disk, inner jet, and cosmic microwave background radiation fields.

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arXiv2001

Neutrinos from active galactic nuclei as a diagnostic tool

Active galactic nuclei (AGN) are known as sources of high energy gamma-rays. The emission probably results from non-thermal radiation of relativistic jets belonging to the AGN. Earlier investigations of these processes have suggested that neutrinos are among the radiation products of the jets and may be used to discriminate between hadrons and leptons as primary particles for the production of the high energy emission. It is shown that the neutrino flux is correlated with the flux of TeV gamma-rays. This allows to distinctly search for neutrino emissions from the jets of AGN by using the TeV gamma-ray light curves to drastically reduce the temporal and spatial parameter space. Given the observed TeV photon fluxes from nearby BL Lacs the neutrino signal from AGN may be detectable with future neutrino observatories as least as sensitive as IceCube.

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arXiv2001

Channeled blast wave behavior based on longitudinal instabilities

To address the important issue of how kinetic energy of collimated blast waves is converted into radiation, Pohl & Schlickeiser (2000) have recently investigated the relativistic two-stream instability of electromagnetic turbulence. They have shown that swept-up matter is quickly isotropized in the blast wave, which provides relativistic particles and, as a result, radiation. Here we present new calculations for the electrostatic instability in such systems. It is shown that the electrostatic instability is faster than the electromagnetic instability for highly relativistic beams. However, even after relaxation of the beam via the faster electrostatic turbulence, the beam is still unstable with respect to the electromagnetic waves, thus providing the isotropization required for efficient production of radiation. While the emission spectra in the model of Pohl and Schlickeiser have to be modified, the basic characteristics persist.

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arXiv2001

Radiation from collision-dominated relativistic pair fireballs

It is generally accepted that gamma-ray bursts (GRBs) are initiated by a relativistic pair fireball, converting its internal energy into kinetic energy of a relativistically moving plasmoid and subsequently into radiation. Here, we investigate the early stages of this evolution, after the pair fireball has become optically thin to gamma-gamma pair production. We show that for a short period of time, ~ 0.1 - a few seconds after the initial explosion, the pair plasmoid evolution might be dominated by collisional processes prior to the formation of a collisionless shock. We simulate these processes during the early pair plasmoid evolution and calculate the expected radiative signatures. We show that the radiation from the collision-dominated pair plasmoid phase results in a short (~ a few ms) flash of thermal soft X-ray emission, followed by a transition phase of < 1 s during which the fireball turns Thomson thin, but its radiation remains dominated by thermal Comptonization, peaking at around E_pk ~ 100 MeV - a few GeV. While the very early thermal emission could be associated with the quasi-thermal radiation signatures found in the very early phases of several bright BATSE GRBs, the predicted subsequent flash of high-energy emission should be easily detectable with the GLAST satellite.

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arXiv2001

Excess GeV radiation and cosmic ray origin

Recent EGRET observations of the diffuse Galactic gamma-ray emission reveal a spectrum which is incompatible with the assumption that the cosmic ray spectra measured locally hold throughout the Galaxy: the spectrum above 1 GeV, where the emission is supposedly dominated by pi^0-decay, is harder than that derived from the local cosmic ray proton spectrum. We demonstrate that in case of a SNR origin of cosmic ray nucleons part of this gamma-ray excess may be attributed to the dispersion of the spectral indices in these objects. In global averages, as are gamma-ray line-of-sight integrals, this dispersion leads to a positive curvature in the composite spectrum, and hence to modified pi^0-decay gamma-ray spectra.

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arXiv2000

Proton and Electron Acceleration through Magnetic Turbulence in Relativistic Outflows

Low frequency electromagnetic turbulence is generated in relativistically outflowing plasma that sweeps up particles from the surrounding environment. Electrons are energized by stochastic gyroresonant acceleration with the turbulence produced by the isotropization of captured protons and charged dust. Protons are accelerated stochastically by the dust-induced turbulence. Analytical solutions for the proton and electron energy distributions are obtained and used to calculate broadband synchrotron emission. The solutions are compared with generic spectral behavior of blazars and gamma-ray bursts. Dust captured by a blast wave can generate turbulence that could accelerate protons to very high energies.

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arXiv1999

Cosmic-Ray Momentum Diffusion In Magnetosonic Versus Alfvenic Turbulent Field

Energetic particle transport in a finite amplitude magnetosonic and Alfvenic turbulence is considered using Monte Carlo particle simulations, which involve an integration of particle equation of motion. We show that in a low-Betha plasma cosmic ray can be the most important damping process for magnetosonic waves. Assuming such conditions we derive the momentum diffusion coefficient for relativistic particles in the presence of anisotropic finite-amplitude turbulent wave field, for flat and Kolmogorov-type turbulence spectra. We confirm the possibility of larger values of a momentum diffusion coefficient occuring due to transit-time damping resonance interaction in the presence of isotropic fast-mode waves in comparison to the Alfven waves of the same amplitude.

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arXiv1998

Transrelativistic pair plasmas in AGN jets

Models of relativistic jets filled with ultrarelativistic pair plasma are very successful in explaining the broadband radiation of $γ$-ray blazars. Assuming that the initial injection and cooling of ultrarelativistic pair plasma in an AGN jet has occurred, producing the observed high-energy $γ$-ray radiation, we investigate the further evolution of the pair plasma as it continues to move out from the central engine. The effects of thermalization and reacceleration, the emission of pair bremsstrahlung and annihilation radiation and the bulk Compton process, and the possible application to MeV blazars are discussed. A model calculation to the special case of PKS~0208-512 is presented.

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arXiv1997

Cosmic Ray Momentum Diffusion in Magnetosonic versus Alfvenic Turbulent Fields

The acceleration of energetic particle transport in high amplitude magnetosonic and Alfvenic turbulence is considered using the method of Monte Carlo particle simulations, involving integration of particle equations of motion. We derive the momentum diffusion coefficient in the presence of anisotropic turbulent wave in the high-magnetized plasma, for a flat and Kolmogorov-type turbulence spectrum. We confirm the quasilinear result (Shlickeiser & Miller (1997) of enhanced values of the momentum diffusion coefficient due to transit-time resonance interaction in the presence of isotropic fast-mode waves as compared to the case of slab Alfvenic waves of the same amplitude. The relation of the momentum diffusion coefficient to the turbulence amplitude and anisotropy is investigated.

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arXiv1997

Spectral modeling of gamma-ray blazars

We present model calculations reproducing broadband spectra of $γ$-ray blazars by a relativistic leptonic jet, combining the EIC and the SSC model. To this end, the evolution of the particle distribution functions inside a relativistic pair jet and of the resulting photon spectra is investigated. Inverse-Compton scattering of both external (EIC) as well as synchrotron photons (SSC) is treated using the full Klein-Nishina cross-section and the full angle-dependence of the external photon source. We present model fits to the broadband spectra of Mrk~421 and 3C279 and the X-ray and $γ$-ray spectrum of PKS~1622-297. We find that the most plausible way to explain both the quiescent and the flaring states of these objects consists of a model where EIC and SSC dominate the observed spectrum in different frequency bands. For both Mrk~421 and 3C279 the flaring states can be reproduced by a harder spectrum of the injected pairs.

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arXiv1997

The Pair Production Spectrum from Photon-Photon Annihilation

We present the first completely analytical computation of the full differential γ-γpair production rate from compact radiation fields, exact to 2nd order QED, and use this result to investigate the validity of previously known approximations.

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arXiv1997

Gamma-ray emission and spectral evolution of pair plasmas in AGN jets

The evolution of the particle distribution functions inside a relativistic jet containing an e^-e^+ pair plasma and of the resulting $γ$-ray and X-ray spectra is investigated. The first phase of this evolution is governed by heavy radiative energy losses. For this phase, approximative expressions for the energy-loss rates due to inverse-Compton scattering, using the full Klein-Nishina cross section, are given as one-dimensional integrals for both cooling by inverse-Compton scattering of synchrotron photons (SSC) and of accretion disk photons (EIC). We calculate instantaneous and time-integrated $γ$-ray spectra emitted by such a jet for various sets of parameters, deducing some general implications on the observable broadband radiation. Finally, we present model fits to the broadband spectrum of Mrk 421. We find that the most plausible way to explain both the quiescent and the flaring state of Mrk 421 consists of a model where EIC and SSC dominate the observed spectrum in different frequency bands. In our model the flaring state is primarily related to an increase of the maximum Lorentz factor of the injected pairs.

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arXiv1997

The Beaming Pattern of Doppler Boosted Thermal Annihilation Radiation: Application to MeV Blazars

The beaming pattern of thermal annihilation radiation is broader than the beaming pattern produced by isotropic nonthermal electrons and positrons in the jets of radio-emitting active galactic nuclei which Compton scatter photons from an external isotropic radiation field. Thus blueshifted thermal annihilation radiation can provide the dominant contribution to the high-energy radiation spectrum at observing angles theta > 1/Gamma, where Gamma is the bulk Lorentz factor of the outflowing plasma. This effect may account for the spectral features of MeV blazars discovered with the Compton Telescope on the Compton Gamma Ray Observatory. Coordinated gamma-ray observations of annihilation line radiation to infer Doppler factors and VLBI radio observations to measure transverse angular speeds of outflowing plasma blobs can be used to determine the Hubble constant.

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arXiv1996

Cosmic ray diffusive acceleration at shock waves with finite upstream and downstream escape boundaries

In the present paper we discuss the modifications introduced into the first-order Fermi shock acceleration process due to a finite extent of diffusive regions near the shock or due to boundary conditions leading to an increased particle escape upstream and/or downstream the shock. In the considered simple example of the planar shock wave we idealize the escape phenomenon by imposing a particle escape boundary at some distance from the shock. Presence of such a boundary (or boundaries) leads to coupled steepening of the accelerated particle spectrum and decreasing of the acceleration time scale. It allows for a semi-quantitative evaluation and, in some specific cases, also for modelling of the observed steep particle spectra as a result of the first-order Fermi shock acceleration. We also note that the particles close to the upper energy cut-off are younger than the estimate based on the respective acceleration time scale. In Appendix A we present a new time-dependent solution for infinite diffusive regions near the shock allowing for different constant diffusion coefficients upstream and downstream the shock.

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