arXiv subjects

Jorg P. Rachen

Publications and source records attributed to Jorg P. Rachen.

9 recordsLinked to original sources

arXiv2019

A physical approach to modelling large-scale galactic magnetic fields

A convenient representation of the structure of the large-scale galactic magnetic field is required for the interpretation of polarization data in the sub-mm and radio ranges, in both the Milky Way and external galaxies. We develop a simple and flexible approach to construct parametrised models of the large-scale magnetic field of the Milky Way and other disc galaxies, based on physically justifiable models of magnetic field structure. The resulting models are designed to be optimised against available observational data. Representations for the large-scale magnetic fields in the flared disc and spherical halo of a disc galaxy were obtained in the form of series expansions whose coefficients can be calculated from observable or theoretically known galactic properties. The functional basis for the expansions is derived as eigenfunctions of the mean-field dynamo equation or of the vectorial magnetic diffusion equation. The solutions presented are axially symmetric but the approach can be extended straightforwardly to non-axisymmetric cases. The magnetic fields are solenoidal by construction, can be helical, and are parametrised in terms of observable properties of the host object, such as the rotation curve and the shape of the gaseous disc. The magnetic field in the disc can have a prescribed number of field reversals at any specified radii. Both the disc and halo magnetic fields can separately have either dipolar or quadrupolar symmetry. The model is implemented as a publicly available software package GalMag which allows, in particular, the computation of the synchrotron emission and Faraday rotation produced by the model's magnetic field. The model can be used in interpretations of observations of magnetic fields in the Milky Way and other spiral galaxies, in particular as a prior in Bayesian analyses. (Abridged.)

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arXiv2018

IMAGINE: A comprehensive view of the interstellar medium, Galactic magnetic fields and cosmic rays

In this white paper we introduce the IMAGINE Consortium and its scientific background, goals and structure. Our purpose is to coordinate and facilitate the efforts of a diverse group of researchers in the broad areas of the interstellar medium, Galactic magnetic fields and cosmic rays, and our goal is to develop more comprehensive insights into the structures and roles of interstellar magnetic fields and their interactions with cosmic rays. To achieve a higher level of self-consistency, depth and rigour can only be achieved by the coordinated efforts of experts in diverse areas of astrophysics involved in observational, theoretical and numerical work. We present our view of the present status of this topic, identify its key unsolved problems and suggest a strategy that will underpin our work. The backbone of the consortium is the Interstellar MAGnetic field INference Engine, a publicly available Bayesian platform that employs robust statistical methods to explore the multi-dimensional likelihood space using any number of modular inputs. It provides an interpretation and modelling framework that has the power and flexibility to include a variety of observational, theoretical and numerical lines of evidence into a self-consistent and comprehensive picture of the thermal and non-thermal interstellar media. An important innovation is that a consistent understanding of the phenomena that are directly or indirectly influenced by the Galactic magnetic field, such as the deflection of ultra-high energy cosmic rays or extragalactic backgrounds, is made an integral part of the modelling. The IMAGINE Consortium, which is informal by nature and open to new participants, hereby presents a methodological framework for the modelling and understanding of Galactic magnetic fields that is available to all communities whose research relies on a state-of-the-art solution to this problem. (Abridged.)

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arXiv2008

Ultra-high energy cosmic rays from radio galaxies revisited

A striking concentration of ultra-high energy cosmic ray (UHECR) events observed by the Pierre Auger Observatory around the direction of the nearby radio galaxy Centaurus A revives the idea that radio galaxies may be dominant sources of UHECR. In this paper, we give a brief overview about processes which may accelerate protons and nuclei in radio galaxies, and their relation to jet power, radio morphology and cosmic source density. We argue that, except for the most powerful FR-II radio galaxies, processes in radio lobes are unlikely to explain the origin of UHECR. However, Fermi acceleration of protons at internal shocks in the "blazarzone" of all radio galaxies, and their photohadronic conversion into neutrons, may lead to the ejection of "UHECR-beams", which remain collimated over several Mpc. Consequences of this hypothesis for the interpretation of the UHECR event distribution, in particular for the special case of Centaurus A, are discussed.

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arXiv2000

Hadronic Correlated Flares from Mrk 501

We discuss the possibility of explaining the extraordinary, correlated X-ray/TeV flares observed during April 1997 from Mrk 501, by synchrotron-pair cascades injected by synchrotron radiation from ultra-high energy protons and muons. Evaluating the jet conditions required to explain the observed features of the flares, the allowed region for this model in the parameter space of jet magnetic field and Doppler factor is identified, and compared with the parameter choices of other, both hadronic and leptonic models presented in the literature. The present model requires magnetic fields similar to other hadronic models for gamma-ray blazars (B >~ 30 G), but a lower Doppler factor (D =~ 3).

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arXiv2000

Hadronic blazar models and correlated X-ray/TeV flares

The hypothesis that AGN jets might be the sources of the ultra-high energy cosmic rays has originally motivated the venture of TeV gamma ray astronomy. Surprisingly, after the discovery of TeV emission from blazars the attention has shifted to more traditional explanations which do not involve energetic hadrons, and there is even common believe that a hadronic interpretation is disfavored by observations. It is shown here that this is not the case, and that the currently observed spectra and variability features of blazars can be perfectly understood within hadronic blazar models. I also discuss how hadronic models might be observationally distinguished from common leptonic models, and point out some interesting aspects which could be relevant for the understanding of the differences between blazar classes.

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arXiv1999

The relation of extragalactic cosmic ray and neutrino fluxes: the logic of the upper bound debate

In a recent paper, Waxman and Bahcall (PRD 59, 023002) claimed that the present data on ultra-high energy cosmic rays imply a model-independent upper bound on extragalactic neutrino fluxes of 2*10^-8 GeV cm^-2 s^-1 ster^-1, independent of neutrino energy. Mannheim, Protheroe and Rachen (astro-ph/9812398) have repeated this calculation and confirmed the WB-bound, within a factor of 2, only at a very limited energy range of E_ν\sim 10^(16-18) eV, while at other energies the neutrino flux is mainly limited by the extragalactic gamma ray background to a level about two orders of magnitude higher than the WB bound. In this paper we present a simple, (almost) no-math approach to the problem, and discuss under which astrophysical assumptions the WB-bound and the MPR-bound, respectively, apply. Then we discuss to which respect these assumptions apply to presently discussed models of extragalactic neutrino production. We note that, averaged over the observed luminosity function, blazars are sufficiently opaque to ultra-high energy neutrons that there is no conflict of the predicted neutrino fluxes with the cosmic ray data, and that these models are rather constrained by their contributions to the extragalactic gamma ray background. At present, no modifications are implied to the predicted neutrino events from these models in active or planned neutrino detectors.

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arXiv1998

Cosmic Rays and Neutrinos from Gamma Ray Bursts

We review the hypothesis that the acceleration of protons at internal shocks in Gamma Ray Bursts (GRB) could be the origin of the ultra-high energy cosmic rays (UHECR) observed at earth, E_max > 10^19 eV. We find that, even though protons may be accelerated to such energies, their ejection into the interstellar/intergalactic medium is problematic because it is likely to be accompanied by considerable adiabatic losses in the expanding shell. The problem is circumvented by neutrons produced in photohadronic interactions, which are not magnetically bound and thus effectively ejected in the moment of their production. They can be both produced in sufficient number and be able to leave the emission region if the optical depth of the emission region to photohadronic interactions is of order 1. We show that this requirement can be fulfilled under the same conditions which allow acceleration of protons to the highest energies. The production of neutrinos in this process correlates the fluxes of cosmic rays and neutrinos, and makes the hypothesis of UHECR origin in GRBs testable.

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arXiv1998

Photohadronic Neutrinos from Transients in Astrophysical Sources

We investigate the spectrum of photohadronically produced neutrinos at very high energies (VHE, >10^14 eV) in astrophysical sources whose physical properties are constrained by their variability, in particular jets in Active Galactic Nuclei (blazars) and Gamma-Ray Bursts (GRBs). We discuss in detail the various competing cooling processes for energetic protons, as well as the cooling of pions and muons in the hadronic cascade, which impose limits on both the efficiency of neutrino production and the maximum neutrino energy. If the proton acceleration process is of the Fermi type, we can derive a model independent upper limit on the neutrino energy from the observed properties of any cosmic transient, which depends only on the assumed total energy of the transient. For standard energetic constraints, we can rule out major contributions above 10^19 eV from current models of both blazars and GRBs; and in most models much stronger limits apply in order to produce measurable neutrino fluxes. For GRBs, we show that the cooling of pions and muons in the hadronic cascade imposes the strongest limit on the neutrino energy, leading to cutoff energies of the electron and muon neutrino spectrum at the source differing by about one order of magnitude. We also discuss the relation of maximum cosmic ray energies to maximum neutrino energies and fluxes in GRBs, and find that the production of both the highest energy cosmic rays and observable neutrino fluxes at the same site can only be realized under extreme conditions; a test implication of this joint scenario would be the existence of strong fluxes of GRB correlated muon neutrinos up to ultra high energies, >10^17 eV. Secondary particle cooling also leads to slightly revised estimates for the neutrino fluxes from (non-transient) AGN cores.

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arXiv1997

Propagation of UHE cosmic rays in a structured universe

In a gravitationally unstable universe, the structure of dark matter and galaxies, intergalactic gas and magnetic field can have severe impact on the propagation of ultra high energy cosmic rays (UHECR) (Ryu & Kang, this proceedings). The possible effects include spatial confinement and directional focusing along the supergalactic matter sheets, as well as universal re-acceleration at large scale shock fronts, and spectral modification due to energy dependent leakage into cosmic voids. As a result, the GZK-cutoff may be less pronounced and occur at a higher energy, where the stochastic nature of both acceleration and energy loss processes has to be taken into account.

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