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Register a new userCosmic-Ray Propagation in Light of Recent Observation of Geminga
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Added by
Gudlaugur J\\'ohannesson
Publication date
2019
fields
Physics
and research's language is
English
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The High Altitude Water Cherenkov (HAWC) telescope recently observed extended emission around the Geminga and PSR~B0656+14 pulsar wind nebulae (PWNe). These observations have been used to estimate cosmic-ray (CR) diffusion coefficients near the PWNe that appear to be more than two orders of magnitude smaller than that typically derived for the interstellar medium from the measured abundances of secondary species in CRs. Two-zone diffusion models have been proposed as a solution to this discrepancy, where the slower diffusion zone (SDZ) is confined to a small region around the PWN. Such models are shown to successfully reproduce the HAWC observations of the Geminga PWN while retaining consistency with other CR data. It is found that the size of the SDZ influences the predicted positron flux and the spectral shape of the extended $gamma$-ray emission at lower energies that can be observed with the {it Fermi} Large Area Telescope ({it Fermi} LAT). If the two observed PWNe are not unique, then it is likely that there are similar pockets of slow diffusion around many CR sources elsewhere in the Milky Way. The consequences of such picture for Galactic CR propagation is explored.
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Information on cosmic-ray (CR) composition comes from direct CR measurements while their distribution in the Galaxy is evaluated from observations of their associated diffuse emission in the range from radio to gamma rays. Even though the main interaction processes are identified, more and more precise observations provide an opportunity to study more subtle effects and pose a challenge to the propagation models. GALPROP is a sophisticated CR propagation code that is being developed for about 20 years. It provides a unified framework for interpretations of data from many different types of experiments. It is used for a description of direct CR measurements and associated interstellar emissions (radio to gamma rays), thereby providing important information about CR injection and propagation in the interstellar medium. By accounting for all relevant observables at a time, the GALPROP code brings together theoretical predictions, interpretation of the most recent observations, and helps to reveal the signatures of new phenomena. In this paper we review latest applications of GALPROP and address ongoing and near future improvements. We are discussing effects of different propagation models, and of the transition from cylindrically symmetrical models to a proper 3D description of the components of the interstellar medium and the source distribution.
We review numerical methods for simulations of cosmic ray (CR) propagation on galactic and larger scales. We present the development of algorithms designed for phenomenological and self-consistent models of CR propagation in kinetic description based on numerical solutions of the Fokker-Planck equation. The phenomenological models assume a stationary structure of the galactic interstellar medium and incorporate diffusion of particles in physical and momentum space together with advection, spallation, production of secondaries and various radiation mechanisms. The self-consistent propagation models of CRs include the dynamical coupling of the CR population to the thermal plasma. The CR transport equation is discretized and solved numerically together with the set of magneto-hydrodynamic (MHD) equations in various approaches treating the CR population as a separate relativistic fluid within the two-fluid approach or as a spectrally resolved population of particles evolving in physical and momentum space. The relevant processes incorporated in self-consistent models include advection, diffusion and streaming well as adiabatic compression and several radiative loss mechanisms. We discuss applications of the numerical models for the interpretation of CR data collected by various instruments. We present example models of astrophysical processes influencing galactic evolution such as galactic winds, the amplification of large-scale magnetic fields and instabilities of the interstellar medium.
Recently, the High Altitude Water Cherenkov (HAWC) collaboration reported the discovery of the TeV halo around the Geminga pulsar. The TeV emission is believed to originate from inverse Compton scattering of pulsar-injected electrons/positrons off cosmic microwave background photons. In the mean time, these electrons should inevitably radiate X-ray photons via the synchrotron radiation, providing a useful constraint on the magnetic field in the TeV halo. In this work, we analyse the data of XMM-Newton and Chandra, and obtain an upper limit for the diffuse X-ray flux in a region of $600$ around the Geminga pulsar, which is at a level of $lesssim 10^{-14}rm erg,cm^{-2}s^{-1}$. Through a numerical modelling on both the X-ray and the TeV observations assuming isotropic diffusion of injected electrons/positrons, we find the magnetic field inside the TeV halo is required to be $<1mu$G, which is significantly weaker than the typical magnetic field in the interstellar medium. The weak magnetic field together with the small diffusion coefficient inferred from HAWCs observation implies that the Bohm limit of particle diffusion may probably have been achieved in the TeV halo. We also discuss alternative possibilities for the weak X-ray emission, such as the hadronic origin of the TeV emission or a specific magnetic field topology, in which a weak magnetic field and a very small diffusion coefficient might be avoided.
Cosmic-ray (CR) sources temporarily enhance the relativistic particle density in their vicinity over the background distribution accumulated from the Galaxy-wide past injection activity and propagation. If individual sources are close enough to the solar system, their localised enhancements may present as features in the measured spectra of the CRs and in the associated secondary electromagnetic emissions. Large scale loop like structures visible in the radio sky are possible signatures of such nearby CR sources. If so, these loops may also have counterparts in the high-latitude $gamma$-ray sky. Using $sim$10 years of data from the Fermi Large Area Telescope, applying Bayesian analysis including Gaussian Processes, we search for extended enhanced emission associated with putative nearby CR sources in the energy range from 1 GeV to 1 TeV for the sky region $|b| > 30^circ$. We carefully control the systematic uncertainty due to imperfect knowledge of the interstellar gas distribution. Radio Loop~IV is identified for the first time as a $gamma$-ray emitter and we also find significant emission from Loop~I. Strong evidence is found for asymmetric features about the Galactic $l = 0^circ$ meridian that may be associated with parts of the so-called Fermi Bubbles, and some evidence is also found for $gamma$-ray emission from other radio loops. Implications for the CRs producing the features and possible locations of the sources of the emissions are discussed.
This work has the main objective to provide a detailed investigation of cosmic ray propagation in magnetohydrodynamic turbulent fields generated by forcing the fluid velocity field at large scales. It provides a derivation of the particle mean free path dependences in terms of the turbulence level described by the Alfvenic Mach number and in terms of the particle rigidity. We use an upgrade version of the magnetohydrodynamic code {tt RAMSES} which includes a forcing module and a kinetic module and solve the Lorentz equation for each particle. The simulations are performed using a 3 dimension periodical box in the test-particle and magnetostatic limits. The forcing module is implemented using an Ornstein-Uhlenbeck process. An ensemble average over a large number of particle trajectories is applied to reconstruct the particle mean free paths. We derive the cosmic ray mean free paths in terms of the Alfvenic Mach numbers and particle reduced rigidities in different turbulence forcing geometries. The reduced particle rigidity is $rho=r_L/L$ where $r_L$ is the particle Larmor radius and $L$ is the simulation box length related to the turbulence coherence or injection scale $L_{inj}$ by $L sim 5 L_{inj}$. We have investigated with a special attention compressible and solenoidal forcing geometries. We find that compressible forcing solutions are compatible with the quasi-linear theory or more advanced non-linear theories which predict a rigidity dependence as $rho^{1/2}$ or $rho^{1/3}$. Solenoidal forcing solutions at least at low or moderate Alfvenic numbers are not compatible with the above theoretical expectations and require more refined arguments to be interpreted. It appears especially for Alfvenic Mach numbers close to one that the wandering of field lines controls perpendicular mean free path solutions whatever the forcing geometry.
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