GeV Cosmic Rays Research Articles

CHARACTERIZING COSMIC-RAY PROPAGATION IN MASSIVE STAR-FORMING REGIONS: THE CASE OF 30 DORADUS AND THE LARGE MAGELLANIC CLOUD

Using infrared, radio, and gamma-ray data,we investigate the propagation characteristics of cosmic-ray (CR) electrons and nuclei in the 30 Doradus (30\,Dor) star-forming region in the Large Magellanic Cloud (LMC) using a phenomenological model based on the radio-far-infrared correlation within galaxies. Employing a correlation analysis, we derive an average propagation length of \sim 100-140 pc for \sim 3 GeV CR electrons resident in 30 Dor from consideration of the radio and infrared data. Assuming that the observed gamma-ray emission towards 30 Dor is associated with the star-forming region, and applying the same methodology to the infrared and gamma-ray data, we estimate a \sim 20 GeV propagation length of 200-320 pc for the CR nuclei. This is approximately twice as large as for\sim 3 GeV CR electrons, corresponding to a spatial diffusion coefficient that is \sim 4 times higher, scaling as (R/GV)^{\delta} with \delta \approx 0.7-0.8 depending on the smearing kernel used in the correlation analysis. This value is in agreement with the results found by extending the correlation analysis to include \sim 70 GeV CR nuclei traced by the 3-10 GeV gamma-ray data (\delta \approx 0.66+/-0.23). Using the mean age of the stellar populations in 30 Dor and the results from our correlation analysis, we estimate a diffusion coefficient D_{R} \approx 0.9-1.0 \times10^27 (R/GV)^{0.7} cm^2 s^-1. We compare the values of the CR electron propagation length and surface brightness for 30 Dor and the LMC as a whole with those of entire disk galaxies. We find that the trend of decreasing average CR propagation distance with increasing disk-averaged star formation activity holds for the LMC, and extends down to single star-forming regions, at least for the case of 30 Dor.

THE EDDINGTON LIMIT IN COSMIC RAYS: AN EXPLANATION FOR THE OBSERVED LACK OF LOW-MASS RADIO-LOUD QUASARS AND THEM•–M⋆RELATION

We present a feedback mechanism for supermassive black holes and their host bulges that operates during epochs of radio-loud quasar activity. In the radio cores of relativistic quasar jets, internal shocks convert a fraction of ordered bulk kinetic energy into randomized relativistic ions, or in other words cosmic rays. By employing a phenomenologically-motivated jet model, we show that enough 1-100 GeV cosmic rays escape the radio core into the host galaxy to break the Eddington limit in cosmic rays. As a result, hydrostatic balance is lost and a cosmic ray momentum-driven wind develops, expelling gas from the host galaxy and thus self-limiting the black hole and bulge growth. Although the interstellar cosmic ray power is much smaller than the quasar photon luminosity, cosmic rays provide a stronger feedback than UV photons, since they exchange momentum with the galactic gas much more efficiently. The amount of energy released into the host galaxy as cosmic rays, per unit of black hole rest mass energy, is independent of black hole mass. It follows that radio-loud jets should be more prevalent in relatively massive systems since they sit in galaxies with relatively deep gravitational potentials. Therefore, jet-powered cosmic ray feedback not only self-regulates the black hole and bulge growth, but also provides an explanation for the lack of radio-loud activity in relatively small galaxies. By employing basic known facts regarding the physical conditions in radio cores, we approximately reproduce both the slope and the normalization of the M_{BH}-M_{Bulge} relation.

Drift Effects and the Cosmic Ray Density Gradient in a Solar Rotation Period: First Observation with the Global Muon Detector Network (GMDN)

We present for the first time hourly variations of the spatial density gradient of 50 GeV cosmic rays within a sample solar rotation period in 2006. By inversely solving the transport equation, including diffusion, we deduce the gradient from the anisotropy that is derived from the observation made by the Global Muon Detector Network (GMDN). The anisotropy obtained by applying a new analysis method to the GMDN data is precise and free from atmospheric temperature effects on the muon count rate recorded by ground based detectors. We find the derived north-south gradient perpendicular to the ecliptic plane is oriented toward the Helioshperic Current Sheet (HCS) (i.e. southward in the toward sector of the Interplanetary Magnetic Field (IMF) and northward in the away sector). The orientation of the gradient component parallel to the ecliptic plane remains similar in both sectors with an enhancement of its magnitude seen after the Earth crosses the HCS. These temporal features are interpreted in terms of a local maximum of the cosmic ray density at the HCS. This is consistent with the prediction of the drift model for the $A<0$ epoch. By comparing the observed gradient with the numerical prediction of a simple drift model, we conclude that particle drifts in the large-scale magnetic field play an important role in organizing the density gradient, at least in the present $A<0$ epoch. We also found that corotating interaction regions did not have such a notable effect. Observations with the GMDN provide us with a new tool for investigating cosmic ray transport in the IMF.

Wave Damping by Magnetohydrodynamic Turbulence and Its Effect on Cosmic‐Ray Propagation in the Interstellar Medium

Cosmic rays scatter off magnetic irregularities (Alfven waves) with which they are resonant, that is, waves of wavelength comparable to their gyroradii. These waves may be generated either by the cosmic rays themselves, if they stream faster than the Alfven speed, or by sources of MHD turbulence. Waves excited by streaming cosmic rays are ideally shaped for scattering, whereas the scattering efficiency of MHD turbulence is severely diminished by its anisotropy. We show that MHD turbulence has an indirect effect on cosmic-ray propagation by acting as a damping mechanism for cosmic-ray-generated waves. The hot (coronal) phase of the interstellar medium is the best candidate location for cosmic-ray confinement by scattering from self-generated waves. We relate the streaming velocity of cosmic rays to the rate of turbulent dissipation in this medium for the case in which turbulent damping is the dominant damping mechanism. We conclude that cosmic rays with up to 10^2 GeV could not stream much faster than the Alfven speed but 10^6 GeV cosmic rays would stream unimpeded by self-generated waves, unless the coronal gas were remarkably turbulence-free.

Modulation over a 22-year cosmic ray cycle: On the tilt angles of the heliospheric current sheet

The ‘tilt angle’ of the wavy current sheet in the hehosphere has been used very successfully as a proxy for solar activity in cosmic ray (CR) modulation models with particle drifts included. The “Hoeksema tilt angles α” are calculated using two potential field methods: (1) The classic ‘line-of-sight’ computation and (2) the more recent ‘radial’ photospheric field computation. The question of which set of ‘tilt angles’ is the more appropriate to use in modulation models is addressed by utilizing them in a comprehensive time-dependent drift model applied to the 22-year modulation cycle of 1.2 GeV cosmic ray electrons. We found that the ‘classic’ α are better suited than the ‘new’ α as a proxy for solar activity for periods of decreasing α and vice versa for periods of increasing α. By comparing the model with data it was shown that the set of α with the smallest rate of change over a period of decreasing or increasing solar activity provides the best compatibility with CR observations.

Supernova remnants and the origin of cosmic radiation: evidence from low-energy gamma rays

Our model, in which cosmic rays are accelerated by shocks in supernova remnants (SNR) and then propagate by diffusion through the galaxy, has been developed and applied to the case of the low-energy (Eγ ⩽ 10 GeV) diffuse gamma ray background. Attention is given to the well-known ‘GeV-excess’ problem, i.e. the gamma ray spectrum from the inner galaxy is flatter (and in the outer galaxy is correspondingly steeper) in the range 1–10 GeV, than would have been expected on the basis of gamma rays coming from the interactions of the cosmic rays observed locally with gas and radiation fields. Attention is also given to the related problem of the ‘cosmic ray gradient’, i.e. the dependence of the low-energy cosmic ray intensity on galactocentric distance; the gradient being deduced from the distribution of >0.1 GeV gamma rays which is flatter than the known SNR distribution.We argue that both problems can be solved in the framework of the conception that SN explosions are the source of the turbulence in the galactic disk leading to a radial dependence of the form of the turbulence spectrum. We contend that the propagation of cosmic rays through the turbulent interstellar medium (ISM) is governed by the so-called anomalous diffusion. This form of diffusion leads to a flatter energy spectrum of cosmic rays in the inner galaxy and a steeper spectrum in the outer galaxy. Anomalous diffusion reduces also the radial gradient of GeV cosmic rays in the inner galaxy; for the outer galaxy we invoke re-entry of cosmic rays from the halo.In the halo, where there are no SNR, the turbulence is driven by cosmic rays themselves and their energy spectrum has to be as steep as in the outer galaxy.

Excess GeV radiation and cosmic ray origin

Particle acceleration at supernova remnant (SNR) shock waves is regarded as the most probable mechanism for providing Galactic cosmic rays at energies below 10 15 eV. The Galactic cosmic ray hadron component would in this picture result from the injection of relativistic particles from many SNRs. It is well known that the superposition of individual power law source spectra with dispersion in the spectral index value, which behaviour is observed in the synchrotron radio spectra of shell SNR, displays a positive curvature in the total spectrum and in particular shows a hardening at higher energies. Recent observations made with the EGRET instrument on the Compton Gamma-Ray Observatory of the diffuse Galactic γ -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 γ -ray excess may be attributed to the dispersion of the spectral indices in these objects. In global averages, as are γ -ray line-of-sight integrals, this dispersion leads to a positive curvature in the composite spectrum, and hence to modified $\pi^0$-decay γ -ray spectra.

Acceleration of Ultra High Energy Cosmic Rays

Some general features of cosmic ray acceleration are summarized along with some inferences that can be drawn concerning the origin of the UHE component. The UHE luminosity density is found to be similar to that derived for GeV cosmic rays and its slope suggests a distinct origin. Reports of clustering on small angular scale, if confirmed, would rule out most proposed source models. More generally, it is argued that the highest energy particles can only be accelerated in sites that can sustain an EMF ℰ ≳ 3 × 1020 V and an associated power Lmin ≳ ℰ2/Z ~ 1039 W, where Z is the characteristic, electrical impedance, typically ≲ 100 Ω. Shock acceleration, unipolar induction and magnetic flares are the three most potent, observed, acceleration mechanisms and radio jet termination shocks, γ-ray blast waves, dormant black holes in galactic nuclei and magnetars are the least implausible, “conventional” manifestations of these mechanisms that have been invoked to explain the UHE cosmic rays. Each of these models presents problems and deciding between these and “exotic” origins for UHE cosmic rays, including those involving new particles or defects will require improved statistical information on the energies, arrival times and directions, as should be provided by the AUGER project.

CME geometry in relation to cosmic ray anisotropy

Strong enhancements of the cosmic ray anisotropy were observed before and during the January 1997 CME/magnetic cloud. From a multi‐station analysis of neutron monitor data, we conclude that “B × ∇n” drift is a primary source of CME‐related anisotropies for 5 GeV cosmic rays. Evolution of the cosmic ray density and density gradients is closely linked to magnetic properties of the ejecta, and provides information on the magnetic cloud and related features as they approach and pass Earth. Strong enhancement of the field‐aligned anisotropy was observed primarily during the 9 hours prior to shock arrival. If typical, this phenomenon should prove useful for space weather forecasting.

Search for Nonthermal X-Rays from Supernova Remnant Shells

The suggestion that the shocks of supernova remnants (SNR's) are cosmic ray acceleration sites dates back more than 40 years. While observations of nonthermal radio emission from SNR shells indicate the ubiquity of GeV cosmic ray production, there is still theoretical debate about whether SNR shocks accelerate particles up to the well-known “knee” in the primary cosmic ray spectrum at ~3,000 TeV. Recent X-ray observations of SN1006 and other SNR's may have provided the missing observational link between SNR shocks and high energy cosmic ray acceleration. We discuss these observations and their interpretation, and summarize our ongoing efforts to find evidence from X-ray observations of cosmic ray acceleration in the shells of other SNR's.

Cosmic ray diffusion at energies of 1 MeV to 105 GeV

A supernova remnant accelerates cosmic rays to energies somewhat above 106 GeV by the time that the free expansion phase of its evolution has come to an end. As the remnant’s outer shock slows, these highest energy cosmic rays diffuse away from the shock along a magnetic flux tube with a radius comparable to that of the remnant at the end of its free expansion phase and which eventually (over a distance of the order of a kiloparsec) bends into the Galactic halo. A similarity solution exists for the temporal and spatial variations, in such a tube, of both the number density for these ∼ 105 GeV cosmic rays and the energy density of the waves on which they resonantly scatter. Wave-wave interactions probably do not dominate the evolution of the energy density of these lowest frequency waves, but we assume that they do establish a Kraichnan wave spectrum at higher wavenumber. Although we cannot rigorously justify this assumption, it does receive some support from the analysis of pulsar signals. There is a large body of observations to which such a model can be applied, yielding constraints that must be met. With the model that we develop here we obtain the following results: 1 The local intensity of ∼ 105 GeV cosmic rays implies that the flux tube which currently surrounds the Solar System last contained a remnant in the free expansion phase several times 107 years ago. We comment on the rough agreement between this age and that inferred from Be10 data. 2 The theoretical value of the cosmic ray diffusion coefficient at ∼ 1 GeV in the tube corresponding to that time is in harmony with the value of the diffusion coefficient inferred from cosmic ray composition and synchrotron measurements.

Solar modulation of cosmic ray electrons 1978–1983

The ISEE 3 spacecraft has provided over 4 years of continuous data from nearby interplanetary space. By using these data together with prelaunch calibration data and precise electron spectra from balloon flights in 1979 and 1982 at Thompson, Manitoba, Canada, we monitor the flux level of approximately 1 GeV cosmic ray electrons with 1 month time resolution. The steplike onset of modulation observed for positive particles in 1979 is also seen in electrons, confirming that the dominant modulation mechanism is not sensitive to the charge of the particle. Hysteresis effects observed in the previous solar maximum by Burger and Swanenburg (1973) are not reproduced at the present solar maximum. A return to the relatively higher electron/proton ratios of the 1960's is a strong possibility. Conclusions must be drawn with caution, however, since the overall time structure of the present solar maximum is significantly different from that of the previous one.

Limits to extragalactic cosmic rays from gamma-ray fluxes

Models involving the production of cosmic-ray protons and heavier nuclei of energies in the range 1-10 GeV in extragalactic space are considered and the fluxes of gamma rays expected from their interactions with gas in clusters of galaxies are derived. Comparison with observation allows limits to be set on the fraction of the cosmic-ray flux detected at the Earth which is derived from extragalactic sources. It is found that for all reasonable models of extragalactic origin an excess of gamma -ray flux would result; thus, the bulk of the 1-10 GeV cosmic rays detected appear to be of Galactic origin.

Existence of the fragmentation region

Inclusive log tg θ spectra from multiparticle production in p-p collisions at 28.5 GeV from bubble chamber data are compared with 28.5 GeV p-N, 60 GeV π−-N, ∼400 GeV cosmic ray and ∼3000 GeV cosmic-ray emulsion data to demonstrate the existence of the fragmentation region.

Observation of ∼15 GeV cosmic ray photons by means of a spark-emulsion-chamber

A new method of detecting high energy events has been developed by exposing an apparatus to cosmic ray electrons and photons at Mt. Norikura (730 g/cm 2). The apparatus of consists of an emulsion chamber and spark chambers, and an electronic cascade shower developed in the former can be located with the aid of black spots marked on X-ray films inserted in the latter. It has been demonstrated that a cascade shower of energy down to ∼ 15 GeV can be detected with considerable ease in the emulsion chamber. This method is thus proved useful in the investigation of high energy interactions.

8Li fragments associated with nuclear disintegration

This paper is a preliminary report on the emission of heavy fragments in nuclear disintegration. It is assumed that the probability of emission of a heavy fragment depends on the nuclear temperature (T) and the potential barrier (V) of the excited nucleus. It is the purpose of this investigation to study the dependence of the differential energy spectrum on the excitation energy. The data collected were from four stacks of Ilford G-5 emulsions, exposed to (a) a stopped pion beam; (b) 400 MeV proton beam; (c) (4÷5) GeV pion beam and (d) cosmic rays with an exposure in Texas, Magnetic latitude of 41° N, and on the Island of Guam at the magnetic equator. In this report emphasis will be placed on the emission of8Li as the heavy fragment. The results of the experiment indicate that for high excitation energy there is a collimation of lithium and beryllium fragments in the forward direction which may be explained on the basis of evaporation theory by assuming the fragment emission to take place during the flight of a highly excited nucleus. In the case of low excitation energy the angular distribution of8Li fragments is isotropic and the energy distribution indicates that the majority result from the absorption of aπ−-meson by a light nucleus.