Active galactic nuclei and pulsars as cosmic ray sources

Relativistic e^± particles and cosmic rays are accelerated in the magnetospheres of supermassive black holes and neutron stars. The possibility of synchrotron radiation with extremely high intensity inside the deepest regions of magnetospheres is investigated. Very high brightness temperatures are expected for such radiation by relativistic protons, which can be made even higher in the presence of non-stationary conditions, Doppler boosting and coherent processes. The main parameters for models of such high-brightness-temperature radiation are determined. Two types of active galactic nuclei (AGNs) are expected. One type is associated with the acceleration and ejection of relativistic e^± particles only (probably non-IDV sources and FR-I radio galaxies). The second type of AGN is also associated with e^± acceleration, but is dominated by the contribution of relativistic protons (probably IDV sources and FR-II radio galaxies). Analogous objects for pulsars are plerion and shell supernova remnants with neutron stars or pulsars without synchrotron nebulae, respectively.     

Propagation of H and He cosmic ray isotopes in the Galaxy: astrophysical and nuclear uncertainties

Observations of light isotopes in cosmic rays provide valuable information on their origin and propagation in the Galaxy. Using the data collected by the AMS-01 experiment in the range ∼0.2–1.5 GeV nucleon⁻¹, we compare the measurements on ¹ H, ² H, ³ He, and ⁴ He with calculations for interstellar propagation and solar modulation. These data are described well by a diffusive-reacceleration model with parameters that match the B/C ratio data, indicating that He and heavier nuclei such as C–N–O experience similar propagation histories. Close comparisons are made within the astrophysical constraints provided by the B/C ratio data and within the nuclear uncertainties arising from errors in the production cross section data. The astrophysical uncertainties are expected to be dramatically reduced by the data upcoming from AMS-02, so that the nuclear uncertainties will likely represent the most serious limitation on the reliability of the model predictions. On the other hand, we find that secondary-to-secondary ratios such as ² H/³ He, ⁶ Li/⁷ Li or ¹⁰ B/¹¹ B are barely sensitive to the key propagation parameters and can represent a useful diagnostic test for the consistency of the calculations.     

Ultrahigh energy cosmic ray acceleration in Seyfert galactic nuclei

We propose a model for the particle acceleration to energy E≈10²¹ eV in Seyfert galactic nuclei. The model is based on the theory of active galactic nuclei by Vilkoviskij et al. (1999). The acceleration takes place in hot spots of relativistic jets, which decay in a dense stellar kernel at a distance of 1–3 pc from the center. The maximum energy and chemical composition of the accelerated particles depend on the jet magnetic-field strength. Fe nuclei acquire the largest energy, E≈8×10²⁰ eV, if the jet field strength is B≈16 G. At a field strength B～5–40 G, the nuclei with Z≥10 acquire energy E≥2×10²⁰ eV; the lighter nuclei are accelerated to E≤10²⁰ eV. In a field B～1000 G, only the particles with Z≥23 gain energy E≤10²⁰ eV. The protons are accelerated to E<4×10¹⁹ eV, and they do not fall within the energy range concerned at any field strength B. Interactions with infrared photons do not affect the accelerated-particle escape from the sources if the galactic luminosity L≤10⁴⁶ erg s^?1 and if the angle between the normal to the galactic plane and the line of sight is sufficiently small, i.e., if the galactic-disk axial ratio is comparatively large. The particles do not lose their energy through magnetodrift radiation if their deflection from the jet axis does not exceed 0.03–0.04 pc at a distance R≈40–50 pc from the center. The synchrotron losses are small, because the magnetic field frozen in the galactic wind at R≤40–50 pc is directed (as in the jet) predominantly along the motion. If this model is correct, then the detected protons are nuclear fragments or are accelerated in other sources. The jet magnetic fields can be estimated by using the cosmic-ray energy spectrum and chemical composition.     

The fermi mechanism and the source spectrum of cosmic ray nuclei

It is shown that the velocity term, occurring in the expression for the rate of energy gain by the Fermi mechanism of acceleration, is to be taken into account in case of acceleration of non-relativistic particles. A spectral form of accelerated particles is derived on this basis and is called the ‘Fermi Spectrum’. At non-relativistic energies this spectral form is significantly different from the currently used forms of power law in total energy per nucleon and in rigidity, and lies about midway between them. It is shown that using this form of source spectrum of cosmic ray nuclei, satisfactory agreement can be obtained between the calculated values and the observed ones of the ratios of H²/He⁴ and He³/He⁴, and the energy spectra of protons and helium nuclei near the Earth.     

Ionization states and the origin of low energy cosmic ray nuclei

The origin of the new component of cosmic ray nuclei in 1–30 MeV amu^–1 recently detected through space vehicles in interplanetary space is investigated in detail. It is assumed that these particles may originate from nearby sources, e.g., from novae type explosions, which have peculiar C, N and O compositions. These particles are further assumed to be accelerated and modulated within the heliosphere. The charged states of these ions in the interstellar space have been calculated in detail and it is shown that the same charged states are preserved in the heliosphere when they are accelerated to energies of the order of 10⁷eV amu^–1 from energies of 10⁵ ev amu^–1. Modulation of these ions are calculated and it is found that because of low charged states of the ions these have high rigidities and are modulated in such a way as to enhance the O-ion abundances as compared to C-ions. A comparison is made of the demodulated composition of C to Si-ions with available abundance data of some novae.     

On the modulation and energy spectrum of highly charged cosmic ray nuclei

The intensity and energy spectrum of cosmic ray VH-nuclei (20Z30) has been measured in a stack of nuclear emulsions exposed over Fort Churchill in 1968. The integral intensity above 300 MeV/nucleon was 1.04±0.04 nuclei m^–2 sr^–1 s^–1 and three differential intensities were measured below 750 MeV/nucleon. Because of the current controversy regarding the true intensities of helium nuclei at this phase of the solar cycle we have also measured these nuclei, obtaining results intermediate between those quoted by other workers. Comparison of these results on the VH-and helium nuclei with those obtained in previous observations made at times of low solar modulation leads to the conclusion that there is no significant charge dependence in the modulation process. This conclusion is in conflict with an earlier analysis but depends on results of improved statistical weight and greater reliability for the VH nuclei and on our measurement of the helium nuclei in the same detector.Supported by the Office of Naval Research under Contract No. N00014-67-A-0113-0021.     

A diffusion model for cosmic ray propagation in the Galaxy

A diffusion model for the propagation of relativistic nuclear cosmic rays in the Galaxy is developed. The model has two nonstandard features: The escape of cosmic-ray particles from the Galaxy is simulated by a term in the diffusion equations, rather than the imposition of boundary conditions on the diffusion solution at the surface of the confinement region. And an age-dependent, locally-averaged effective gas distribution is employed in the diffusion equations. The model simulates free-particle outflow at the Galactic boundary. The model is fit to chemical composition data in the 0.3–5 GeV per nucleon range. It is then consistent with the large-scale Galactic -ray data, radio halo data, energy constraints on the assumed supernova sources, and, when extended to very high energies, cosmic-ray anisotrophy data. From the fit we conclude that the cosmic rays are confined in a large flattened or quasis-pherical halo with a scale height in the range 3–6 kpc and an average Galactic escape time of 10⁸ yr.     

Devgaon (H3) chondrite: Classification and complex cosmic ray exposure history

Abstract— The Devgaon meteorite fell in India on February 12, 2001 and was immediately collected. It is an ordinary chondrite having a number of SiO₂‐rich objects and some Ca, Al‐rich inclusions. Olivines (Fa_17–19) are fairly equilibrated, while pyroxenes (Fs_4–20) are unequilibrated. Occasionally, shock veins are visible, but the bulk rock sample is very weakly shocked (S2). Chondrules and chondrule fragments are abundant. Based on chemical and petrological features, Devgaon is classified as an H3.8 group chondrite. Several cosmogenic radionuclides ranging in half‐lives from 5.6 d (⁵²Mn) to 7.3 times 10⁵ yr (²⁶Al), noble gases (He, Ne, Ar, Kr, and Xe), and particle track density have been measured. The track density in olivines from five spot samples varies between (4.6 to 9) × 10⁶ cm^?2 showing a small gradient within the meteorite. The light noble gases are dominated by cosmogenic and radiogenic components. Large amounts of trapped gases (Ar, Kr, and Xe) are present. In addition, (n, γ) products from Br and I are found in Kr and Xe, respectively. The average cosmic ray exposure age of 101 ± 8 Ma is derived based on cosmogenic ³⁸Ar, ⁸³Kr, and ¹²⁶Xe. The track production rates correspond to shielding depths of about 4.9 to 7.8 cm, indicating that the stone suffered type IV ablation. Low ⁶⁰Co, high (²²Ne/²¹Ne)_c, and large neutron produced excesses at ⁸⁰Kr, ⁸²Kr, and ¹²⁸Xe indicate a complex exposure history of the meteoroid. In the first stage, a meter‐sized body was exposed for nearly 10⁸ yr in the interplanetary space that broke up in ?50 cm‐sized fragments about a million years ago (stage 2), before it was captured by the Earth.     

A comparison of the energy spectra of cosmic ray helium and heavy nuclei

Recent observations of the spectra of cosmic ray helium, M, LH and VH nuclei in the energy range from 200 MeV/nuc to>22 GeV/nuc are reported. The differential spectra of all of these groups of nuclei are found to have a maximum at 300–400 MeV/nuc at sunspot minimum. The average exponents on the integral rigidity spectra in the range 5 to 50 GV are 1.54±0.03 for He nuclei, 1.50±0.04 for M nuclei, 1.47±0.06 for LH nuclei and 1.40±0.08 for VH nuclei. The spectra of these groups of nuclei are compared and it is found that the average He/M, He/LH and He/VH ratios are 16±1, 70±3 and 200±15 respectively. These values are reasonably constant from the highest down to the lowest energies measured although some evidence for a dip is present in the 500–1000 MeV/nuc range for both the He/LH and He/VH ratios. Solar modulation effects on these ratios are discussed and it is concluded that the ratios measured at earth are representative of those existing in interstellar space only if energy loss processes in interplanetary space are unimportant. The influence of interstellar propagation on the spectra and charge ratios at low energies arising from ionization energy loss and nuclear spallation during matter traversal is examined. It is found that propagation models that contain a large number of relatively short path lengths significantly modify the expected effects of ionization energy loss at low energies. Specifically it is suggested that the presently measured charge ratios are consistent with the passage of the average cosmic radiation through enough matter to reproduce the abundances of the so-called fragmentation nuclei, Li, Be, B and He³. Two component models are not required to explain our data. Rather we feel that a better representation of the situation results from considering a continuous spatial distribution of sources which, along with the actual interstellar propagation conditions, leads to a particular distribution of matter path lengths. It is pointed out that large differences exist in the approaches used in the literature to calculate the effects of matter traversal in interstellar space at low energies. These differences play an important role in the interpretation of the experimental results. Significant modifications of the charge ratios at low energies can also be obtained by requiring that some of the matter traversal occur in the cosmic ray sources themselves during the cosmic ray acceleration process. This may be sufficient to produce charge ratios that are essentially flat at low energies even in the presence of interstellar ionization loss.     

Charge dependence of the solar modulation of multiply charged cosmic ray nuclei

The intensity and energy spectra of multiply charged cosmic ray nuclei, in the energy interval 250–1500 MeV/n, were studied at three different levels of solar activity, viz. in 1963, 1964 and 1967. The same detectors, nuclear emulsion stacks flown from Fort Churchill, Canada, were used to determine simultaneouslty the energy spectra of helium, C, N, O as well as H (Z=10–28) nuclei. An analysis of the measured spectra indicates that these can be interpreted in terms of: (a) the source spectrum as a Fermi spectrum with a spectral index of 2.65; (b) the interstellar propagation as in a Gaussian distribution of path lengths with a mean path length of 4 g cm^–2 and (c) the interplanetary propagation as given by the numerical solution of the Fokker-Planck equation incorporating diffusion, convection and adiabatic deceleration. On comparing the measured ratios of He to H-nuclei (mean Z14) with the theoretically calculated values for the three levels of solar activity, it is found that within experimental uncertainties, the solar modulation is essentially the same for nuclei of same mass to charge ratio and is not dependent on the charge of the nuclei.On leave from Tata Institute of Fundamental Research, Bombay.     

A study of solar modulation of medium energy primary cosmic ray nuclei

The energy spectra of primary cosmic rays were studied in the energy interval 150 to 450 MeV/nucl by using balloon-borne cellulose-nitrate solid-state plastic detector. Effects of solar modulation were studied using the theoretical spectrum ofH ₁ nuclei near the solar minimum in 1964 as the demodulated spectrum. The ‘force-field’ potential which fit the experimental results was estimated to be 270 MeV/nucl.     

Nonlinear perturbation theory for cosmic ray propagation in random magnetic fields

A nonlinear perturbation theory is applied to the problem of pitch angle diffusion of energetic particles in random magnetic fields. To keep the analysis simple, the discussion is restricted to fluctuation fields, consisting of Alfvén waves. It is shown that the failure of quasilinear theory at small particle velocities parallel to the average field can be overcome by a statistically exact treatment of the particle orbits in the first order fields. In fact, for spherical power spectra which, in addition, do not fall off too steeply with increasing frequency, the conventional perturbation theory also leads to formally convergent expressions for the scattering mean free path. These results are shown to be quite satisfactory, even in a quantitative sense. For more general physically realistic power spectra, however, a divergence-free diffusion theory is indispensible. A simple representation for the resulting pitch-angle diffusion coefficient is suggested.     

Anisotropic solar cosmic ray propagation in an inhomogeneous medium,II

The author's model for anisotropic solar cosmic ray propagation gives 2 coupled, partial differential equations for the intensity and anisotropy of solar cosmic rays propagating with finite speed V in an inhomogeneous medium. The model is used to study the effect of the solar shell on solar cosmic ray propagation. It predicts an exponential decay, regardless of the observer's position. It predicts that when the observer is near the center of the shell, t _D/t ₀ 20 to 30, (t _D= decay time, t ₀ = onset time) and A _m(anisotropy) 15%, if t _m/t ₀ 3 to 5 (t _m= time of maximum), consistent with observations of relativistic particles on Feb. 23, 1956. When the observer is between the shell and the sun, the model predicts that oscillations might be observed near maximum intensity. When the observer moves away from the sun and the shell, the propagation is diffusive, but there is an increasingly large persistent anisotropy which serves as a measure of the width of the shell.     

On the contribution of nearby sources to the observed cosmic ray nuclei

The presence of nearby discrete cosmic ray (CR) sources can lead to many interesting effects on the observed properties of CRs. In this paper, we study about the possible effects on the CR primary and secondary spectra and also the subsequent effects on the CR secondary-to-primary ratios. For the study, we assume that CRs undergo diffusive propagation in the Galaxy and we neglect the effect of convection, energy losses and reacceleration. In our model, we assume that there exists a uniform and continuous distribution of CR sources in the Galaxy generating a stationary CR background at the Earth. In addition, we also consider the existence of some nearby sources which inject CRs in a discrete space–time model. Assuming a constant CR source power throughout the Galaxy, our study has found that the presence of nearby supernova remnants (SNRs) produces noticeable variations in the primary fluxes mainly above ∼100 GeV n⁻¹, if CRs are assumed to be released instantaneously after the supernova explosion. The variation reaches a value of ∼45 per cent at around 10⁵ GeV n⁻¹. Respect to earlier studies, the variation in the case of the secondaries is found to be almost negligible. We also discuss about the possible effects of the different particle release times from the SNRs. For the particle release time of ∼10⁵ yr, predicted by the diffusive shock acceleration theories in SNRs, we have found that the presence of the nearby SNRs hardly produces any significant effects on the CRs at the Earth.     

Relative abundances of cosmic ray nuclei from neon to iron over Fort Churchill

The relative abundances of the nuclei from neon to iron in the energy interval 150–400 MeV/n have been estimated by using a balloon borne cellulose-nitrate plastic detector. The source abundances are obtained by extrapolating the near-earth abundances using leaky box model of cosmic ray propagation in the interstellar space. The results are compared with those of other investigators and a general agreement is obtained. However, a discrepancy arises especially in the case of Al which is not detected in the present investigation.     

Radio-emitting electrons and cosmic ray confinement

The propagation of cosmic ray electrons in the framework of the Disk-Halo diffusion model in which the diffusion coefficientD z E (wherez is the distance from the galactic plane andE is the energy), and the magnetic fieldHz ^– has been examined by making use of the recently available radio data up to 8 GHz toward the Anticenter (A) and Halo Minimum (M). The following inferences are then made. From the difference in the frequency at which steepening occurs in the radio spectra towardA andH, it is found that the observations are consistent with the magnetic field decreasing withz such that =0.24–0.37. An electron injection spectrum with a single power law down to energies well below 1 GeV cannot explain satisfactorily the observed radio spectra. All observations, however, can be understood in a self consistent way if the observed steepening of the radio spectra, and hence the interstellar electron spectrum, is due partly to the deviation in the power law electron injection spectrum below a few GeV and partly to the first break arising from electron energy losses occurring in the same energy region. In this case, using the value of obtained above and a value of =0.3–0.6, it is found that the spectral index ₀ of the injected electrons above a few GeV has a value between 1.9 and 2.3 and the index a value between 0.5 and 1. Further, if the electrons and protons have the same spectral shape at injection, then ₀=2.1–2.3.NASA-NRC, Senior Research Associate on leave from Tata Institute of Fundamental Research, Bombay, India.     

On the role of plasma effects in the cosmic ray propagation and isotropization in the Galaxy

Cosmic ray (c. r.) propagation in interstellar magnetic fields is often considered in the diffusion approximation, i.e. by the diffusion equation in the coordinate space. Cosmic ray momentum distribution in this case is considered isotropic when the space gradients of c.r density are absent. This approach, with the use of an unfixed effective diffusion coefficientD independent of the energyE enables one to describe all the data available However, neither the diffusion mechanism nor the limits of applicability of the diffusion approximation is clear particularly ifD is independent ofE. Furthermore, the diffusion coefficientD must be expressed through the characteristics of the interstellar medium and possibly through the flux velocity and density of c.r. etc. One of the possible approaches for the analysis of the mechanism and characteristic features of c.r. distribution and isotropization is the account taken of the plasma effects and specifically, the study of c.r. flux instability arising when c.r. are moving in the interstellar plasma. As a result of such instability c.r. may generate waves of different types (magnetohydrodynamic, high-frequency plasma and other waves). Generation of waves and scattering on them result in isotropization of cosmic rays while their propagation under certain conditions turns out similar to that under diffusion.An attempt is made here to systematically analyse the avove mentioned plasma effects and to find out to what extent they are responsible for the behaviour of c.r. in the Galaxy. It turns out that c.r. In any case this is true if this mechanism is regarded as the only c.r. isotropization mechanizm within a wide energy range from 1 to 1000 GeV. Isotropization and spatial diffusion of c.r. up toE100–1000 GeV on the waves from external sources (for example, on the waves from the supernova shells) also proved impossible if the diffusion coefficient is assumed to be independent of c.r. energy. Some new possibilities of c.r. isotropization are also considered.A List of Notations D cosmic ray (c.r.) space diffusion coefficient - degree of c.r. anyisotropy - E,E _kin total and kinetic particle energy - p,p particle momentum and its absolute value - angle between the particle momentum direction and the magnetic field direction (z-axis) - cos - v, particle velocity and its absolute value - c light velocity - f(p),f(E) momentum and energy particle distribution function - N( > E) = N( > p) = f(p) dp/(2)³ = _E f dE c.r. particle density - c.r. spectrum index,N(>E)=KE ^–+1 - n _H neutral particle density - n=n _e=n _i ion and electron density - H niagnetic field - T temperature - thermal velocities of electrons and ions - Boltzmann constant - Alfén velocity - M, m proton and electron masses - e electron charge - wave frequency - _H =eH/Mc, = _H (Mc ²/E) gyrofrequency of a plasma proton and relativistic particle - _H =eH/mc gyrofrequency of an electron - plasma frequency - v _ii,v _ei,v _en,v _in collision frequencies between ions, electrons and ions, electrons and neutrals, ions and neutrals - growth rate of wave amplitude - k,k wave vector and its absolute value - angle between the directions of the vectorsk andH - wave energy density     

Progenitor model of cosmic ray knee

The primary energy spectrum of cosmic rays exhibits a knee at about 3 Pe V where a change in the spectral index occurs. Despite many efforts, the origin of such a feature in the spectrum is not satisfactorily solved yet. Here it is proposed that the steepening of the spectrum beyond the knee may be a consequence of the mass distribution of the progenitor of the cosmic ray source. The proposed speculative model can account for all the major observed features of cosmic rays without invoking any fine tuning to match flux or spectra at any energy point. The prediction of the proposed model regarding the primary composition scenario beyond the knee is quite different from most of the prevailing models of the knee, and thereby can be discriminated from precise experimental measurement of the primary composition.     

The nature of cosmic ray sources

The limitations on the nature of cosmic ray acceleration regions and processes, as deduced from cosmic ray measurements and propagation studies, are reviewed. The power requirements for these acceleration regions are estimated from measurements of the local cosmic ray energy density, anisotropy and spallation-deduced pathlength. Possible constraints on the acceleration spectrum of the cosmic rays and on a charge dependence of the acceleration process, implied by the measured cosmic ray spectrum and composition, are considered. Various suggested sources and processes of cosmic ray acceleration are discussed in the light of these limitations.Astrophysics and Space Science Review Paper.