The relevance of molecular observations to gamma-ray astronomy

The interaction of cosmic rays with interstellar clouds may produce some of the observed gamma-ray sources. The use of molecular observations to estimate the cloud masses, which are used to derive cosmic-ray fluxes, is reviewed. Molecular diagnostics of high cosmic-ray ionization rates are discussed, and a detailed application of those diagnostics is summarised and presented as evidence that second-order Fermi acceleration is important in old supernova remnants and can produce cosmic rays of too low energy to induce gamma-ray emission.Proceedings of the XVIII General Assembly of the IAU: Galactic Astrophysics and Gamma-Ray Astronomy, held at Patras, Greece, 19 August 1982.Royal Society Jaffé Donation Fellow.     

The Energy Spectra and Anisotropies of Cosmic Rays

The existing paradigm of the origin of Galactic cosmic rays places strong supernovae shocks as the acceleration site for this material. However, although the EGRET gamma-ray telescope has reported evidence for GeV gamma rays from some supernovae, it is still unclear if the signal is produced by locally intense cosmic rays. Although non-thermal X-ray emissions have been detected from supernova remnants and interpreted as synchrotron emission from locally intense electrons at energies up to 100 TeV, the inferred source energy spectral slopes seem much steeper than the electron source spectrum observed through direct measurements. It remains the case that simple energetics provide the most convincing argument that supernovae power the bulk of cosmic rays. Two characteristics which can be used to investigate this issue at high energy are the source energy spectra and the source composition derived from direct measurements.     

The origin of cosmic rays

In the following we describe recent progress in our understanding of the origin of cosmic rays. We propose that cosmic rays originate mainly in three sites, a) normal supernova explosions into the interstellar medium, b) supernova explosions into stellar winds, and c) hot spots of powerful radio galaxies. The proposal depends on an assumption about the scaling of the turbulent diffusive transport in cosmic ray mediated shock regions; the proposal also uses a specific model for the interstellar transport of cosmic rays. The model has been investigated in some detail and compared to i) the radio data of OB stars, Wolf Rayet stars, radio supernovae, radio supernova remnants, Gamma-ray line and continuum emission from starforming regions, and the cosmic ray electron spectrum, ii) the Akeno air shower data over the particle energy range from 10 TeV to EeV, and iii) the Akeno and Fly's Eye air shower data from 0.1 EeV to above 100 EeV.     

Spallogenic Light Elements and Cosmic-ray Origin

We discuss the new information that the light elements, particularly Be, have brought to cosmic-ray studies, specifically to the issue of the origin of the seed material of the cosmic rays. The primary nature of the Be evolution strongly suggests that supernova ejecta are the sources of this material. We discuss the superbubble models that emerged as the most likely site for the acceleration of supernova ejecta, and we review the arguments that support the model in which the present epoch cosmic rays have the same origin as those that produce the light elements throughout the evolutionary history of the Galaxy. These arguments include the facts that the bulk of the Galactic supernovae are confined within the interiors of superbubbles, where their ejecta could dominate the metallicity, and that high velocity grains, which condense out of the cooling and expanding ejecta, serve as the injection source for shock acceleration, via sputtering of grain material and scattering of volatile gas atoms. We also review the evolutionary calculations that show that a secondary origin for the evolution of Be as a function of the O abundance is energetically untenable, and unnecessary if cosmic-ray transport is properly taken into account.     

Shell type supernova remnants

The possibility of observing gamma ray emission from supernova remnants is discussed. It is shown that this could be possible in the 100 MeV band accessible to satellite instruments, but that confusion with the Galactic background is a major problem. At TeV energies and with modern imaging atmospheric cherenkov telescopes the situation should be much better and at least some of the nearby remnants may be detectable. Positive detections in both bands would provide a decisive test of current theoretical ideas on particle acceleration in supernova remnants and the origin of the Galactic cosmic rays.     

Galactic Cosmic Rays and the Light Elements

The study of the light elements abundances in low metallicity stars offers a unique way to learn about the past content of our Galaxy in energetic particles (EPs). This study teaches us that either the light elements are not produced by cosmic rays interactions in the interstellar medium (ISM), as has been thought for 30 years, or the cosmic rays are not what one usually thinks they are, namely standard interstellar material accelerated by the shock waves generated by supernova explosions. In any case, we have to revise our understanding of the EPs in the Galaxy. Relying on the observational evidence about Li, Be and B Galactic evolution as well as about the distribution of massive stars, we show that most of the EPs responsible for the production of light elements must be accelerated inside superbubbles, as is probably the case for the standard Galactic cosmic rays as well.     

Particle Acceleration and Nonthermal Phenomena in Superbubbles

Models of nonthermal particle acceleration in the vicinity of active star forming regions are reviewed. We discuss a collective effect of both stellar winds of massive stars and core collapsed supernovae as particle acceleration agents. Collective supernova explosions with great energy release in the form of multiple interacting shock waves inside the superbubbles are argued as a favourable site of nonthermal particle acceleration. The acceleration mechanism provides efficient creation of a nonthermal nuclei population with a hard low-energy spectrum, containing a substantial part of the kinetic energy released by the winds of young massive stars and supernovae. We discuss a model of temporal evolution of particle distribution function accounting for the nonlinear effect of the reaction of the accelerated particles on the shock turbulence inside the superbubble. The model illustrates that both the low-energy metal-rich nonthermal component and the standard galactic cosmic rays could be efficiently produced by superbubbles at different evolution stages.     

Microphysics of Cosmic Ray Driven Plasma Instabilities

Energetic nonthermal particles (cosmic rays, CRs) are accelerated in supernova remnants, relativistic jets and other astrophysical objects. The CR energy density is typically comparable with that of the thermal components and magnetic fields. In this review we discuss mechanisms of magnetic field amplification due to instabilities induced by CRs. We derive CR kinetic and magnetohydrodynamic equations that govern cosmic plasma systems comprising the thermal background plasma, comic rays and fluctuating magnetic fields to study CR-driven instabilities. Both resonant and non-resonant instabilities are reviewed, including the Bell short-wavelength instability, and the firehose instability. Special attention is paid to the longwavelength instabilities driven by the CR current and pressure gradient. The helicity production by the CR current-driven instabilities is discussed in connection with the dynamo mechanisms of cosmic magnetic field amplification.     

Energetic Particle Characteristics in the High-Latitude Heliosphere Near Solar Maximum

This article reviews observations on the large-scale distribution of various constituents of the interstellar medium. We subsequently discuss several theoretical issues related directly to Galactic cosmic rays: the Galactic hydrostatic equilibrium, the Parker instability of the Galactic disk, and the problem of the origin of the large-scale Galactic magnetic field.     

Acceleration of particles by shocks in a cosmic plasma

The theory and observational evidence pertaining to particle acceleration by shock waves in astrophysical objects and in space are systematized. Recent works showing observational and theoretical aspects of the problem dealing with shocks in turbulent media are emphasized. The acceleration of particles by shocks in turbulent media is observed in interplanetary space. This acceleration mechanism is of particular interest from the point of view of the origin of cosmic rays, providing the degree form of the spectrum. The index of the spectrum is close to the observable one for galactic cosmic rays. It depends slightly on specific conditions in the acceleration region. Electron and nucleus acceleration in supernova remnants and in radiogalaxies is discussed, and theory and observational data are compared. The theory of particle acceleration by supersonic turbulence is outlined.     

Cosmic Ray Production in Supernovae

We give a brief review of the origin and acceleration of cosmic rays (CRs), emphasizing the production of CRs at different stages of supernova evolution by the first-order Fermi shock acceleration mechanism. We suggest that supernovae with trans-relativistic outflows, despite being rather rare, may accelerate CRs to energies above \(10^{18}\mbox{ eV}\) over the first year of their evolution. Supernovae in young compact clusters of massive stars, and interaction powered superluminous supernovae, may accelerate CRs well above the PeV regime. We discuss the acceleration of the bulk of the galactic CRs in isolated supernova remnants and re-acceleration of escaped CRs by the multiple shocks present in superbubbles produced by associations of OB stars. The effects of magnetic field amplification by CR driven instabilities, as well as superdiffusive CR transport, are discussed for nonthermal radiation produced by nonlinear shocks of all speeds including trans-relativistic ones.     

The Composition of Cosmic Rays and the Mixing of the Interstellar Medium

The differences between the composition of Galactic cosmic rays and that of the interstellar medium are manifold, and they contain a wealth of information about the varying processes that created them. These differences reveal much about the initial mixing of freshly synthesized matter, the chemistry and differentiation of the interstellar medium, and the mechanisms and environment of ion injection and acceleration. Here we briefly explore these processes and show how they combine to create the peculiar, but potentially universal, composition of the cosmic rays and how measurements of the composition can provide a unique measure of the mixing ratio of the fresh supernova ejecta and the old interstellar medium in this initial phase of interstellar mixing. In particular, we show that the major abundance differences between the cosmic rays and the average interstellar medium can all result from cosmic ray ion injection by sputtering and scattering from fast refractory oxide grains in a mix of fresh supernova ejecta and old interstellar material. Since the bulk of the Galactic supernovae occur in the cores of superbubbles, the bulk of the cosmic rays are accelerated there out of such a mix. We show that the major abundance differences all imply a mixing ratio of the total masses of fresh supernova ejecta and old interstellar material in such cores is roughly 1 to 4. That means that the metallicity of ∼3 times solar, since the ejecta has a metallicity of ∼8 times that of the present interstellar medium.     

Electron-Ion Temperature Equilibration in Collisionless Shocks: The Supernova Remnant-Solar Wind Connection

Collisionless shocks are loosely defined as shocks where the transition between pre-and post-shock states happens on a length scale much shorter than the collisional mean free path. In the absence of collision to enforce thermal equilibrium post-shock, electrons and ions need not have the same temperatures. While the acceleration of electrons for injection into shock acceleration processes to produce cosmic rays has received considerable attention, the related problem of the shock heating of quasi-thermal electrons has been relatively neglected. In this paper we review the state of our knowledge of electron heating in astrophysical shocks, mainly associated with supernova remnants (SNRs), shocks in the solar wind associated with the terrestrial and Saturnian bowshocks, and galaxy cluster shocks. The solar wind and SNR samples indicate that the ratio of electron temperature, (T _e ) to ion temperature (T _p ) declining with increasing shock speed or Alfvén Mach number. We discuss the extent to which such behavior can be understood on the basis of waves generated by cosmic rays in a shock precursor, which then subsequently damp by heating electrons, and speculate that a similar explanation may work for both solar wind and SNR shocks.     

New Experimental Data and What It Tells Us About the Sources and Acceleration of Cosmic Rays

This paper summarizes new data in several fields of astronomy that relate to the origin and acceleration of cosmic rays in our galaxy and similar nearby galaxies. Data from radio astronomy shows that supernova remnants, both in our galaxy and neighboring galaxies, appear to be the sources of most of the accelerated electrons observed in these galaxies. -ray measurements also reveal several strong sources associated with supernova remnants in our galaxy. These sources have -ray spectra that are suggestive of the acceleration of cosmic-ray nuclei. Cosmic-ray observations from the Voyager and Ulysses spacecraft suggest a source composition that is very similar to the solar composition but with distinctive differences in the ⁴He, ¹²C,¹⁴ N and ²²Ne abundances that are the imprint of giant W-R star nucleosynthesis. Injection effects which depend on the first ionization potential (FIP) of the elements involved are also observed, in a manner similar to the fractionization observed between the solar photosphere and corona and also analogous to the preferential acceleration observed for high FIP elements at the heliospheric solar wind termination shock. Most of the ⁵⁹Ni produced in the nucleosynthesis of Fe peak nuclei just prior to a SN explosion appears to have decayed to ⁵⁹Co before the cosmic rays have been accelerated, suggesting that the⁵⁹ Ni is accelerated at least 10⁵ yr after it is produced. The decay of certain K capture isotopes produced during cosmic-ray propagation has also been observed for the first time. These measurements suggest that re-acceleration after an initial principal acceleration cannot be large. The high energy spectral indices of cosmic-ray nuclei show a significant charge dependent trend with the index of hydrogen being -2.76 and that of Fe -2.61. The escape length dependence of cosmic rays from our galaxy can now be measured up to ~300 GeV nucl^-1 using the Fe sec/Fe ratio. This escape length is P ^-0.05 above 10 GeV nucl^-1 leading to a typical source spectral index of (2.70±0.10) -0.50 = -2.20 for nuclei. This is similar to the source index of -2.3 inferred for electrons within the errors of ±0.1 in the index for both components. Spacecraft measurements in the outer heliosphere suggest that the local cosmic-ray energy density is ~2eV cm^-3 – larger than previously assumed. Gamma-ray measurements of electron bremsstrahlung below 50 MeV from the Comptel experiment on CGRO show that fully 20–30% of this energy is in electrons, several times that previously assumed. New estimates of the amount of matter traversed by cosmic rays using measurements of the B/C ratio are also higher than earlier estimates – this value is now ~10 g cm^-2 at 1 GeV nucl^-1. Thus altogether cosmic rays are energetically a more important component of our galaxy than previously assumed. This has implications both for the types of sources that are capable of accelerating cosmic rays and also for the role that cosmic rays may play in ionizing the diffuse interstellar medium.     

Cosmic Rays in Galactic and Extragalactic Magnetic Fields

We briefly review sources of cosmic rays, their composition and spectra as well as their propagation in the galactic and extragalactic magnetic fields, both regular and fluctuating. A special attention is paid to the recent results of the X-ray and gamma-ray observations that shed light on the origin of the galactic cosmic rays and the challenging results of Pierre Auger Observatory on the ultra high energy cosmic rays. The perspectives of both high energy astrophysics and cosmic-ray astronomy to identify the sources of ultra high energy cosmic rays, the mechanisms of particle acceleration, to measure the intergalactic radiation fields and to reveal the structure of magnetic fields of very different scales are outlined.     

A cosmochemical view of cosmic rays and solar particles

The composition of cosmic rays and solar particles is reviewed with emphasis on the question of whether they are representative samples of Galactic and solar matter. The composition of solar particles changes with energy and from flare to flare. A strong excess of heavy elements at energies below a few MeV/nuc decreases with energy, and at energies above 15 MeV/nuc the composition of solar particles resembles that of galactic cosmic rays somewhat better than that of the solar atmosphere. The elements Ne through Pb have remarkably similar abundances in cosmic ray sources and in the matter of the solar system. The lighter elements are depleted in cosmic rays, whereas U and Th may be enriched or not, depending on whether the meteoritic or solar abundance of Th is used. Two prototype sources of cosmic rays are considered: gas with solar system composition but enriched in elements with Z > 8 during acceleration and emission (by analogy with solar particle emission), and highly evolved matter enriched in r-process elements such as U, Th and transuranic elements. The energy-dependence of cosmic ray composition suggests that both sources may contribute at different energies.Miller Institute Professor, 1972–73.     

Solar cosmic rays

The first observations of solar cosmic rays were made simultaneously by many investigators at worldwide cosmic-ray stations in the periods of powerful chromospheric flares on February 28 and March 7, 1942. The discovery of these and the investigation of cosmic-ray solar-daily variations with maximum time near noon led some authors (Richtmyer and Teller, 1948; Alfvén, 1949, 1950) to a model of apparent cosmic-ray solar origin. We present here the results of the properties of solar cosmic rays from ground events (experimental and theoretical investigations). We also discuss important information from solar experimental data relating to these ground events observed in September and October 1989 and May 1990. Some experimental evidence of acceleration processes in associated phenomena with flares and long-term (solar cycle) variation of the average flux of solar cosmic rays is discussed as also cornal and interplanetary propagation, and that in the terrestrial magnetosphere. Note that the energy spectrum of solar cosmic rays varied very strongly from one flare to another. What are the causes of these phenomena? What is the nature of chemical and isotopic contents of solar cosmic rays? How can its changes occur in the energy spectrum and chemical contents of solar cosmic rays in the process of propagation? Is it possible to recalculate these parameters to the source? What makes solar cosmic rays rich in heavy nucleus and³He? The important data about electrons, positrons, gamma-quanta and neutrons from flares will be discussed in a subsequent paper (Dorman and Venkatesan, 1992). The question is: What main acceleration mechanism of solar flare and associated phenomena are reliable? These problems are connected with the more general problem on solar flare origin and its energetics. In Dorman and Venkatesan (1993) we will consider these problems as well as the problem of prediction of radiation hazard from solar cosmic rays (not only in space, but also in the Earth's atmosphere too).     

Insights into Cosmic-Ray Acceleration from the Study of Anomalous Cosmic Rays

Cosmic rays from many sources and in many locations exhibit similar, inverse-power-law energy spectra, which suggests a common origin for most cosmic rays. Diffusive shock acceleration appears at present to be this common accelerator. Hence, anomalous cosmic rays, thought to be accelerated at the solar-wind termination shock, provide a relatively accessible laboratory for the study of the mechanism of cosmic-ray acceleration. Observations showing a transition from singly-charged anomalous cosmic-ray oxygen to multiply-charged at an energy of some 250 MeV support the picture of acceleration at the quasi-perpendicular termination shock. Such acceleration may be important in other sources, as well. The basic physics of this acceleration process is discussed in some detail. This revised version was published online in June 2006 with corrections to the Cover Date.     

Synchrotron nebulae

The most frequent manifestation of synchrotron nebulae is the radio emission emanating from radio galaxies and supernova remnants. In general the synchrotron spectra of these objects do not extend into optical and x-ray domains presumably because the high energy electrons needed to sustain such emission are too short-lived. In fact, we knew of only one class of objects in which synchrotron nebulae are observed at frequencies above the radio, namely Crab-like supernova remnants (SNR). In these instances, a central pulsar is presumed to continually accelerate electrons up to the requisite energies, thus balancing the high synchrotron loss rate. The first part of this talk will discuss the available x-ray observations of these sources as well as some of the difficulties in their interpretation. The last part of the talk will be concerned with a new class of synchrotron nebulae associated with binary star systems.     

Corotating Particle Events

The corotating particle events give us a unique opportunity to probe the three-dimensional structures of the heliosphere. This is especially true if we have observations over a period of extreme stability of the CIRs, such as existed over the recent solar minimum. We discuss how the observations fit into the context of current heliospheric magnetic field models. The energetic particle signatures of CIRs throughout the regions of the heliosphere covered by the deep-space missions are reviewed. The CIRs accelerate these particles and at the same time modulate both the high energy galactic cosmic rays and the anomalous cosmic rays.