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.     

Nonlinear Shock Acceleration and Cosmic-Ray Production in Young Supernova Remnants

A number of young supernova remnants (SNRs) are now known to have nonthermal X-ray spectra. The steepness of the X-ray emission suggests that it is synchrotron from TeV electrons, and if this is the case, efficient shock acceleration is likely occurring in these objects. Here we use a model of nonlinear diffusive shock acceleration to fit the broad-band emission from SN1006, Tychos, and Keplers SNRs. Our fits confirm that all of these SNRs are producing TeV particles, but also show that the electron and ion spectra do not extend as a power law above a few TeV, well below the cosmic ray `knee at 10¹⁵ eV.     

A study of M100 in X-rays

During a search for X-ray emission from Supernova 1979c, the parent galaxy M100 (NGC 4321) was repeatedly observed with the IPC and HRI instruments aboard the Einstein X-ray Observatory. The X-ray data reveal two possible sources in the arms of the spiral galaxy, two components in the nuclear bulge and extended X-ray emission from the central part of the galaxy (160x160 square arc seconds centered on the nucleus). We find that the estended X-ray emission cannot be explained in terms of inverse Compton effect on radio, optical or 3 K blackbody photons but rather it is likely to originate from supernova remnants (M100 is indeed a prolific supernova producer) and/or early type stars. As for M100 as a whole, the ratio of X-ray to optical liminosity places it half way between normal galaxies e.g. M31 or M33 and peculiar or active galaxies.     

Nonlinear Kinetic Theory of Cosmic-ray Acceleration in Supernova Remnants

A review of kinetic nonlinear theory for cosmic-ray (CR) acceleration and subsequent -ray production due to CR nuclear component in supernova remnants (SNRs) is presented. The correspondence of the expected spectrum and composition of CRs produced inside SNRs in the Galaxy with the experimental data is discussed. Possible explanations of negative results in searching high energy -ray emission from nearby SNRs are analyzed.     

EXOSAT PSD and CMA observations of the SNR PKS 1209-52

EXOSAT PSD images and spectra are presented of the supernova remnant (SNR) PKS 1209-52 (G296.5+9.7. Milne 23). This source was observed for 8.5 hours in June, 1983. PSD images constructed in different energy intervals reveal that the spatial structure of the SNR is energy dependent. Comparison of the PSD and CMA images with the latest radio map of PKS 1209–52 shows some interesting correlations, especially between the X-ray and radio Hot Spots. The PSD spectrum of the SNR is fitted with a Raymond and Smith line-emission model: the best fit temperature is found to be 1.7×l0⁶ K and the absorbing column is less than 2×10²¹ cm^–2.A compact X-ray source lies within the radio shell of PKS 1209–52, near the centre of the remnant. The PSD spectrum of this object is somewhat harder than that of the SNR, but does not require a significantly different absorbing column density. The possible association of the SNR and the compact object is briefly discussed.     

Spectral observation of the composite supernova remnant G 29.7-0.3

The X-ray properties of the supernova remnant G 29.7-0.3 are discussed based on spectral data from the EXOSAT satellite. In the 2 to 10 keV range a featureless power-law spectrum is obtained, the best-fit parameters being: energy spectral index =-0.77, hydrogen column density on the line of sight N_H=2.3.10²² cm^–2. The incident X-ray flux from the source is (3.6±0.1) 10¹¹ erg cm^–2 s^–1 in the 2 to 10 keV range corresponding to an intrinsic luminosity of about 2. 10³⁶ erg s^–1 for a distance of 19 kpc. The source was not seen with the imaging instrument thus constraining the hydrogen column density to be N_H=(3.3 ±0.3) 10²² cm^–2 and the energy spectral index =1.0±0.15. This new observation is consistent with emission by a synchroton nebula presumably fed by an active pulsar. An upper limit of 1.5% for the pulsed fraction in the range of periods 32ms to 10⁴ s has been obtained.     

Recent results from COMPTEL

The COMPTEL telescope aboard the Compton Gamma Ray Observatory has put MeV -rays into the midst of astronomy. Among recent highlights are the discovery of intense MeV emission from blazar-type active galactic nuclei, the surprising non-detection of any Seyfert galaxy at MeV energies, the first image of the Milky Way in the light of the²⁶Al line at 1.809 MeV (possibly including a detection of the Vela supernova remnant), the discovery of 3–7 MeV emission from the Orion complex, which can be identified with nuclear interaction lines of¹²C and¹⁶O at 4.44 and 6.13 MeV, the detection of the⁴⁴Ti line at 1.15 MeV from the supernova remnant Cas A, and the first results on the spectrum and propagation of low-energy (1–100 MeV) cosmic-ray electrons.also Leiden Observatory     

Mapping of the Cygnus Loop with EXOSAT

Three 2.10⁴ s observations were carried out with EXOSAT on three fields following the shock from the north to the east of the Cygnus Loop supernova remnant. Due to the softness of the source, most of the photons were collected with the LE package (CMA). For each exposure we used at least three filters (4000 Å lexan, Al-Par, boron) in order to extract the maximum spectral information from the data. The few photons gathered with the boron filter are particularly important in that respect. The total count number collected with the other filters allows a statistically significant overall mapping of the fields with 1*1 pixels, but a better resolution can be achieved on the brightest areas of the Loop. Interesting details are revealed, such as bright small spots. Irregularities are also evident both in the shock front and inside the remnant, specially in the northern and eastern fields.     

Magnetic Fields in Supernova Remnants and Pulsar-Wind Nebulae

We review the observations of supernova remnants (SNRs) and pulsar-wind nebulae (PWNe) that give information on the strength and orientation of magnetic fields. Radio polarimetry gives the degree of order of magnetic fields, and the orientation of the ordered component. Many young shell supernova remnants show evidence for synchrotron X-ray emission. The spatial analysis of this emission suggests that magnetic fields are amplified by one to two orders of magnitude in strong shocks. Detection of several remnants in TeV gamma rays implies a lower limit on the magnetic-field strength (or a measurement, if the emission process is inverse-Compton upscattering of cosmic microwave background photons). Upper limits to GeV emission similarly provide lower limits on magnetic-field strengths. In the historical shell remnants, lower limits on B range from 25 to 1000?μG. Two remnants show variability of synchrotron X-ray emission with a timescale of years. If this timescale is the electron-acceleration or radiative loss timescale, magnetic fields of order 1?mG are also implied. In pulsar-wind nebulae, equipartition arguments and dynamical modeling can be used to infer magnetic-field strengths anywhere from ～5?μG to 1?mG. Polarized fractions are considerably higher than in SNRs, ranging to 50 or 60% in some cases; magnetic-field geometries often suggest a toroidal structure around the pulsar, but this is not universal. Viewing-angle effects undoubtedly play a role. MHD models of radio emission in shell SNRs show that different orientations of upstream magnetic field, and different assumptions about electron acceleration, predict different radio morphology. In the remnant of SN 1006, such comparisons imply a magnetic-field orientation connecting the bright limbs, with a substantial density gradient across the remnant.     

The synthesis of helium and CNO isotopes in massive stars

We present helium and CNO isotopic yields for massive mass-losing stars in the initial mass range 15M M _i 50M . We investigate their dependence on assumptions about mass loss rates, internal mixing processes, and metallicity, and specify the contributions from stellar winds and from supernova ejecta.     

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.     

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.     

Periodic or random acceleration in solar flares?

The issue whether acceleration and injection of electron beams is coherently modulated by a single quasi-periodic source, or whether the injection is driven by a stochastic process in time or (eventually fragmented) in space, is investigated by menas of a periodicity analysis of metric type III bursts.We analyze 260 continuous type III groups observed byIkarus (ETH Zurich) in the frequency range of 100–500 MHz during 359 solar flares with simultaneous 25 keV hard X-ray emission, in the years 1980–1983. Pulse periods have been measured between 0.5 and 10 s, and can be described by an exponential distribution, i.e.N(P) e ^–P/1.0s. We measure the mean periodP and its standard deviation p in each type III group, and quantify the degree of periodicity by the dimensionless parameter p/P. The representative sample of 260 type III burst groups shows a mean periodicity of p/P=0.37±0.12, while Monte-Carlo simulations of an equivalent set of truly random time series show a distinctly different value of p/P=0.93±0.26. This result suggests that the injection of electron beams is periodically modulated by a particle acceleration source which is either compact or has a global organization on a time scale of seconds.     

Nucleosynthesis in Supernovae

We present the status and open problems of nucleosynthesis in supernova explosions of both types, responsible for the production of the intermediate mass, Fe-group and heavier elements (with the exception of the main s-process). Constraints from observations can be provided through individual supernovae (SNe) or their remnants (e.g. via spectra and gamma-rays of decaying unstable isotopes) and through surface abundances of stars which witness the composition of the interstellar gas at their formation. With a changing fraction of elements heavier than He in these stars (known as metallicity) the evolution of the nucleosynthesis in galaxies over time can be determined. A complementary way, related to gamma-rays from radioactive decays, is the observation of positrons released in \(\beta^{+}\)-decays, as e.g. from \(^{26}\mbox{Al}\), \(^{44}\mbox{Ti}\), \(^{56,57}\mbox{Ni}\) and possibly further isotopes of their decay chains (in competition with the production of \(e^{+}e^{-}\) pairs in acceleration shocks from SN remnants, pulsars, magnetars or even of particle physics origin). We discuss (a) the role of the core-collapse supernova explosion mechanism for the composition of intermediate mass, Fe-group (and heavier?) ejecta, (b) the transition from neutron stars to black holes as the final result of the collapse of massive stars, and the relation of the latter to supernovae, faint supernovae, and gamma-ray bursts/hypernovae, (c) Type Ia supernovae and their nucleosynthesis (e.g. addressing the \(^{55}\mbox{Mn}\) puzzle), plus (d) further constraints from galactic evolution, \(\gamma\)-ray and positron observations. This is complemented by the role of rare magneto-rotational supernovae (related to magnetars) in comparison with the nucleosynthesis of compact binary mergers, especially with respect to forming the heaviest r-process elements in galactic evolution.     

Observations of fragmented energy release

Energy release into coronal plasmas is observable in the forms of heating and acceleration. In flares and active stars, heating and acceleration have been found to be related as indicated by an approximately constant ratio of microwave (synchrotron) and soft X-ray (thermal) emission. The discovery suggests a flare-like heating process for the quiescent coronae of active stars.The energy release in solar flares involves several time scales: (i) The largest is the rate of homologous flares in an active region of the order of one per five hours. (ii) Hard X-ray andH emissions suggest a total flare duration of ten minutes, (iii) with individual episodes of contiguous acceleration of one minute. (iv) Elementary hard X-ray peaks have 5–10 s duration, corresponding to groups of beams observable as type III radio bursts. (v) The effective injection time of these beams is of the order 0.1 s. (vi) The smaller time scale is observed in narrowband radio spikes in the 0.2–8 GHz range with durations of a few times 0.01 s.     

Wolf–Rayet star nucleosynthesis and the isotopic composition of the Galactic Cosmic Rays

There is now strong observational evidence that the composition of the Galactic Cosmic Rays (GCRs) exhibits some significant deviations with respect to the abundances measured in the local (solar neighbourhood) interstellar medium (ISM). Two main scenarios have been proposed in order to account for these differences (`anomalies). The first one, referred to as the `two-component scenario, invokes two distinct components to be accelerated to GCR energies by supernova blast waves. One of these components is just made of ISM material of `normal solar composition, while the other one emerges from the wind of massive mass-losing stars of the Wolf–Rayet (WR) type. The second model, referred to as the `metallicity-gradient scenario, envisions the acceleration of ISM material whose bulk composition is different from the local one as a result of the fact that it originates from inner regions of the Galaxy, where the metallicity has not the local value. In both scenarios, massive stars, particularly of the WR type, play an important role in shaping the GCR composition. After briefly reviewing some basic observations and predictions concerning WR stars (including s-process yields), this paper revisits the two proposed scenarios in the light of recent non-rotating or rotating WR models.     

Self-acceleration of nuclear flames in supernovae

The speed of the laminar flame in a degenerate carbon-oxygen white dwarf is too slow to produce a Type I supernova. We draw attention to the role of the Landau instability in wrinkling the flame front. This roughening of the front surface can cause an appreciable enhancement in the flame velocity. For typical presupernova conditions the density jump across the front is small. This implies the applicability of the potential approximation for describing the flow pattern and allows derivation of the Frankel equation for flame propagation. We report the results of our numerical experiments with this equation in a highly nonlinear regime. These calculations, coupled with analytic approximations allow us to determine the dependence of the fractal dimension of the front surface on the density jump across the front. Astrophysical implications of our results are discussed.     

Cosmic-ray acceleration and transport,and diffuse galactic γ-ray emission

Cosmic-ray acceleration and transport is considered from the point of view of application to diffuse galactic -ray sources. As an introduction we review several source models, in particular supernovae exploding inside or near large interstellar clouds. The complex problem of cosmic ray transport in random electromagnetic fields is reduced to three cases which should be sufficient for practical purposes. As far as diffusive acceleration is concerned, apart from reviewing the basic physical principles, we point out the relation between shock acceleration and 2nd order Fermi acceleration, and the relative importance of the two processes around interstellar shock waves. For -ray source models the interaction of cosmic rays with dense clouds assumes great importance. Past discussions had been confined to static interactions of clouds with the ambient medium in the sense that no large scale mass motions in the ambient interstellar medium were considered. The well-known result then is that down to some tens of MeV or less, cosmic-ray nucleons should freely penetrate molecular clouds of typical masses and sizes. The self-exclusion of very low energy nucleons however may affect electron transport with consequences for the Bremsstrahlung -luminosity of such clouds.In this paper we consider also the dynamical interaction of dense clouds with a surrounding hot interstellar medium. Through cloud evaporation and accretion there exist mass flows in the cloud surroundings. We argue that in the case of (small) cloud evaporation the galactic cosmic rays will be essentially excluded from the clouds. The dynamic effects of cosmic rays on the flow should be minor in this case. For the opposite case of gas accretion onto (large) clouds, cosmic-ray effects on the flow will in general be large, limiting the cosmic-ray compression inside the cloud to dynamic pressure equilibrium. This should have a number of interesting and new consequences for -ray astronomy. A first, qualitative discussion is given in the last section.Proceedings of the XVIII General Assembly of the IAU: Galactic Astrophysics and Gamma-Ray Astronomy, held at Patras, Greece, 19 August 1982.     

SN 1987A and SN 1993J: Testing stellar evolution theory?

Nearby supernovae like SN 1987A and SN 1993J provide valuable constraints on the late evolution of massive stars. For this purpose, we review evolutionary models for the progenitor of SN 1987A and confront them with five observational/theoretical tests we devised. We show that single-star models (with the possible exception of rapid-rotation models) fail at least two of these tests, while two binary models (accretion and merger models) are consistent with all available constraints. We conclude that it is most likely that the progenitor of SN 1987A had a binary companion, either at the time of the explosion or at least in the not-too-distant past, and that SN 1987A should therefore not be used to calibrate single stellar evolution theory. For SN 1993J, we infer from the presupernova photometry and the early light curve that its progenitor was a 15M star that lost almost all of its hydrogen-rich envelope prior to the supernova. This seems to require that the progenitor underwent stable case C mass transfer. We discuss future observational tests of binary models for both supernovae.     

He-like Ions as Practical Astrophysical Plasma Diagnostics: From Stellar Coronae to Active Galactic Nuclei

We review X-ray plasma diagnostics based on the line ratios of He-like ions. Triplet/singlet line intensities can be used to determine electronic temperature and density, and were first developed for the study of the solar corona. Since the launches of the X-ray satellites Chandra and XMM-Newton, these diagnostics have been extended and used (from C?v to Si?xiii) for a wide variety of astrophysical plasmas such as stellar coronae, supernova remnants, solar system objects, active galactic nuclei, and X-ray binaries. Moreover, the intensities of He-like ions can be used to determine the ionization process(es) at work, as well as the distance between the X-ray plasma and the UV emission source for example in hot stars. In the near future thanks to the next generation of X-ray satellites (e.g., Astro-H and IXO), higher-Z He-like lines (e.g., iron) will be resolved, allowing plasmas with higher temperatures and densities to be probed. Moreover, the so-called satellite lines that are formed closed to parent He-like lines, will provide additional valuable diagnostics to determine electronic temperature, ionic fraction, departure from ionization equilibrium and/or from Maxwellian electron distribution.