Particle Acceleration In Supernova Remnants Research Articles

X-ray polarization: A view deep inside cosmic ray driven turbulence and particle acceleration in supernova remnants

X-ray polarization: A view deep inside cosmic ray driven turbulence and particle acceleration in supernova remnants

Electron Acceleration in Middle-age Shell-type γ-Ray Supernova Remnants

Abstract Over the past decade, γ-ray observations of supernova remnants (SNRs) and accurate cosmic-ray (CR) spectral measurements have significantly advanced our understanding of particle acceleration in SNRs. In combination with multiwavelength observations of a large sample of SNRs, it has been proposed that the highest energy particles are mostly accelerated in young remnants, and the maximum energy that middle-age and old SNRs can accelerate particles to decreases rapidly with the decrease in shock speed. If SNRs dominate the CR flux observed at Earth, a large number of particles need to be accelerated in old SNRs for the soft CR spectrum even though they cannot produce very high-energy CRs. With radio, X-ray, and γ-ray observations of seven middle-age shell-type SNRs, we derive the distribution of high-energy electrons trapped in these remnants via a simple one-zone leptonic emission model and find that their spectral evolution is consistent with such a scenario. In particular, we find that particle acceleration by shocks in middle-age SNRs with age t can be described by a unified model with the maximum energy decreasing as t −3.1 and the number of GeV electrons increasing as t 2.5 in the absence of escape from SNRs.

Anomalous Distributions of Primary Cosmic Rays as Evidence for Time-dependent Particle Acceleration in Supernova Remnants

Abstract Recent precise measurements of cosmic-ray (CR) spectra show that the energy distribution of protons is softer than those of heavier nuclei, and there are spectral hardenings for all nuclear compositions above ∼200 GV. Models proposed for these anomalies generally assume steady-state solutions of the particle acceleration process. We show that if the diffusion coefficient has a weak dependence on the particle rigidity near shock fronts of supernova remnants (SNRs), time-dependent solutions of the linear diffusive shock acceleration at two stages of SNR evolution can naturally account for these anomalies. The high-energy component of CRs is dominated by acceleration in the free expansion and adiabatic phases with enriched heavy elements and a high shock speed. The low-energy component may be attributed to acceleration by slow shocks propagating in dense molecular clouds with low metallicity in the radiative phase. Instead of a single power-law distribution, the spectra of time-dependent solutions soften gradually with the increase of energy, which may be responsible for the “knee” of CRs.

Particle acceleration and non-thermal emission during the V407 Cygni nova outburst

On March 2010, the symbiotic binary V407 Cyg erupted as a result of a nova explosion. The event gave rise to a two-week long burst of >100MeV gamma-rays detected by Fermi/LAT, a unique observation testifying to particle acceleration in the system. The outburst can be considered a scaled-down supernova, with short dynamical time scale, and thus can constitute a test case for theories of the origin of galactic cosmic rays. We aim at determining the properties of the accelerated particles and identifying the origin of the high-energy radiation. We developed a model for diffusive shock acceleration and non-thermal emission in V407 Cyg, complemented by an evaluation of the thermal emission from the shocked plasma. We considered both leptonic and hadronic contributions to the non-thermal processes, and investigated the effect of many binary and nova parameters. The gamma-ray emission is mostly of leptonic origin and arises predominantly from inverse-Compton scattering of the nova light. Upscattering of the red giant photons is a minor contribution. Matching the light curve requires gas accumulation in the vicinity of the white dwarf, as a consequence of wind accretion, while the spectrum imposes particle scattering close to the Bohm limit in the upstream equipartition magnetic field. The nova accelerated protons (respectively electrons) with energies up to ~300GeV (respectively ~20GeV), for a total non-thermal energy 10^43 erg after two weeks, representing ~10% of the initial nova kinetic energy. The electron-to-proton ratio at injection is 6%. The V407 Cyg eruption can be understood from the same principles that are invoked for particle acceleration in supernova remnants, although without the need for strong magnetic field amplification. The population of novae in symbiotic systems is a negligible source of Galactic cosmic rays, and most likely not a class of TeV-emitters.

Observational Signatures of Particle Acceleration in Supernova Remnants

We evaluate the current status of supernova remnants as the sources of Galactic cosmic rays. We summarize observations of supernova remnants, covering the whole electromagnetic spectrum and describe what these observations tell us about the acceleration processes by high Mach number shock fronts. We discuss the shock modification by cosmic rays, the shape and maximum energy of the cosmic-ray spectrum and the total energy budget of cosmic rays in and surrounding supernova remnants. Additionally, we discuss problems with supernova remnants as main sources of Galactic cosmic rays, as well as alternative sources.

A census of high-energy observations of Galactic supernova remnants

We present the first public database of high-energy observations of all known Galactic supernova remnants (SNRs). In Section 1 we introduce the rationale for this work motivated primarily by studying particle acceleration in SNRs, and which aims at bridging the already existing census of Galactic SNRs (primarily made at radio wavelengths) with the ever-growing but diverse observations of these objects at high-energies (in the X-ray and γ-ray regimes). In Section 2 we show how users can browse the database using a dedicated web front–end (http://www.physics.umanitoba.ca/snr/SNRcat). In Section 3 we give some basic statistics about the records we have collected so far, which provides a summary of our current view of Galactic SNRs. Finally, in Section 4, we discuss some possible extensions of this work. We believe that this catalogue will be useful to both observers and theorists, and timely with the synergy in radio/high-energy SNR studies as well as the upcoming new high-energy missions. A feedback form provided on the website will allow users to provide comments or input, thus helping us keep the database up-to-date with the latest observations.

Numerical simulations of diffusive shock acceleration in SNRs

A new numerical model of the nonlinear diffusive shock acceleration is presented. It is used for modeling of particle acceleration in supernova remnants. The model contains coupled spherically symmetric hydrodynamic equations and the transport equations for energetic protons, ions and electrons. The forward and reverse shocks are included in the consideration. The spectra of cosmic rays released into interstellar medium from a supernova remnant are determined. The role of the reverse shock in the production of cosmic ray ions and positrons is discussed.

Time-dependent particle acceleration in supernova remnants in different environments

We simulate time-dependent particle acceleration in the blast wave of a young supernova remnant (SNR), using a Monte Carlo approach for the diffusion and acceleration of the particles, coupled to an MHD code. We calculate the distribution function of the cosmic rays concurrently with the hydrodynamic evolution of the SNR, and compare the results with those obtained using simple steady-state models. The surrounding medium into which the supernova remnant evolves turns out to be of great influence on the maximum energy to which particles are accelerated. In particular, a shock going through a $\rho \propto r^{-2}$ density profile causes acceleration to typically much higher energies than a shock going through a medium with a homogeneous density profile. We find systematic differences between steady-state analytical models and our time-dependent calculation in terms of spectral slope, maximum energy, and the shape of the cut-off of the particle spectrum at the highest energies. We also find that, provided that the magnetic field at the reverse shock is sufficiently strong to confine particles, cosmic rays can be easily re-accelerated at the reverse shock.

Particle acceleration in supernova remnants

Supernova remnants (SNR) are the most likely source of galactic cosmic rays (CRs) up to the ‘knee’ in the spectrum at a few PeV. The theory of diffusive shock acceleration nicely supplies a power law energy distribution with approximately the desired spectral index and with suitably high efficiency. For a SNR blast wave expanding into a typical interstellar magnetic field the predicted maximum CR energy falls short of 1 PeV, but a non-resonant plasma instability allows the CRs themselves to amplify the magnetic field by orders of magnitude to a level capable of accelerating CRs to the knee.

A model of polarized X-ray emission from twinkling synchrotron supernova shells

Synchrotron X-ray emission components were recently detected in many young supernova remnants (SNRs). There is even an emerging class - SN1006, RXJ1713.72-3946, Vela Jr, and others - that is dominated by non-thermal emission in X-rays, also probably of synchrotron origin. Such emission results from electrons/positrons accelerated well above TeV energies in the spectral cut-off regime. In the case of diffusive shock acceleration, which is the most promising acceleration mechanism in SNRs, very strong magnetic fluctuations with amplitudes well above the mean magnetic field must be present. Starting from such a fluctuating field, we have simulated images of polarized X-ray emission of SNR shells and show that these are highly clumpy with high polarizations up to 50%. Another distinct characteristic of this emission is the strong intermittency, resulting from the fluctuating field amplifications. The details of this "twinkling" polarized X-ray emission of SNRs depend strongly on the magnetic-field fluctuation spectra, providing a potentially sensitive diagnostic tool. We demonstrate that the predicted characteristics can be studied with instruments that are currently being considered. These can give unique information on magnetic-field characteristics and high-energy particle acceleration in SNRs.

A kinetic approach to cosmic-ray-induced streaming instability at supernova shocks

We show that a purely kinetic approach to the excitation of waves by cosmic rays in the vicinity of a shock front leads to predict the appearance of a non-Alfvenic fast-growing mode which has the same dispersion relation as that previously found by Bell in 2004 by treating the plasma in the magnetohydrodynamic approximation. The kinetic approach allows us to investigate the dependence of the dispersion relation of these waves on the microphysics of the current which compensates the cosmic ray flow. We also show that a resonant and a non-resonant mode may appear at the same time and one of the two may become dominant on the other depending on the conditions in the acceleration region. We discuss the role of the unstable modes for magnetic field amplification and particle acceleration in supernova remnants at different stages of the remnant evolution.

Particle Acceleration in Supernova Remnants and the Production of Thermal and Nonthermal Radiation

If highly efficient, cosmic ray production can have a significant effect on the X-ray emission from SNRs as well as their dynamical evolution. Using hydrodynamical simulations including diffusive shock acceleration, we produce spectra for both the thermal and nonthermal forward shock emission. For a given ambient density and explosion energy, we find that the position of the forward shock at a given age is a strong function of the acceleration efficiency, providing a signature of cosmic-ray production. Using an approximate treatment for the ionization state of the plasma, we investigate the effects of slow vs. rapid heating of the postshock electrons on the ratio of thermal to nonthermal X-ray emission at the forward shock. We also investigate the effects of magnetic field strength on the observed spectrum for efficient cosmic-ray acceleration. The primary effect of a large field is a considerable flattening of the nonthermal spectrum in the soft X-ray band. Spectral index measurements from X-ray observations may thus be indicators of the postshock magnetic field strength. The predicted gamma-ray flux from inverse-Compton (IC) scattering and neutral pion decay is strongly affected by the ambient conditions and, for the particular parameters used in our examples, the IC emission at E ~ 1 TeV exceeds that from pion decay, although at both lower and higher energies this trend is reversed for cases of high ambient density. More importantly, high magnetic fields produce a steepening of the electron spectrum over a wide energy range which may make it more difficult to differentiate between IC and pion-decay emission solely by spectral shape.

Shocks and particle acceleration in supernova remnants: observational features

The last 10 years a number of observational advances have substantially increased our knowledge of shock phenomena in supernova remnants. This progress has mainly been made possible by the recent improvements in X-ray and γ-ray instrumentation. It has become clear that some shell-type supernova remnants, e.g., SN 1006, have X-ray emission dominated by synchrotron radiation, proving that electrons are accelerated up to 100 TeV. This is still an order of magnitude below 3 × 10 15 eV, at which energy the ion cosmic-ray spectrum at earth shows a spectral break. So one of the major goals is to test whether supernova remnants are indeed capable of accelerating ions at least up that energy. Here I review the evidence that ions and electrons are accelerated up to energies ∼100 TeV in supernova remnants, and, in addition, the recent progress that has been made in understanding the physics of collisionless shock fronts and the magnetic fields inside supernova remnants.

Shocks and particle acceleration in SNRs: theoretical aspects

Energetic particles are an essential component in supernova remnants (SNRs). Non-thermal particle acceleration in supernova shocks is expected to be an efficient process at different evolutional stages of SNRs. Non-linear wave–particle interactions being the governing process of the SNR collisionless shock formation are responsible for both shock heating and compression of the thermal gas as well as for creation of energetic particle population. The gas temperature behind such a shock could depart strongly from that predicted by the standard Rankine–Hugoniot law. We overview current models of collisionless supernova shocks with an emphasis on their confrontation with observed multi-wavelength spectra of SNRs from the radio to γ-rays. Simple scaling relations for post-shock ion temperature in the shocks with efficient particle acceleration are presented. A special attention is paid to the connections with high resolution SNR spectra currently obtained with XMM-Newton and Chandra. We discuss X-ray line emission from fast moving supernova ejecta fragments, arguing that they could contribute substantially to the observed line emission of the galactic ridge. We also consider the non-thermal appearance of SNRs interacting with molecular clouds.

Particle Acceleration in Supernova Remnants (SNRs)(SNR,Prospects of High Energy Astrophysics)

Particle Acceleration in Supernova Remnants (SNRs)(SNR,Prospects of High Energy Astrophysics)

Particle acceleration in SNRs

We present the results of hard X-ray emissions from the shell of supernova remnants (SNR) revealed with the ASCA imaging and spectroscopic observations. The hard X-ray spectra and morphologies are consistent with synchrotron emission from extremely high energy electrons of 10–100 TeV. We infer that significant fractions of the Galactic cosmic rays below the energy of 10 14.5 eV are produced by the Fermi acceleration mechanisms at the shocked shell of SNRs.

Kinetic theory of cosmic rays and gamma rays in supernova remnants. I. Uniform interstellar medium

Abstract Kinetic models of particle acceleration in supernova remnants (SNRs) are used to determine the cosmic ray (CR) nucleon and, for the first time, also the associated γ-ray spectrum during SN shock propagation in a uniform interstellar medium. SNR evolution is followed numerically taking into account the backreaction of accelerated CRs on the overall dynamics. The high energy CRs also produce π0-decay γ-rays. The model for SNRs includes injection of suprathermal particles at the shock front and heating of the thermal plasma due to the dissipation of Alfven waves in the precursor region. It is shown that the CRs are accelerated with very high efficiency. About 50% of the explosion energy is absorbed by CRs at maximum during the SNR evolution even for relatively low injection rates. The maximum energy achieved by accelerated CR protons is about 1014 eV for a Bohm-limit diffusion coefficient. The main flux of high energy γ-rays is produced during the early Sedov phase and decreases thereafter. The results are compared with earlier models based on the hydrodynamic approximation for CR transport and test particle estimates. For a moderate ambient gas density of 0.3 H-atoms cm−3, corresponding to a warm interstellar medium, a magnetic field strength of 5 μG, and an explosion energy of 1051 erg, the integral TeV γ-ray flux from a SNR at a distance of 1 kpc exceeds 10−11 photons cm−2s−1 at peak luminosity. This is higher than previously estimated peak fluxes by factors of the order of 7, and is primarily due to the large shock compression during the sweep-up phase. Spatially, SNRs without a central compact object are shell sources also in γ-rays throughout all their evolutionary phases. Given that such SNRs exist close-by they should be observable in particular with sensitive ground based instruments.

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.

Numerical studies of diffusive particle acceleration in supernova remnants

The time-dependent acceleration of cosmic ray particles is analysed by the Fermi, first-order diffusive shock process in supernova remnants. The convection-diffusion transport equation for the cosmic ray particles has been numerically solved self-consistently with the gas dynamic equations for the underlying flow. A model SNR characterised by an explosion energy of 10 51 ergs, an ejected mass of 10 M ○. and an ISM gas density of 3×10 -3 cm -3 is followed through the adiabatic, Sedov-Taylor phase. A wide range of diffusion coefficients is considered.

On particle acceleration in supernova remnants

The applicability of first and second order Fermi acceleration to electrons in supernova remnants is briefly examined.