Numerical Studies of Cosmic Ray Injection and Acceleration

A numerical scheme that incorporates a thermal leakage injection model into a combined gas dynamics and cosmic ray (CR, hereafter) diffusion-convection code has been developed. The particle injection is followed numerically by filtering the diffusive flux of suprathermal particles across the shock to the upstream region according to a velocity-dependent transparency function that controls the fraction of leaking particles. We have studied CR injection and acceleration efficiencies during the evolution of CR modified planar shocks for a wide range of initial shock Mach numbers, $M_0$, assuming a Bohm-like diffusion coefficient. The injection process is very efficient when the subshock is strong, leading to fast and significant modification of the shock structure. As the CR pressure increases, the subshock weakens and the injection rate decreases accordingly, so that the subshock does not disappear. Although some fraction of the particles injected early in the evolution continue to be accelerated to ever higher energies, the postshock CR pressure reaches an approximate time-asymptotic value due to a balance between fresh injection/acceleration and advection/diffusion of the CR particles away from the shock. We conclude that the injection rates in strong parallel shocks are sufficient to lead to rapid nonlinear modifications to the shock structures and that self-consistent injection and time-dependent simulations are crucial to understanding the non-linear evolution of CR modified shocks.Comment: 28 pages, To appear in ApJ November 1, 2002 issu

Efficiency of Nonlinear Particle Acceleration at Cosmic Structure Shocks

We have calculated the evolution of cosmic ray (CR) modified astrophysical shocks for a wide range of shock Mach numbers and shock speeds through numerical simulations of diffusive shock acceleration (DSA) in 1D quasi- parallel plane shocks. The simulations include thermal leakage injection of seed CRs, as well as pre-existing, upstream CR populations. Bohm-like diffusion is assumed. We model shocks similar to those expected around cosmic structure pancakes as well as other accretion shocks driven by flows with upstream gas temperatures in the range $T_0=10^4-10^{7.6}$K and shock Mach numbers spanning $M_s=2.4-133$. We show that CR modified shocks evolve to time-asymptotic states by the time injected particles are accelerated to moderately relativistic energies (p/mc \gsim 1), and that two shocks with the same Mach number, but with different shock speeds, evolve qualitatively similarly when the results are presented in terms of a characteristic diffusion length and diffusion time. For these models the time asymptotic value for the CR acceleration efficiency is controlled mainly by shock Mach number. The modeled high Mach number shocks all evolve towards efficiencies $\sim 50$%, regardless of the upstream CR pressure. On the other hand, the upstream CR pressure increases the overall CR energy in moderate strength shocks ($M_s \sim {\rm a few}$). (abridged)Comment: 23 pages, 12 ps figures, accepted for Astrophysical Journal (Feb. 10, 2005

Shock Acceleration of Cosmic Rays - a critical review

Motivated by recent unsuccessful efforts to detect the predicted flux of TeV gamma-rays from supernova remnants, we present a critical examination of the theory on which these predictions are based. Three crucial problems are identified: injection, maximum achievable particle energy and spectral index. In each case significant new advances in understanding have been achieved, which cast doubt on prevailing paradigms such as Bohm diffusion and single-fluid MHD. This indicates that more realistic analytical models, backed by more sophisticated numerical techniques should be employed to obtain reliable predictions. Preliminary work on incorporating the effects of anomalous transport suggest that the resulting spectrum should be significantly softer than that predicted by conventional theory.Comment: 8 pages, invited review presented at the 17th ECRS, Lodz, July 2000; to appear in Journal of Physics G: Nuclear and Particle Physic

T Cell Activation Results in Conformational Changes in the Src Family Kinase Lck to Induce Its Activation

n/

Nonthermal Radiation from Type Ia Supernova Remnants

We present calculations of expected continuum emissions from Sedov-Taylor phase Type Ia supernova remnants (SNRs), using the energy spectra of cosmic ray (CR) electrons and protons from nonlinear diffusive shock acceleration (DSA) simulations. A new, general-purpose radiative process code, Cosmicp, was employed to calculate the radiation expected from CR electrons and protons and their secondary products. These radio, X-ray and gamma-ray emissions are generally consistent with current observations of Type Ia SNRs. The emissions from electrons in these models dominate the radio through X-ray bands. Decays of \pi^0 s from p-p collisions mostly dominate the gamma-ray range, although for a hot, low density ISM case (n_{ISM}=0.003 cm^{-3}), the pion decay contribution is reduced sufficiently to reveal the inverse Compton contribution to TeV gamma-rays. In addition, we present simple scalings for the contributing emission processes to allow a crude exploration of model parameter space, enabling these results to be used more broadly. We also discuss the radial surface brightness profiles expected for these model SNRs in the X-ray and gamma-ray bands.Comment: 37 pages, 7 figures, accepted in MNRA

A Multi-Frequency Radio Study of Supernova Remnant G292.0+1.8 and its Pulsar Wind Nebula

(Abridged) We present a detailed radio study of the young supernova remnant (SNR) G292.0+1.8 and its associated pulsar PSR J1124-5916, using the Australia Telescope Compact Array at observing wavelengths of 20, 13 and 6 cm. We find that the radio morphology of the source consists of three main components: a polarized flat-spectrum central core coincident with the pulsar J1124-5916, a surrounding circular steep-spectrum plateau with sharp outer edges and, superimposed on the plateau, a series of radial filaments with spectra significantly flatter than their surroundings. HI absorption argues for a lower limit on the distance to the system of 6 kpc. The core clearly corresponds to radio emission from a pulsar wind nebula powered by PSR J1124-5916, while the plateau represents the surrounding SNR shell. The plateau's sharp outer rim delineates the SNR's forward shock, while the thickness of the plateau region demonstrates that the forward and reverse shocks are well-separated. Assuming a distance of 6 kpc and an age for the source of 2500 yr, we infer an expansion velocity for the SNR of ~1200 km/s and an ambient density ~0.9 cm^-3. We interpret the flat-spectrum radial filaments superimposed on the steeper-spectrum plateau as Rayleigh-Taylor unstable regions between the forward and reverse shocks of the SNR. The flat radio spectrum seen for these features results from efficient second-order Fermi acceleration in strongly amplified magnetic fields.Comment: 11 pages of text, plus 7 embedded EPS figures. Accepted to ApJ. Added missing units on x-axis of Fig